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Brakes

BRAKE PADS
STRUCTURE AND GEOMETRY

The pad is essentially a piece of material designed to rub against the disc surface in order to convert mechanical energy into thermal energy. In this sense it is no different from the linings in a drum brake. Its distinguishing feature, however, is that the friction surface is flat. We can imagine calipers where pads are nothing more than a piece of friction material.

Section of a pad

In reality the pad is rather more complicated as it is made up of numerous parallel layers produced from different materials. The thickest layer is the true friction material that comes into contact with the disc and gradually wears down. On the opposite side is the support or plate, a flat plate of mild steel about 5 mm thick. Its main purpose is to distribute the force exerted by the piston over the pad's entire contact surface. In fact friction materials are quite fragile and subject to breakage, the direct contact of the piston on a limited area would risk damaging them. The thickness of the support is therefore calculated so that under maximum force it has an imperceptible flexing distortion that does not cause the material to wear unevenly. The purpose of the support is also to secure and position the pad. In particular, sections of this metal plate rest against the caliper during braking. This is because the disc tends to drag the pad in the direction of rotation.

Different shapes for pads.

Many types of pad have an additional layer about 2 mm thick between the support and the friction material: this is known as the substrate. It has a number of functions: it bridges the dilation coefficients of the steel and the lining, it acts as a thermal insulator so preventing an excessive flow of heat towards the piston and brake fluid, it absorbs noise and vibrations. This layer does not have a friction function as it is mandatory for the pad to be replaced before wear exposes it. During production a layer of adhesive is interposed between the substrate and the support in order to improve the adherence: the quality of the adhesive is most important as it conditions the pad's cutting resistance. Behind the support there may also be a sheet of what is usually rubber coated steel an anti-noise layer which has the function of minimizing the transmission of vibrations. After manufacture most pads are fitted with a wear indicator that normally consists of an insulated electric wire which comes into contact with the disc when the maximum pad wear level is reached. The shape of the pad is a compromise between two types of geometry. Firstly, the pad must rub against the disc surface and therefore its natural shape is a section between two arcs of a circle. The piston has a circular section; the isobar curves are therefore circumferences. This shape enables a balanced distribution of forces to be obtained.

The shape of a pad is a compromise between two geometrical forms.

The definitive form is the result of this compromise, but it is also based on calculations. Calculations performed on finished elements not only make it possible to determine pressure distribution but also provide useful information relative to both localized stresses that may possible cause breakage, and heat diffusion.

Finite element modeling of a pad. The red area is that subjected to higher pressure.

COMPOSITION
In order to understand the relative complexity of pad composition reference must be made to specifications summarizing the qualities required. In addition to basic geometrical and mechanical aspects the concepts of safety, comfort and economy play a part. As each vehicle differs from others in this respect it is often necessary to develop a product that is exactly suited to a new model. This takes place during the vehicle development stage and is the result of a long series of modifications and tests.

Volume composition of a pad.

When describing the composition of today's pads in detail it is difficult not to refer to old formulas that contained a large proportion of asbestos. This component, now no longer present in European products, had a profound effect on how friction products were conceived and used. Asbestos is a mechanical and heat-resistant mineral fibre. However, as it was not replaced by an alternative material, it became necessary to modify the composition of friction materials considerably. Furthermore, simultaneous with the disappearance of asbestos-based products, new needs made themselves felt as regards safety and comfort. It is therefore difficult to compare old and new products since, seen overall, the latter have substantially improved properties.
The composition of products clearly also depends on the company conducting the research and producing them. In spite of this, the same broad categories of constituent substances are found in all formulations: active products, diluters, modifiers and consolidators.
The components that confer the main braking properties are the solid abrasives and lubricants. In the composition these components are normally never of one single type as certain are more active when cold and others when hot. A number of very abrasive or very lubricating compounds are added to the mixture in order to achieve the ideal balance. As in the case of pharmaceutical products, these active elements are diluted using mechanical and chemically resistant loads, however, which have only a slight abrasive or lubricating action as far as cast iron is concerned. The physical properties of this initial mixture are modified by the addition of elastomers that give the product a certain elasticity and reduce its rigid and fragile nature. Heat transfer properties are also modified by adding powders or metal fibres.
Consolidation of this mixture -essentially comprising powders- is achieved by adding fibres which are the reinforcement of the finished product. Lastly, a binder is required that can bind these various particles together so that the mixture becomes a compact solid. The basic binder used by all manufacturers is a thermo-hardening phenolic resin.
 

 CHARACTERISTICS OF A DISC

 COMPOSITION DISC

Formula 1 carbon disc.

The two main functions of a brake disc or drum are the transmission of a considerable mechanical force and dissipation of the heat produced, that implies functioning at medium or high temperatures. From a theoretical standpoint numerous materials would be able to fulfill these functions. In reality, for reasons of performance stability , cost of raw materials and ease of production, cast iron is the material universally used. However, other materials are used for specific braking applications. For example, composite carbon matrix materials are employed in the production of brake discs for competition cars and airplanes, although their particular performance level and cost make them inappropriate for use on standard vehicles. Also aluminum alloys containing silicon carbide can be considered as they afford a significant reduction in weight, although their inability to support high temperatures means that brakes have to be oversized, a factor which partly cancels out the weight advantage. Cast iron in one of its numerous forms therefore remains the preferred material.
 

CAST IRON DISC

Cast iron is the first product obtained in steel making when smelting iron ore. It is the result of the reduction of ferrous oxides under the action of the carbon in metallurgical coke. Molten cast iron is in reality a carbon solution in molten iron. When it is cooled a very small part of the carbon remains in the ferrous solution whereas the majority of it precipitates to form small nodules scattered throughout the structure of the metal. Usually this unrefined cast iron is not suitable for the majority of applications but must undergo both physical and chemical processes in order to create the vast and well-known range of ferrous alloys. The main products are the many types of stainless steel, cast iron and mild steels which have a very low carbon content and are widely used because of the property whereby they can be shaped by means of bending and rolling.

Cast iron is defined as any alloy with a greater than 2% carbon content. In standard cast irons this content is always more than 3% in terms of weight.

It is this that gives the material a special structure since carbon -hardly soluble at all in a solid state in iron- precipitates under various forms and, given its density , represents between 12-15% of total volume, This high carbon, and above all graphite content gives the alloy good thermal conductivity , but also a certain fragility , Three broad families of cast iron can be identified, White cast iron is very hard, breaks easily and is difficult to work, Nodular or spheroidal grey cast iron contains, as the name indicates, precipitated carbon in the form of small nodules.
Lamellar graphite grey cast iron is the most common type and is used for the examination of production of discs and drums in braking systems. As can readily be observed in lamellar the micrograph analyses, the graphite is present in the form of small plates that structured grey seem like threads as they are only seen in section.
These various cast irons all have a rather similar composition and therefore the actions taken to obtain the different structures involve the additives and, above all, thermal treatment conditions during the processing and cooling stages. In order for the product to be considered good quality for disc production it must be homogeneous from the micrographic standpoint: thin plates of graphite dispersed in the perlite.
Perlite itself is a succession of very thin cementite (Fe3C) plates within the ferrite, that is to say, a very pure iron. Cast irons that contain large inclusions of only ferrite or cementite are discarded. In fact cementite has a very high hardness rating, equal to more than 650 HV1 (VICKERS hardness scale that corresponds to 750 HB on the BRINELL scale).

Micrographic examination of lamellar structured gray cast iron. x100 magnification.

Its presence would create localized hard points and make it inappropriate for friction material as these would act as a powerful abrasive. On the contrary ferrite has a hardness of around 100 HB, much lower than that of cast iron which is about 200 HB. Another factor that comes into play in friction is the dimension of the graphite plates that normally range between 15 mm and 500 mm. The arrangement of these plates must be random as opposed to organized. The latter case can arise during the disc production process if cooling is not properly controlled. The cast iron becomes fragile and discs made from it are not appropriate for use.
 

SPECIAL CAST IRONS
A cast iron can be characterized by a large number of chemical and physical parameters. For brake disc applications, however, only a limited number of these are normally taken into account and are closely linked to its use. Grey cast iron, less fragile than white cast iron, is however not malleable or ductile enough. Elasticity is minimal and it rapidly reaches breaking point. The value for breaking resistance varies on the basis of composition and structure, but the order of magnitude is 25 daN/mm2. This value is checked systematically and is also necessary for standardization. The same can be said for hardness which is much easier to measure and much more representative of the structure and homogeneity .As far as grey cast irons are concerned the Brinell hardness values lie between 170 and 250 HB. Other parameters are measured indirectly: for example, casting, machining and milling properties, or capability to dampen vibrations and therefore noise. It is in fact possible to measure the Young dynamic modulus and the damping factor for samples or perform overall measurements on finished discs. It can be noted that for every type of vehicle and disc it would be necessary to use a type of cast iron with specific properties. Although this is not always possible, numerous qualities of cast iron are used, depending on the specific characteristics required. This choice is part of a brake disc designer's know-how. Two aspects come into play when modifying a cast iron: its composition and thermal treatment during casting. A typical grey cast iron contains between 93-94% of iron in the mass. This iron, plus the three main additives, constitutes more than 99.5% of the overall mass, although this does not mean that the remaining additives do not play a part. In addition to carbon the main additive in the alloy we find silicon, the content of which must be carefully controlled because although on the one hand it has the benefit of improving casting properties, on the other it also reinforces graphitising and increases fragility. Metallurgical experts usually speak in terms of carbon equivalent content. Certain additives in fact fulfill a similar role to that obtained by increasing the carbon content. In the case of the carbon illustrated here, the carbon content is 3.35% whereas the carbon equivalent content is 4.1%. Manganese, found as a result of the metallurgical process, must only be present in very limited quantity. In fact this element combines with sulphur to form Manganese sulphide (MnS) that gives rise to very hard, abrasive granules which, in addition, make machining very difficult. The maximum manganese sulphide content is 1 %. Other elements found in the alloy are almost always the result of impurities in the ore of the compounds used in casting. Nickel, chrome and copper in particular all affect the metallurgical structure and therefore hardness. Their content normally does not exceed 0.2% although certain nickel of copperrich cast irons have been developed to satisfy specific technical requirements. Depending on the quality of cast iron sought, the maximum and minimum values for additive content must fall within a very narrow range of the batch concerned should be immediately discarded.

Basic composition of a grey cast iron.

 

SHAPE
While it is true that some discs were and still are produced according to simple, flat and circular geometry, their shape is normally more complex and can be broken down into a number of parts, each corresponding to the particular function performed.

Finite element analysis view of a disc.

The braking surface is the area on which the braking action of the friction material takes place. Dimensions are such as to ensure that the specific power output is not too high. A value of 230 Watt per cm2 of braking surface is the basis for calculating size, although this value can change considerably when the disc is very well ventilated and can reach 623 Watt per cm2.
The second function is that of attachment provided by the central part of the disc which has a circular aperture which serves to center the wheel axle. The central part of the disc is surrounded by a number of holes for the hub screws and wheel bolts.
The part that serves to connect the braking surface to the central part almost always has the shape of a horizontal cylinder. This part, together with the central part, is referred to as the carrier or the hat because of its shape.
The internal surface of the carrier is often used to fit a small drum brake, usually a parking brake, that, as mentioned previously, is known as the Drum-in-Hat technique.
As we will see later, most of the heat produced in braking is transferred to the disc. Since the latter cannot accumulate an infinite quantity of heat, a way of dissipating it must be devised. The most simple manner is to ensure circulation of air which heats up when in contact with the disc and keeps the temperature at an acceptable level in order to keep it intact.
The disc is therefore required to perform two additional tasks: induce air movement like the rotor in a centrifugal fan and, simultaneously, act as a heat exchanger like a radiator. The circular shape of a disc makes it particularly well suited to this dual role. In fact as the disc rotates it sets in motion the laminar stratum of air with which it is in contact. The external part of the disc rotates at a greater linear speed than the part near to the carrier or hat. Here, dynamic pressure acting on the air is greater in as much as it varies with the square of the speed.

A brembo dual-disc caliper.

The result is that air is sucked from the central point towards dual-disc caliper: the periphery; movement is created and the air moving over the surface of the disc gradually heats up which in its turn tends to increase circulation.. This mechanism already exists in the case of solid discs and is sufficient when only small to medium energy levels need to be transferred, as in the case of lighter cars. As thermal energy dissipated in braking increases, the solid disc surfaces are no longer sufficient. It would be necessary , for instance, to increase the radius of the disc, but this would quickly become incompatible with the size of the wheel. It is possible to use multiple discs, as is the case on certain special or heavy vehicles, although this increases the complexity of brakes enormously.
The solution adopted universally is the ventilated disc. In reality the ventilated disc is a dual disc comprising two plates separated by metal bridges which join them together while allowing the passage of air.

Fixed disc.

Ventilated disc.
 
 
 
 
 
 

Four blade shapes.

The outer surfaces only are used for friction purposes, Thanks to air circulation between the plates, cooling takes place not only over the external surfaces (as with solid discs) but also over the internal surfaces. Air could enter from one side ar the other relative to the carrier. For ventilation to be efficient, however, the side opposite the carrier is almost always preferred. In fact, the presence of the wheel obstructs the entry of air. Certain heavy trucks have discs where the entry of air alternates between the two sides.
Air is set in motion between the two plates and exchanges thermal energy with the surfaces of the cast iron. To a great extent this circulation depends on the farm of the metal bridges, known as blades, as in a turbine. The shape of the blades is a compromise between efficiency and the production difficulties they create. The output of a turbine is given by the ratio between energy transmitted ta the gas and the energy required ta make the turbine rotate. This output improves when the blades are shaped and does not obstruct movement of the gas. This is why discs receiving a considerable quantity of energy have shaped blades which, at a given rotational speed, optimize the speed of circulation. There is, however, a limit represented by the speed of heat transfer from within the metal towards the gas. We have to bear in mind that same blades shape requires the production of both specific right and left discs.
 

 MECHANICAL STRESSES
When the vehicle is in motion and the brakes are not applied, the disc is subjected to very little mechanical stress. There is only a traction force created by the centrifugal effect due to rotation of the disc. During braking the disc is subjected to two additional forces. Firstly, compression force as a result of the pads pressing perpendicularly against the surfaces of the disc.

Mechanical stresses exerted on a disc.

In turn this force is the result of the application of brake fluid pressure on the piston surfaces within the caliper. This force, very high at maximum pressure values (for example, 80 bar) creates a compression force on the cast iron to the order of a few Newton per mm2, a very low value for this material even when hot. Instead in the case of ventilated discs, this force is only exerted on the bladed section, increasing the force tenfold at that point: This force is also exerted on the surface between the blades and can bend it -usually in an irreversible manner- if the force remains within the cast iron's elastic limit. It should be noted that the main limit to strong compression is the friction material.
Braking action due to the pads rubbing against the disc surface is translated into a tractive force on the cast iron. In fact the part in contact with the pad is braked, namely is subjected to a force that opposes rotary motion, whereas the part not in contact with the pad is drawn in the direction of the disc's rotation. Even if the entire force is applied at the center of the pad's thrust, traction force values of approximately 1-2 daN/mm2 are obtained which are no comparison to the traction resistance of cast iron, equal to about 200 MPa, namely 20 daN/mm2.

As this force is distributed over the entire pad surface, its value is still lower and remains well below the breaking point. It should be emphasized however that this limit falls with temperature and is very heavily accentuated when cracks start to appear in the cast iron. In such cases breakage can occur. The micro-cracking that may occur after long periods of use is linked to this type of repeated stress and is known as fatigue.
Therefore there is an ample margin between mechanical stresses applied to the disc and the limits, if reached, that could cause breakage. In order to complete the list of forces acting on the disc both flexing that can occur when braking on a bend and the dynamic stresses found when the disc vibrates must also be added.

THERMAL STRESSES

All energy that a vehicle loses during braking ( with the clutch disengaged) is found in the form of heat generated at the disc/pad interface. The flow of heat created as braking begins is considerable; in our example it is of around one hundred kilowatts a very high power indeed. By way of comparison, to have a similar electrical power level available a domestic user would need to equip the network for an intensity of 450 Amps at 220 Volts. In practice this power falls in a linear manner to zero when deceleration is constant. In spite of this, the total energy released by a wheel -equal to around 20 kilojoule- would bring a little over one litre of water to boiling point in 7 seconds.
Heat is generated by contact between pad and disc surfaces. Localized temperature increases are considerable although this is by no means easy to measure. It can, however, be calculated approximately. Given the considerable temperature gradient, heat disperses in the two materials that come into contact to a degree based on their specific property in terms of this phenomenon.
Distribution of heat flows depends on the physical-chemical properties of the two materials; it remains relatively constant as far as cast irons are concerned whereas it tends to vary somewhat in the case of friction materials. It can be seen, however, that in more than 80% of cases the heat generated ends up in the disc.

Heatflow in pad/disc.

Different types of disc ventilation.

Therefor the disc need help to cool down. This occurs as a result of air circulation generated by the vehicle's motion, but above all from air movement induced by the vehicle itself. Depending on the maximum quantity of heat to be eliminated, various methods are used that in turn make the shape of the disc more or less complex. For instance the heat exchange surface can be increased, as in the case of ventilated discs. Air flow can also be increased and performance improved by shaping the blades. Entry of air through the side to which the wheel is attached is generally less efficient since the disc's environment is more confined and creates a circulation of hotter air. An excessive temperature increase in the pads causes their material to deteriorate and also increases the temperature of the piston and, as a consequence, the brake fluid. Moreover, excessive temperature increases in the disc have numerous consequences.
The cast iron can undergo a transformation that leads to the bluing of the surface or a permanent distortion of the disc itself. By conduction, heat is transferred towards the carrier. In this case the disc surface curves, the disc becomes conical and does not return to its original shape on cooling. Lastly, the carrier is in contact with the wheel and, as a consequence, heats the tyre.
 
 

MODELING

Disc distortion due to heat.

The only way to make improvements to a physical system is first to fully understand how it functions. This is why technicians commence by taking a large number of measurements in order to form an idea of the system's reaction to the various stresses. This widely applied approach is costly and only partly effective since it is rather difficult, or often impossible, to obtain precise measurements of moving parts affected by transitory  phenomena. Low cost, powerful computers have made it possible to expand such studies by modeling, also in the brake disc field. The principle is to break down the component,. in a virtual sense, into small parts which are assigned certain pertinent basic characteristics: geometry , weight, mechanical and thermal properties. Following this they are reduced to the form of simplified linear equations that describe all the possible relationships that can exist between the various elements: for example, between heat conduction and elastic properties. Of course, data representing the initial situation must be provided (for instance, the temperature map) and indications are given of the external stresses to which the element under consideration is exposed. All of these data are then processed by what is known as "finite element" software that provides new maps of the stresses and flows. After a small time increment, it is then possible to calculate the new state of the various disc elements being studied before progressing to the examination of braking itself.
The result of these calculations is presented in the form of drawings of the part concerned, where those zones that present the same value for a certain property (for example, all areas with the same temperature, given a tolerance margin of two degrees) are shown in a distinct color. These maps can be processed at regular time intervals and an examination of them makes it possible to form a precise idea of subsequent transformations for a certain property within the disc. The true value of this approach is evident when, for example, a change is made to the input data relative to a geometrical detail such as a dimensional of machining modification. Such changes can be evaluated rapidly without the need to physically produce several intermediate models and then proceed with a long series of tests.
When calculations indicate that a change can bring about an improvement, then at that point the part is produced and multiple tests and measurements carried out.

Calculation of temperature distribution in a disc. Finite element modeling.
 

IMPROVEMENTS

A disc groove.

A precise analysis of shapes and stresses, optimized by calculations and measurements, has led to some to improvements with regard to which examples will be given: carrier temperature reduction, optimizing of blade design, a solution to deformation.
The wheel is normally attached to the outer side of the carrier. If the temperature of the latter is particularly high, so is the wheel temperature, with the risk that tyre rubber is subjected to excessive temperatures. Analysis of dilation also points to the possibility of a conical distortion of the braking surface. It is therefore necessary to reduce the flow of heat towards the carrier as much as possible and diminish the rigidity af the joint. This can be achieved by machining a groove (as channel) where the disc carrier joins the braking surface. As a result the section of this heat transmission point is reduced, the thermal gradient increases and the temperature of the carrier falls. It has also been observed that there is a clear reduction in disc distortion.
Another solution to limit overheating of the carrier is to make cooling apertures in this part of the disc. These apertures limit heat transmission from the braking surface to the carrier. The reduced mass means lower heat conduction and therefore less disc distortion. In ventilated discs, using a different thickness for the two plates (the carrier side plate is the thicker) reduces disc distortion. A further example of improvement is the use af blades created by means af a series of rungs. Compared to rigid disc manufacture, the production of blades on ventilated discs represents a complication, above all when they are shaped.

Illustration of a disc with cooling holes.
 

They can be usefully substituted by a series of metal shaped bridges known as rungs and in this case the additional production complication is offset by a substantial increase in cooling and resistance of the carries to bending stress. Surface contact between the air and the cast iron is increased considerably and the greater turbulence produced improves heat exchange. This technology was developed by BREMBO in 1985.
The mixed disc, also known as the floating disc, represents another innovative solution. This involves using a cast iron ring (carbon is used in Fl) for the braking surfaces while the center is made from aluminum alloy. The two parts are joined together by bushes. When in use the disc has a hot part (the braking surfaces) and a cold part (the carrier).
For particularly demanding situations, for instance in Group A rally cars, floating discs are used to overcome distortion problems. It is important for the braking surface to dilate without distorting or giving rise to stresses that could cause cracks, the first step on the way to breakage. This type of disc means that the braking sections can dilate radially, so avoiding permanent distortion and stresses. Furthermore, this technology represents an advantage both in terms of weight and wear; when worn, only one part of the disc need be replaced. Used above all on motorbikes, floating discs are also used "on the road" for other than competition purposes.
 
 
 

Rung-shaped blades.
 
 

Disc with aluminum carrier.
 

 TESTING
As for all car components, the most reliable tests for discs are those carried out directly on the vehicle for which they were developed. However, such tests present two drawbacks. On the one hand, tests on vehicles only provide overall results since the testing involves the whole brake and also the environment in which it operates. On the other hand, the cost is very high in as much as such tests cannot be automated, a technician must always be at the wheel. This is why technicians have developed laboratory test instruments that enable them to focus attention on one particular performance aspect. This solution means that vehicle tests are freed from a certain number of constraints, enabling greater emphasis to be placed on the user-braking relationship.
 

TESTING INSTRUMENTS
IN THE LABORATORY
The more frequently used and most widespread instrument in the brake sector is the dynamometric bench. As the word tells us, this is a piece of equipment capable of measuring forces, therefore also those involved in braking. There are several sizes and types of dynamometer. Here we will only describe the up-to-date model that permits testing of the entire brake and its surrounding environment and can simulate the braking of a vehicle, or more correctly, that of a wheel. First and foremost an inertia dynamometer comprises an electric motor capable of controlling a variable speed inertia flywheel and, more specifically, the speed of rotation corresponding to the maximum speed of the vehicle for which the brakes are to be tested. The power output of the motor need only be in line with that of the vehicle's engine if the intention is to make a real-time simulation of a series of braking actions taking place rn a very short space of time. In this case it is necessary to communicate a kinetic energy to the inertia masses which is identical to that supplied by the engine to the part of the vehicle affected by the brake being tested. The inertia masses are the flywheels fixed to the crankshaft.

Normally a number of flywheels with different inertia are available so that it is possible to select those that accurately produce the desired total inertia. The disc and possibly the wheel are fitted at the end af the shaft. a dynamometer The caliper is attached to the bench, often with the aid af the axle of the vehicle being tested.

Diagram showing the principle of dynamometer bench.

A torque meter is placed between the caliper and the fixed part of the dynamometer bench in order to quantify the force exerted on the caliper, a force that tends to cause it to rotate when the disc is braked. Modern dynamometers use computer-controlled automatic devices.
A test procedure comprises a series of braking actions performed at various speeds, temperatures and, for instance, pressures. Certain procedures are intended to be representative of the average user when braking while others aim to evaluate the consequences of extreme braking action during which the discs reach temperatures that generate an easily observed red hot color. Programming a braking action requires a certain number of typical stages. Firstly, as previous braking has heated up the disc, there is a pause until the temperature falls ta a pre-established value. To achieve this the disc is rotated at a speed sufficient to ensure effective ventilation and is usually the longest stage of the test. Once the desired temperature is reached, the rotation of the shaft is regulated to correspond to the vehicle's traveling speed, expressed in km/h. At this point the motor is disengaged, for example by cutting off the current, and braking takes place.
Laboratory Braking is carried out by increasing the hydraulic pressure in the circuit. Two thermal types of braking action are normally used: with a constant pressure or with a controlled torque. In the latter case, after applying pressure, the torque is measured and as soon as the desired value is reached, the pressure is then regulated so that the torque remains at the established level (it can be constant or vary according to a predetermined cycle). This method is reliable in as much as it corresponds to a constant deceleration that is very similar to a driver's braking action.

Laboratory thermal shock test.

Throughout the braking procedure the automatic device records the values and trends of various parameters: speed, temperature, pressure and torque. Using these values it is possible to calculate the distance traveled and the friction coefficient at every possible moment. According to requirements other measurements are taken: for instance, the temperature at various points throughout the disc, the temperature of the pads, disc distortion, the amount of liquid introduced into the caliper or the frequency of vibrations.

ON THE VEHICLE
As mentioned, the majority of test procedures can be conducted using a dynamometer. However, such tests are only rarely able to reproduce the exact environment within which the item tested must perform. This is why the final procedures to obtain homologation are always carried out on the vehicle itself. In fact the vehicle brings stresses into play that are almost impossible to reproduce in the laboratory. Among these are transfers of mass and efficiency between the front and rear sections, the environment (temperature), mechanical stresses, deformity caused by contact with the ground and the effect of vibrations that are not produced by the brake itself. In addition, vehicle tests make it possible to highlight specific instances of wear due to the type of route and the brake's sensitivity to water. Although they are not really tests on brake components, ABS and ASR tests are also carried out on vehicles.
In order to perform braking tests, certain modifications must be made to the vehicle. However, first and foremost the vehicle must be available for testing and this is not always possible. For instance this may be the case during the development stage when the vehicle does not yet exist or when it cannot leave the manufacturer's plant for reasons of secrecy. In such cases the brake to be tested is fitted to a similar vehicle in terms of weight and concept: this is often a prototype equipped with the definitive suspension and wheel assembly. The brake fluid circuit is modified by fitting solenoids so that either only the front or the rear brake is activated. Braking action is then controlled by hand, where-as for safety reasons using the pedal to brake overrides the manual control and acts on all four brakes.

Test vehicle.
 

Interior of a test vehicle.

The tests sometimes include sharp braking, at the limit of wheel grip. In such cases the ABS action is excluded. As the prime objective of tests is to carry out measurements, the vehicle is fitted with instruments that enable reliable and accurate results to be obtained. The disc surface temperature is measured with the aid of thermocouples that either rub against it or are inserted in holes in the braking surface created for this purpose. Fluid pressure values are converted to an electrical current measurement by means of a transducer. The vehicle's speed and the distance traveled during braking can be obtained from the on-board instruments available (speedometer and odometer). It is accepted that these values are not precise as they count wheel turns and do not take into account tyre slip against the road surface. If a higher level of accuracy is required then a fifth wheel -similar to a bicycle wheel- can be fitted to the vehicle and given the fact that it bears: no weight it is not subject to slip. Deceleration can be calculated using the previous data, although the use of an inertia mass accelerometer is usually preferred. This equipment creates an electrical signal proportional to acceleration or deceleration and a device governed by the on-board computer is used to record these data. In addition to data acquisition and storage, this device is used to give sequential indications to the driver as to the type of braking action to be performed. A method that is particularly useful when long and complicated procedures are involved.
 
 

LABORATORY TEST PROCEDURES
These tests are performed when, for various reasons, either an expert opinion is required or -and this is the more frequent case- during development of a disc for a new use. A complete report includes at least ten tests, each of which refers to a specific property of the disc. In the majority of cases the same test procedure brings a number of properties into play. Results need to be cross-referenced for evaluating properties separately.
Certain tests aim to evaluate disc performance during the braking process. Such tests must be conducted extremely carefully as a variation in pad composition and structure produces a greater effect than modifications to a disc. This is why it is important to have a standard for pads, so that comparisons can be made between discs. These pads must also be rigorously checked (sensitivity to speed, to pressure and to temperature).
In this manner it is possible to establish differences in friction efficiency based on the composition of discs and the state of their surfaces. Disc ventilation is a factor that seriously affects temperature and, as a consequence, friction. Comparison of temperatures reached by different discs during testing conducted in a perfectly identical manner provides useful information on cooling efficiency. As the friction coefficient is temperature-sensitive, this effect will emerge from the braking results. Three types of such dynamometer tests exist. Procedures based directly on European testing regulations are too general to be able to extract precise information at disc level. A European group of brake producers and manufacturers have developed a series of procedures known as EU- ROSPEC Akn. The purpose of these procedures is to assemble the various companies' numerous protocols, that comprise both performance surveys at several temperature levels (AK1) and detailed measurements of brake efficiency before and after damping at high temperature (AK2). These tests still remain at a very general level as they measure the efficiency of a group of components: caliper, pads and disc. Specifically, they envisage measurement of the quantity of fluid forced into the caliper, a fact that depends on the distortion of the caliper and pads but does not concern the disc. Lastly each company has its own tests that often come to represent their historical standard and benchmark.
The second aspect of laboratory tests concerns the evaluation of mechanical stability. Measurements can be carried out either during a dedicated procedure or during performance evaluation procedures. Such tests consist of taking geometrical measurements during functioning, when hot and turning. These are therefore easier to perform on a bench in the laboratory than on a vehicle.
 

Distortion, undulation, DTV and other measurements are carried out with the aid of capacitative transducers that permit measurement relative to a contact-less reference plane. Measurements of wear are only performed from time to time as this is a slow process. In fact not only does the disc have to be disassembled but also the pads, in order to make a complete survey of thickness and weight reductions. These measurements are taken after the standard procedures mentioned previously although they only give approximate results as a true test procedure for wear must reproduce use and is therefore, by necessity, long-drawn-out. Increasing the speed of wear, obtained by making test conditions more severe (heavier braking action, higher temperatures), does nat always provide very realistic results.

Infrared ray thermograph of a tested disc.

"Extreme" tests are, on the other hand, effectively based on this approach (thermal resistance). Their aim is to destroy the disc by subjecting it to extreme conditions that cause it to degrade. Attempts are above all made to create cracks and even breakage in order to learn the limits that must not be exceeded and therefore are of help in sizing the disc correctly. To achieve this the disc is subjected to rapid, repeated thermal shock where the aim is to create localized thermal stress. During such tests temperature measurements can be taken at various points on the disc's surface in order to highlight the correlation between high temperature points and the location of cracks. As will be seen later, it is also possible to perform laboratory tests to identify and analyze the onset of noises and vibrations.
 

ROAD TEST PROCEDURES
Once the vehicle is ready only two technical issues need to be resolved. First af all, the load. In fact many tests envisage a full load. There is a very convenient method for loading the test vehicle. This entails filling water tanks secured to the rear seats by which means the weight can be regulated precisely. The second issue concerns the test circuit. Certain tests can only be carried out on a private circuit, with no members of the public present for safety reasons (for instance, in the case af high speed tests). Instead many other tests can be performed on the open road provided the vehicle has been properly registered and the tests concerned do not involve breaking the Highway Code. Such tests must be conducted by brake technicians. The peculiarity of tests carried out on a vehicle is that braking torque cannot be easily measured and therefore it is not possible to calculate the friction coefficient.
In the majority of cases it is deemed sufficient to express the results in terms of braking distance and deceleration or, better still, by means of the deceleration/pressure ratio. For a given brake this ratio is proportional to m if the mass remains constant, a circumstance that does not hold true when a high load transfer is involved.
While European regulations (annex 12) establish that certain homologation tests can be conducted using a dynamometer, almost all of them take place on the road. Such homologation tests are more useful in the case of heavy trucks. In the case of cars they only provide information on safety and are rather deficient as far as a precise description of brake performance is concerned, an aspect which is instead required for disc development and by the manufacturer. We should bear in mind that the two main tests are type 0, in which braking distance is measured for cold brakes at various speeds, and type 1, that measures the same parameter but after the brakes have been warmed up.

Test circuit vehicle.
 

BRAKING CHARACTERISTICS
Performance measurements are carried out on a circuit using one of several methods, depending on the manufacturer's requirements. Once more to a large degree such tests involve analysis of the values for three parameters: speed, pressure and temperature. Procedures usually include one or two warm-ups with measurements to determine at which point braking performance is restored as the temperature falls again. Even though these tests involve the brake as a whole they can provide specific information about the disc. For a certain technology and given disc thickness, temperature analysis provides data on ventilation when the disc itself is attached to the wheel. Wear can be measured at the end of the test although after thermal exposure checks are above all made for possible distortion.
Tests to measure wear are rarely performed on the road as they are so time-consuming. Instead thermal shock testing is currently used in order to determine the limit beyond which cracking may, take place.

Characteristic result.
 
 

STUDY OF VIBRATIONS
One of the truly interesting aspects of vehicle testing is the analysis of vibrations and noise. In fact in this particular area of braking performance only tests conducted on vehicles are really significant. A braking system (caliper, disc and pads) may produce noise (its own vibrations) when used on a certain car but instead not give rise to this phenomenon on another model. We might also note that possibly only certain units of a model are subject to vibrations. From this it can be deduced that problems which may appear to be of a design nature are not in reality only the result of the actuator system or peripheral components (suspensions, tyres, hub, etc.), but of conditions of use (climate, route, type, load, user, etc.).

Vibrations.

From the theoretical standpoint there is not much difference between vibrations and noises since the latter are produced by their own vibrations that cause the surrounding air to move at a sufficiently high frequency to be audible to the human ear. Even so we will deal with the two phenomena separately as the causes and methods by which vibrations and noises are generated are very different. As far as braking noise is concerned, this will be covered in a later chapter.
By the term vibrations we mean movements of parts of the vehicle that are felt, as opposed to heard, by the driver, even though certain vibrations are accompanied by noise. The majority of these vibrations are perceived at either the brake pedalor steering wheel level. In simple terms it can be stated that vibrations are essentially due to a succession of impacts between disc and pads. Consequently it is understandable that the frequency of such vibrations is linked to the wheel's speed of rotation and, therefore, that this frequency gradually diminishes with braking, that is, as speed drops. These vibrations are also said to be synchronous. The frequency varies from around one hundred or more Hertz when braking at 130 km/h on the motorway to almost zero when the car has almost come to a halt. If there were one rough patch on the disc then there would only be one impact for each turn of the wheel. Normally however, there are at least two such patches. For example, if we take the case of a badly installed disc that is not parallel to the hub, the first impact will involve the left pad and then, half a turn later, the right pad. In many cases where vibrations are present there are more than two impacts for each turn of the wheel.
In an attempt to better describe vibrations, they are classified on the basis of how they are generated as opposed to their effect. Two large groups of vibrations can be identified: Cold Judder, linked to dimensional anomalies independent of use of the brakes (anomalies that often occur when the brakes are not used) and Hot Judder that occurs after use at high temperature. Cold judder vibrations are due to both geometrical imperfections of the disc itself and to defects caused by installation and excessive play (for instance, in the bearings) and, as a general rule, to anything that may increase disc wobble. This wobbling causes irregular disc and pad wear. Cold judder is a vibration caused typically by wear when the brakes are not in use (long journeys on the motorway). These cold judder vibrations are noticeable during deceleration and at low to medium pressures, situations that occur frequently when slowing down slightly on the motorway. Such vibrations have a high frequency and their effect is even more unpleasant when travelling at high speed. If the disc is really the cause of such vibration this is due to a production defect: a substantial variation in parallelism known as DTV (Disc Thickness Variation) or a wobble or planarity defect which in turn causes a DTV. Instances of this occur very infrequently thanks to the production methods and controls described previously.
When pads press against the braking surface they may meet "cavities" or "bumps". Passing over the latter they are forced back towards the piston and these alternating movements (right pad, left pad) cause vibration. When this phenomenon arises, possibly a few hundred or thousand kilometres after installation, the vibration start gradually but then becomes more intense as the distance travelled increases.
In certain cases the pads oscillate and vibrate in the absence of braking action, for instance when travelling on a motorway. They touch against the disc and cause a facet-type wear. The end result is almost the same as a production-type DTV. In other cases, a new disc that has a pronounced wobble due to installation will cause a similar type of pad wear and after a few thousand kilometres will present the same symptoms. A disc thickness variation equal to or greater than 35 mm is sufficient to make vibrations felt, even though this will differ from car to car.
If a disc is undersized or badly designed, distortion may occur as a result of a significant temperature increase. When it cools down it does not return to its original shape and the wobble this causes will lead to irregular wear and, after a certain distance, the appearance of cold judder-type vibrations.

Undulation and DTV.
 
 

Formation of DTV.
 

If the vibrations appear after or during exposure to high temperature then the phenomenon is known as hot judder. In an attempt to cause such vibration a series of average level deceleration braking actions are performed while speed is increased up to 90% of top speed. For instance this type of test resembles leaving a motorway: speed is high while braking action is average but prolonged since it continues until the vehicle comes to a halt. As a consequence the temperature increases considerably. If friction is uniform over the entire disc braking surface, then energy is evenly distributed and nothing critical happens. If friction is more accentuated in one or more points then the energy exchanged at those points will be greatet at the outset and there will be a rapid, localised temperature increase. Vibrations appear Simultaneously at the hot points, which are normally distributed in a uniform manner over the disc, and these, on cooling, create more or less visible dark patches, or blue points. This transformation mechanism, the cause of which can be attributed to pad material (rapid friction coefficient variation, encrustation, etc.) is extremely detrimental and the disc is damaged beyond repair.
In fact the blue points are a localised conversion of cast iron into cementite an extremely hard substance. This transformation takes place at very high temperature and is non-reversible: as the blue points will be lesss subject to wear than the rest of the surface, the phenamenon will spread with each braking action of the type described. Transformation of the cast iron affects it to such a depth that a reworking of the surface would not resolve the problem. During development of pads for a new vehicle the possibility that a certain composition may cause hot judder is sufficient reason to discard it. As a result, pads homologated by the original equipment manufacturer do not have this defect. It should be noted however that use of discs worn down below their minimum thickness can be the cause of this type of deterioration since the temperatures reached tend to be much higher. The "snatching vibration" phenomenon is the onset of noises that are sometimes accompanied by vibrations. This is due to a variation in the braking torque, the cause of which can be attributed to instability of the pad material's friction coefficient. Incorrect running-in may at the same time be both the cause and the consequence of this. This problem appears in cases of heavy, though not sharp, braking -once again, when the temperature is very high.

"Cold" disc with dark patches.
 
 

Vibrations occur simultaneously with the formation of hot points that, on cooling, create dark patches.
 
 
 

TESTS UNDER EXTREME CONDITIONS
Alpine tests are a special kind of raad test. While perhaps they do nat provide the same quantity of scientific results as labaratory tests, manufacturers use them because, on the one hand, they enable safety checks to be made and, on the other, they allow comparisons of technical levels to be made since they have been performed for decades. The Alps provide a number of suitable routes: the Stelvio in ltaly, the Gross Glockner in Austria and the Ventoux in France. All of these roads lead down from a pass and generally comprise downhill sections with a pronounced, uninterrupted slope (about 10%) for at least ten kilometres and have no flat ar uphill stretches. There are two types of tests: those conducted at average speed that envisage a long series of braking actions so that the temperature increases in an exaggerated manner, and fast descent tests. On the Stelvio, for example, three speeds of descent are used from the pass down to Gomagoi: 23, 21 and 19 minutes. After testing, a detailed analysis of brake components, particularly the disc, is performed.
 
 

Temperature curves recorded during a descent from the Stelvio.
 

Map ofthe Stelvio test route.

It is also interesting to compare two 21-minute descents, each performed using a different driving style. The first descent involves braking a high number of times (130) which means that thte brakes are activated for a total of fully 8 minutes and 5 seconds. This style also means resorting to bursts of rapid acceleration to make up lost time. The other descent involves braking only 65 times, equal, that is, to 3 minutes and 22 seconds. In the latter case the temperature of the brakes, but also of the brake fluid, increases much less. The first type of descent is dangerous because the brake fluid temperature almost reaches the boiling point of a new fluid. We will see later that the boiling point decreases when used fluid contains a little water, a factor that increases the risk of malfunction during braking. These tests under extreme conditions sometimes reflect the driving style of certain users; they can cause irreversible deterioration of the disc and pads.
The comparative test on the Stelvio descent provides a concrete indication of the type of braking to adopt in order to  keep the system in good shape. It is advisable to avoid long, low pressure braking action (when approaching a motorway toll-station, for example), as this increases the temperature of the discs and pads. Whenever possible (depending on road conditions, traffic, load and passengers) try to brake for as brief a period as possible.

The road down from the Stelvio pass.
 

NOISE ANALYSIS, LOCALISATION AND ELIMINATION
The notion of comfort plays a fundamental role in the choice of a car, especially in the case of top of the range models. It is therefore essential to eliminate all repetitive and fastidious noises and vibrations. Engines today run more silently and are better soundproofed, as is the exhaust system, than they once were. In effect components no longer start to vibrate heavily when resonance is produced at high revs. This is why braking noises -should these occur- are not at all well tolerated. While in the majority of cases braking noises are by no means a sign of reduced safety they do force users to visit their garage urgently in the search for a remedy. Noise can be said to be an almost natural consequence of friction. If  you hit something with a hammer there is noise because whatever you hit vibrates. During braking a number of micro-impacts occur between the pads and disc that may cause them to vibrate. If the system becomes unstable at frequency, the braking system produces a whistle at the instability frequency. Another possible comparison is with an old-style gramophone: the brake disc is the record, the pad is the needle whereas the caliper and bodywork act as the horn.
Brake designers have to ensure that there are no noises, but the problem they face is extremely complex. In fact, as we will see, the question of noise involves many aspects since the causes, if not highly numerous, are still many. The disc is the transmission agent although the origin is always friction. As no miraculous cure exists for this problem, technicians have developed a number of analysis techniques to identify the origin of noises and also various solutions that can be adapted to each case as it arises.

NOISES
Noise is an acoustic emission that is normally perceived as something unpleasant whereas music and speech -although they are also acoustic emissions- are almost always pleasing sounds to the human ear. There are also two reasons for this: the first is subjective while the the second is one of a more scientific nature.
The first reason regards the level of acoustic emission. In fact, even if a piece of music is considered sublime, if it is played at a level near to the pain threshold it is considered as a noise because listening to it is unpleasant. The second and real reason derives from the fact that a noise is the superimposing of non-harmonic sound frequencies and comprises much discontinuity and phase changes.
Sound is a sinussoidal oscillation of airpressure. This oscillation is created by the vibration of a solid and is perceived by man through to the structure of his inner ear. In the case of a elementary sound, there is one single vibration frequency. A complex sound is due to the superimposing of vibrations having different fequencies. When the frequencies are multiples of the lowest frequency they are said to be harmonic, an instance of this being the sound made by a musical instrument. The difference in terms of distribution based on intensity for the harmonics produced by two instruments is the main reason why didtinction is made bbetween the note C on a piano and that on a violin.
Without going into the rules of harmony in detail, it is sufficient to say that if one were to press two adjacent keys on a piano simultaneously then it would start to bring home the concept of noise. Taking this the extreme case, there is what is known as white noise that comprises all of the frequencies that can be included in a merely random sequence of intensities.

Simple sound, complex sound, noise and white noise.

This means that the intensity at any given moment is by no means the consequence of the intensities produced an instant before. Braking noises are not white noise as has been shown by spectrum analysis.
The intensity of a noise should be measured as a pressure variation. However such a unit would not be easy to use as the human ear does not perceive sound as being proportional to pressure but to the logarithm of the pressure. This is why sound intensity is measured in decibels. This logarithmic scale requires a starting point which has been established as the smallest variation in pressure that the ear is able to perceive, namely, 20 micro-Pascal. On the other hand it is a well-known fact that the threshold of perception depends heavily on the frequency of the sound. An adult with good hearing can only hear sounds well between 15 and 20,000 Hertz.
Recording the intensity of a noise or a complex sound as a function of time gives a completely disorganised view. In order to represent this result in a clear manner it is represented on the basis of a frequency curve as opposed to a time curve. In fact it can be demonstrated that any acoustic signal is the superimposing of different frequencies and intensities of elementary sounds. When the noise persists, the intensities remain constant; when the noise decreases the intensities diminish, each according to the law underlying the cause of its emission. Even an impact -namely, an emission of noise for a very brief period- can be broken down into a series of frequencies distributed in a regular manner. Equipment used to perform this type of analysis employ a mathematical function known by the name FOURIER transform or, more often, by the acronym  FFT (Fast Fourier Transform). Besides intensities and frequencies, a further important data item is the difference in phase between two vibrations. One of the difficulties encountered when analysing complex noises like those in braking is the discontinuity of the emission and the frequent phase change. In fact performing a valid analysis of a noise requires the recording of it for a time corresponding to numerous periods of the sound with the lowest frequency .This period may last just a few seconds during which the highest frequency sounds may change phase many times. Recources therefore has to be made to more complex methods.

Frequency breakdown lor a complex sound and a noise.

BRAKING NOISES
Three factors must exist in order for there to be a noise: an excitation, a resonator and an environment for its propagation. Of course we should not forget that there must be someone there to hear it and that the frequencies perceived fall within this person's hearing range. Example: a bell is excited by the impact of the clapper. A bell is a resonator with its own frequencies, its own note, and the air is the medium through which its acoustic wave propagates. The same happens with a brake: excitation is given by the friction material rubbing against the disc. Energy transfer from the disc to the interface and the pad appears to happen in a continuous manner although in reality it is a succession of more ar less rough impacts. The resonator is almost always the disc although it can be the pad, the caliper ar a combination of these various components. In the section on modal analysis we will see that any object subjected ta an impact, even a slight one, vibrates for a certain length of time. The vibration modalities of a component are not left to chance but are closely linked to the geometry of the item concerned and the physical properties of the material from which it is made (for example, density ar elasticity modulus). Following excitation, vibrations dampen and finally disappear. While normally metals -particularly ferrous metals- are not good dampers, grey cast iron does have a damping effect, although both friction materials and tyres are much better dampers. Noises in brakes are rarely produced by the vibration of a single element since all components come into contact during braking, a fact that makes analysis and identification of the causes much more complex. Numerous researchers have attempted to describe and classify models to represent the various types of noise generation in brakes. Among such studies, that conducted by A.M. LANG, N. MILLNER and M.R. NORTH should be mentioned. The various models proposed give a good explanation of the different origins of braking noises although these will not be examined in detail here. We will only refer to and describe them briefly here, while readers are invited to consult publications listed in the bibliography for more in-depth information. To explain low frequency noises, namely those less than 200 Hz, the best known model is referred to as "non linear stick-slip vibration". This simplified model shows a fragment of pad material rubbing against the disc (here shown as a surface in motion) and connected to the caliper by means of a viscous-elastic system, that is, comprising a spring and a shock-absorber.

Instability of friction coefficient.

This model assumes that the friction coefficient is unstable and varies in a linear manner with the rotational speed of the disc relative to the pad. The equation shows that damped vibrations will be generated, regarding which sensitivity to the speed of the friction material is a significant factor.
Complicating the model, in particular by bearing in mind the possibility of caliper vibration relative to the axle, the authors have highlighted the possibility of vibration emissions in the 200 to 400 Hz range. These noises are sometimes referred to as "Hum" or "Moan". In such cases the manner in which the caliper is attached is the important factor.
Higher frequency noises, in the 500-13.000 Hz range, are often referred to as squeal. The model shows, in principle, how a disc can vibrate of its own accord and that the vibration frequencies noted are the same as those found in the noise produced.
The term squeak is used to describe the mechanism generating even higher frequencies, from 2 to 5 kHz. This mechanisrn brings both the disc and pad jointly into play.
 
 

"Cantilever" model.

When the disc vibrates and distorts toform four orfive lobes, the width af each lobe measures approxirnately the same as the length of the pad. As a consequence, the latter is pushed away alternately fram right ta left (fram tap ta battam in the illustratian): this is the excitatian movement. The pad also  has its own mode and therefore starts tovibrate. As it is in an unstable equilibrium on its own supports relative tothe caliper, the latter vibrates too. Given that the size is less than that of the disc, the frequencies will be higher.
Other noises, far example that referred to as "metal brush", are the result of a superimpasing af a number af squeak-type vibrations, first damped and then again excited. Such noises, that bring into play the components' own modes, have frequencies that are not dependent an the disc's speed of rotation. Lastly it should be noted that from both the theoretical and practical standpoint, an increase in friction-coefficient tends to increase the probability that noises will appear.
 
 

METHODS FOR ANALYSING BRAKING NOISES AND THEIR CAUSES
The first method of analysis naturally consists of measuring the noise intensity with the aid of a phonometer.This method requires generating the noise and placing a microphone at a clearly established point, usually outside of the vehicle. The recording will enable more detailed analysis to be carried out. In essence the different frequencies and their intensities are separated out by means of spectrum analysis using what is today quite commonly used equipment and that includes a computer which can process the signal's FFT. Such equipment is portable and can be installed on board the vehicle.
The frequencies detected by the spectrum will provide initial indications towards finding a solution. This type of analysis can also be performed in the laboratory , for instance using a dynamometer. However it usually requires a more complex brake assembly compared to that used for friction measurements. In fact the brake environment must be reproduced and this, for example, implies fitting a wheel to the disc or even installing the complete suspension. This method provides almost identical results to those achieved on a car and is particularly productive because tests can be automated and therefore a greater number of situations can be analysed.
As we have mentioned, when an object is subjected to impact, however slight, it begins to vibrate and may emit a noise. These vibrations are by no means random and in particular do not depend on the excitation except in terms of their intensity. Let us take the case of a disc. The first type of distortion is the collical shape that develops in the carrier, first in one direction and then in another. The second type is similar to the first, except that the opposed parts of the carrier move in the opposite direction. The following types are more complex and involve the formation of an even number of lobes as if the disc has been subjected to radial undulation. These different folds vibrate alternately. Certain of them move in the same direction and are simultaneous: they are in phase. Others vibrate in a counter-phase manner. The points located on a radius that vibrate with a higher intensity are called "loops" whereas those with zero movement are called "nodes". As far as a disc is concerned, their positions are determined by the point at which the initial inpact occurred, since this is a loop. Each single frequency has its own mode. For example, as the vibration modes all take place simultaneously although with different intensities when excited by an impact, the sound emitted will be the superimposing of all of the discrete frequencies.
If the disc is no longer excited by an impact but by a vibrator, it can be noted that the disc only vibrates at fequency excitation values corresponding to one of its own frequencies. In this case it is said that resonance exists. It is possible to measure such frequencies and distortions, by repeatedly striking the disc at the same point with a hammer and by positioning the accelerometers at various points over the disc. The hammer is fitted with an electronic device that provides the start signal for measurement: Modal analysis is very useful as it illustrates how a disc distorts and what effect either geometrical or weight modifications rnight produce. In recent years other investigation techniques have been developed relative to braking noises and their generation. Using laser holography it is possible to make an object's state of distortion visible at a given moment. Little testing time is required since this corresponds to the time needed to take a photograph, that is more or less one thousandth of a second.

Braking noise spectrum.
Peak: 2250 Hz
112.9 dB

This complex method is particularly interesting in cases where the noises emitted are very unstable as a result of vibrations that are themselves discontinuous. The principle is based on making interference fringes appear on the surface of the vibrating object, not due to height differences but to temporal positioning differences of each point of the object examined. A high-power laser beam is used to produce a hologram, namely a three-dimensional photograph, that is processed to trace the level curves corresponding to the fringes. This is an extremely complex procedure and can only be carried out in a specialised optical laboratory . The coherence of laser light is also used in laser fluximetry. In this case measurement takes only a few seconds and it is much easier to perform. The principle is based on the well-known DOPPLER effect according to which, for example, if one were to stand near to a railway track and listen to the whistle of a train in motion, the frequency, even though originally fixed, changes continually from the standpoint of the observer. The same can be said of light. When a laser beam is directed at an object and reflected, the frequency of the reflected light is different from the original frequency. The difference is proportional to the speed of movement of the object in question. Surface exploration of the object under examination using a laser beam and permanently measuring the variation of the wavelength of the reflected light makes it possible to map the speed of movement of the various points on the object. This method is much easier to perform than the one mentioned previously although it can only be applied if the vibration remains stable for at least the entire duration of one exploration. This becomes a limitation in the case of very complex noises.

Modal analysis.
 
 
 
 

Analysis of a disc using laser fluxometry.
 
 
 
 

SOLUTIONS FOR ELIMINATING NOISES
The aim of all of the analysis techniques we have mentioned is to highlight the cause of noise generation. In fact it is very easy and effective to search for corrective measures once the cause of the problem is known. The first method, although not always easy to apply, consists of modifying aspects of geometry to alter the frequencies in question, so avoiding resonance instability. Machining, feedheads and springs can sometimes resolve the problem without resorting to a complete redesign of the braking system. The second category of modifications concerns the friction material. This solution often has to be adopted for braking systems that have remained unchanged for long periods, but it is not always the best. The technician formulating the pad material will attempt to modify certain characteristics such as stability as a function of speed, the change of aggressiveness due to use at high temperature or will alter certain of the material's physical properties, like density , dynamic elasticity or vibration damping power. It has to be admitted that in many cases this leads to an overall decrease in friction level. Another method is to interrupt the possible transmission of vibrations between brake components and also to its support attachment. Attempts can involve either making contacts more rigid or, alternatively, damping them. In the spirit of this approach friction material producers supply pads with a damping layer attached to the back. This is the socalled anti-noise layer or shim. In the simplest cases it consists of a backing produced from a type of rubber-based paint, but it can also be achieved by bonding on a sheet of complex lamellar rnaterial. Certain suppliers prefer more complicated solutions such as alternating two or three thin metal plates and sheets of rubber, plus a film of bonder. The desired effect may simply be to dampen the transmission of vibrations between the piston and the pads or also to modify the rigidity of the support in the case of a squeak-type noise.

Pad and shim.

Progress made in the automotive field during the latter part of the 20th century has been considerable but would require too much space here to describe fully. It is however useful to underline that the broad issues addressed have been an increase in safety , weight reduction, lowering of fuel consumption, improvements in comfort and a considerable increase in reliability .This latter point has led to two advantages: a longer interval between scheduled service interventions (on average, every 15,000 km) and a reduction in the number of components requiring substitution during the vehicle's useful life.
In spite of this progress, certain parts need to be periodically checked and replaced in order to maintain a high level of safety and reliability for the system as a whole. Almost all the parts that require replacing are subject to wear in the tribological sense of the term. In fact, the purpose of each such part involves a rubbing action against another one, as in the case of windscreen wipers, belts, discs and pads. This, of course, without overlooking items that need to be replaced more often, namely, oil and filters. In fact oil -apart from its role as lubricant, coolant and corrosion protection agent- also gathers waste material lost as a result of wear by parts that move or rub against one another.
The braking system must be checked at every scheduled service and components that have either exceeded recommended wear levels or present signs of deterioration must be replaced. It is essential that such maintenance be carried out by specialised mechanics who possess complete documentation and have period- ically undergone training on the new products for which they are required to provide assistance.
 

ANALYSIS AND DIAGNOSIS

Braking system maintenance can occur in one of two circurnstances: during a general check-up or when the user has a problem. In the latter case, braking system producers provide flowcharts to help identify the cause, however, such flow-charts are very exhaustive and therefore too voluminous to present here. On the other hand, as can be imagined they cover aspects concerning the braking circuit, pads and discs. The ABS is also covered although an attempt to discuss it here would sidetrack us from the rnain issues involved.
This check is normally carried out when the pads are changed. In fact the pistons progressively protrude more from the caliper as the friction material wears down and need to be pressed back before new pads are installed. Before doing this it is advisable to check the level of the brake fluid in the tank. If the latter were already full then pressing back the pistons would cause an overflow of brake fluid. Under no circurnstances must the brake pedal be pressed while performing this operation. It is normal for the fluid level in the tank to gradually reduce as the pads and disc wear down, however, it must never be allowed to fall below the minimum level. Similarly, if after replacing the pads, the level still remains low then this is an indication of a leak in the circuit. When pressing back the pistons a check should be made that they move freely within their cylinders. Were this not the case, then they must at least be "greased". Care however, must be taken that the pad and disc surfaces are not contaminated with grease. If greasing is not sufficient then the caliper and pistons should be dismantled, after first bleeding the fluid from the circuit. The manufacturer's technical manual should be consulted before attempting this operation, particularly since the gaskets will have to be replaced, which is a very delicate operation. First of all clean the parts and examine the caliper carefully, and then -in the case of floating calipers- check that the guides run properly. The initial check to perform on the braking circuit is to establish if there is any loss of brake fluid. This operation first requires that the circuit be cleaned externally. The brake must then be activated at a high pressure if there is a brake servo, as is often the case today. In order for the servo circuit to function, this operation must be performed with the engine running. Poor braking may occur if surface dirt has not been removed from the hub before refitting the disc or if there is play in the bearing. In the latter case, the effect will be the same as that of a disc run out error and it will therefore be necessary to replace the bearing. Defects that may arise due to malfunctioning of the brake servo, master cylinder or ABS will not be described here in detail even though it is recognised that these can cause poor braking performance.

Pistons and gaskets.
 
 

 THE PADS
Pad with wear indicator.

The main cause of pad replacement is normal wear of the friction material. This requirement is provided for and expected since the majority of pads are fitted with a wear indicator. This device sends an electrical signal to the instrument panel when the layer of material covering the metal support pad is less than approximately 3 mm. The most simple type of wear indicator comprises an electric wire located in a hole within the friction material. A current runs through this wire which, when exposed by wear, forms a contact with the disc, completing a circuit through which current passes, so causing the on-board indicator to light up. In reality systems employed today are a little more .sophisticated. In fact they maintain information right from the first contact which means an increase in safety but also prevents the indicator lighting up in an untimely or accidental manner. The material still remaining is normally just the substrate, namely a less abrasive material.
Other situations can occur that almost always necessitate pad replacement. The cause of these anomalies requires more detailed analysis. Although such situations are rare, they do in fact arise: this is why pads must periodically be checked, at least every 10,000 km.
 
 
 
 
 

Check on state of components	Actions to be taken
Wear indicator is in contact with the disc.	The pads need to be replaced.
Wear indicator circuit is interrupted or missing.	Requires repair or replacement.
Pad cracked or flawed.	Requires replacement. Analyse the question more closely.
The pad has detached from its metal support.	Requires replacement.
Trace of brake fluid or grease on friction material.	Requires replacement. Check the loss of fluid.
Grooves or scores (pad and disc). Tolerance for the disc is 0.3 mm.	No action required if pads and disc are within the tolerance.
Wear of one pad is greater than that of the other pad on the same caliper.	Check the calipers and, in particular, movement of the pistons.
Uneven or oblique wear of one or more pads.	Check positioning of the accessories, the correct movement of the pads in their seat and functioning of the caliper.
The assembly accessaries ar dIe anti-noise are loose, missing or damaged.	Requires replacement.

THE DISC
This section deals entirely with amore in-depth analysis of the technical reasons underlying a change in disc performance and, as a consequence, the need for its replacement. We will also summarise the various visual checks and analyses that need to be carried out in order to ensure disc integrity . First and foremost it should be stressed that a mechanic is the most qualified person to check the state of wear of a disc. Such wear occurs with normal. use and must be checked periodically.
Normal disc wear creates a ridge that runs around the external perimeter and corresponds to an unworn part. This ridge is one of the "wear indicators": when it is clearly visible the residual thickness of the disc between the two braking surfaces should be checked and compared with the minimum thickness etched on the outer rim of the disc.
This measurement alone is not enough inasmuch as a check must also be made to ensure that wear is not uneven, namely, that one surface is not worn to a greater degree than the other. The maximum variation, not to be exceeded, is indicated by the manufacturer, although in any event it should not be greater than a few tenths of a millimetre, above all when the disc is quite worn. The caliper must also be checked since wear may be due to defective roll-back.
The disc may have lost other size characteristics when compared to its new state: run out, planarity, DTV. A closer analysis of these changes -causes of vibration-  provides a better understanding of the state of the braking system.
Increase in run out: after fitting and using a new disc, run out can be determined by taking a measurement on the unworn external edge (ridge). Run out can also be measured on the braking surfaces. These measurements are taken with the aid of a DTI Gauge fixed to the suspension by means of a magnetic foot. The onset of run out, not present at the time of installation, may indicate that there is a problem with other components in the braking system. If run out was excessive at the time of installation then the hub and bearings should be checked, because they were probably not checked previously. Change in planarity: exposure to high temperature may compromise the disc's planarity .As the carrier remains cooler than the braking surfaces, dilation of the metal and radial stresses leads to distortion of the disc with the appearance of lobes.

The pad only presses against the outer half of the braking surface.
 
 

The change in the disc needs to be closely analysed.
 

These measurements are taken with the aid of a DTI Gauge fixed to the suspension.

Part of the distortion occurs at the moment of cooling. The DTI Gauge indicates a series of high and low points (three or more). An incorrect tightening torque of the bolts securing the disc to the hub causes loss of planarity. DTV behaviour: different thickness of the braking surfaces, a consequence of uneven wear due to the two problems described previously, causes the onset of vibrations that become considerable when the value of the disc braking surface thickness (DTV) exceeds 35 mm. The disc should be inspected carefully after, or at the same time as, measurements are taken. In certain cases superficial corrosion - namely, rust- can be observed, due to prolonged presence in a damp environment. If the disc is not worn, then such rusting is no cause for replacing the disc. However, a certain number of braking actions should be performed -similar to the running-in procedure- in order to eliminate the corroded layer. If the corrosion remains on only part of the braking surface after normal use this is due to poor caliper functioning: the caliper should therefore be checked and either repaired or repiaced. A presence of deep circular grooves or numerous radial cracks extending for more than a few millimetres are unquestionably indications that the discs must be replaced. Lastly, the presence of dark patches or blue spots is a cause of vibrations. If this phenomenon is rather pronounced and, above all, if the driver is disturbed by them, then the disc must be replaced. The previously mentioned factors indicate that the disc has lost its original mechanical and chemical properties and has become the source of vibrations (hot judder). For simplification purposes, we have often referred to the need to replace "the disc". In effect bath discs on the same axle must be replaced. Furthermore, apart from instances of normal wear and corrosion, cases of abnormal wear do not only involve replacing discs. Such cases in fact point to faulty behaviour by other components within the braking system and this needs to be carefully investigated by a specialisL The latter should moreover give this information to the driver and communicate it to the supplier companies concerned. Clearly this comment does not just apply to brake discs.

BRAKE FLUID
Transmission of force from the pedal to the pad has changed greatly from the times of early cars. Initially it was an entirely mechanical process. It then became pneumatic, and finally hydraulic. Air transmission is still widely used on heavy trucks as it is both efficient and convenient among other factors, it facilitates the connection and disconnection of trailers. On the contrary , compressed air systems occupy a lot of space and are therefore not suitable for use on cars. As a result, the transmission system invented by Malcolm Loughead has become standard. Just as one refers to a "Hoover" to indicate a certain home appliance, the term Lockheed is often used in the brake field to indicate the fluid Lockheed was the name of the company that Loughead formed. Brake fluid must meet a certain number of requirements:
-it must be a non-compressibie fluid under normal use conditions;
-it must have a high boiling point in order to remain in a liquid state even in extremely severe braking conditions;
-it must have a low viscosity even at temperatures near to its freezing point, which must not be higher than -40°C;
-it must be a lubricant so as not to cause seizure of moving parts (master cylinder and pistons);
-it must be chemically stable and not have an aggressive action on braking system components in order to avoid corrosion;
-it must be an inert fluid as far as rubber parts the gaskets are concerned and furthermore must not act as a solvent.

As modern fluids possess all of these properties it could be thought that they do not need to be checked or maintained. Nothing is further from the truth! All liquids that meet these requirements are essentially of organic origin and their molecules have polar properties as, for instance, in the case of glycol or its esters. These properties render such liquids hydrophilic or hygroscopic. This means that in presence of water vapour the fluid fixes water molecules by means of weak curves bonds, a fact that affects the boiling point enormously. In fact the system is not airtight and so damp air can enter the tank as the fluid level drops. Humidity is progressively absorbed by the fluid up to a certain limited percentage which is, however, enough to lower the limit at which the first gas bubbles -essentially, water vapour- appear. Unlike liquids, one of the properties of gases is that they can be compressed. When bubbles are present within a circuit, pressing the brake pedal causes them to compress, slack increases enormously and pressure in the system does not reach the required  values; braking action is thus completely inefficient. This phenomenon is known as Vapour-Lock.

Brake fluid boiling point curves.
 

Brake fluid testequipment.

The only way to avoid this very serious problem is ta check and replace the fluid periodically. Manufacturers' maintenance manuals recommend that this be done annually. All mechanics should have boiling point measurement equipment to check the state of fluids. The rate at which a fluid absorbs water is ta a large degree determined by the climate. A value af 3% can be reached in one ar two years, corresponding ta a decrease of about 80°C in the fluid's boiling point. Furthermore the presence of water increases the risk af corrosion within the circuit. After changing brake fluids it is absolutely essential to bleed the circuit since, in this instance, there could be air bubbles that may impede reaching high pressures. There are a number of types af fluid that have been homologated by the US Department Of Transport (DOT), referred to as DOT3, DOT4 and DOTS based on their performance.
 
 

REASONS FOR REPLACING DISCS
Experience indicates that drivers believe discs are hardly ever replaced. As a result, when a mechanic says that he has done so, customers suspect that either the braking system is defective or that he is being cheated. Usually this is not the case since wear is a normal occurrence. There should be no hesitation in replacing discs once they have worn down to minimum thickness. As we will show, safety is not compromised immediately when a disc is too worn, however, it can be in a very short time.
 

WEAR AND CRACKING
While a disc's braking surfaces wear down systematicaIly, even though the speed at which this occurs varies, cracking, on the contrary , does not always appear. It is by no means unusual however to find this type of deterioration. This "ageing", that represents a transformation of the cast iron, normaIly occurs when the disc is well worn. This is why it is discovered when discs are replaced. Very complex and in-depth studies have been conducted on this phenomenon that can become very serious. Firstly, it can be ascertained that cracking occurs when the disc's surface is subjected to very high forces and energy transformations as a result of braking action. Therefore, as this is essentially a problem of dimensions., such deterioration is found more rarely on cars when the braking system has been correctly dimensioned and includes an ample margin. Even so technicians still manage to devise very tough tests that cause cracks to appear in new discs. Cracking as such is not detrimental to safety , but it can be an early sign of a breakage to come and this is a much mofe dangerous matter.
The mechanism by which cracks are formed has been studied very carefully. Here, we will merely outline the principles. As we have seen, during braking the disc's surface temperature is much higher than the internal temperature and, as a consequence, surface dilation is much greater. The surface is also subjected to astrong compression force exerted by the pads. If this force exceeds the material's limit of elasticity then distortion begins to take place when the disc cools down, and cracking occurs. Clearly this does not occur as a fesult of the first thermal excursion but after a large number of cycles. This is what is normally referred to as thermal fatigue. Examination under the microscope reveals that small cracks very often start at a point where there is a heavy concentration of graphite scales. A very homogeneous cast iron will therefore be less subject to this phenomenon. Cracking takes place gradually. Initially the cracks are very small and what is referred to as fissuring can be observed. Instead when cracks are more evident they can be seen to run radially across the disc's surface. This occurs because of a complementary mechanism. In the first chapter we saw that the sufface of the disc is subjected to traction forces during braking, perpendicular to the direction of pad movements. This mechanism tends to create cracks: it is known as mechanical fatigue. Studies on a number of sections have shown that the depth of ceacks increases moere rapidly than their lenght.
The so-called "thermal shock" or "thermal fatigue" tests clearly show that cracks proliferate with an increase in braking. Moreover it can be noted that the disc thickness has a considerable effect on the crack' speed of growth.
Comparison between a new and a worn disc is sufficient to show this. In order to speed up tests, a new disc is prepared that is already at the minium thickness level. It is evident that when cracks a few tenths of a millimetre long can be noted on the disc surface, the component must be replaced, since the widening of such cracks can provoke breakage, often in the space between two blades in the disc's braking surface area.

Cracking in the form of fissures.

Stresses within the castiron of  the disc.
 
 

Size of cracks in terms of width and length.
 
 

Impact of disc thickness to proliferation of cracks.

Apart from the fact that braking efficiency may be compromised, this also causes rapid pad deterioration as the disc effectively acts like a rasp. The studies mentioned previously have highlighted the main causes for development and proliferation of cracks. Certain solutions exist to resolve this problem.
The thermal conductivity of the main components in the braking system has to be increased so that lower temperatures are reached; in particular, pad material conductivity must be higher,though without creating vapourlock; the pad's YOUNG modulus -namely, its elasticity- also has a certain effect in as much as the friction surfaces have to be increased in order to reduce localised mechanical stresses. To achieve this the two surfaces must complement each other, both in terms of the wear effect and as far as distortion is concerned.
In addition to these recommendations, clearly every possible action must be taken to reduce temperature: above all, the cooling of the disc and its ' dimensions.

Crack / temperature relationship.
 
 

WEAR, MINIMUM THICKNESS AND TEMPERATURE
Wear is a normal occurrence although it is not mandatory in order for the brake to function properly. A great deal of progress has already been made in the materials' area and it is quite likely that there will be further progress in the future. In spite of this, however, we have not yet discovered a friction system that has all the characteristics necessary for correct braking but that is not subject to wear. First a distinction must be made between cast iron wear and friction material wear, above all because too often there is a tendency to attribute both of these types of wear to the pads. This is due to the fact that disc material remains the same ( variations in the composition of the various alloys are minimal compared to mixtures used for pads), whereas to achieve certain compromises, pad materials vary much more. As far as wear alone is concerned, materials have been identified that show little wear but that cause a lot of wear in the disc, and vice versa. Faced with such a choice, car manufacturers tend to choose the second option.
We will also see that there is an consequences of interaction between the two types of wear and that replacement of the discs is just as important as similar action with regard to the pads, both in terms of braking safety and comfort levels.
The concept of minimum thickness for a disc is so important that the value is etched indelibly on its outer rim. This value is set by the manufacturer for at least three reasons. First and foremost for mechanical reasons: in the case of ventilated discs the cast iron plate must not distort as a result of pressure. Secondly, for reasons to do with the remaining mass of cast iron: a reduced mass leads to high temperatures. Lastly, for mere geometrical reasons: in fact if, with time, the pads wear down and the disc is allowed to fall below its minimum thickness, then the serious consequences noted below will occur. First of all the piston's dust- shield may tear. This does not create an immediate danger, but in the case of dirty and muddy conditions, abrasive powders rapidly damage the gaskets which in turn affects the air-tightness of the fluid circuit.

New disc / new pads.

Worn disc / worn pads.

In certain cases the pads may no longer be guided and come out of their seat. In such cases braking action will quickly deteriorate or, even worse, not exist at all. If the pisto's position is too advanced it can be ripped out of the caliper on the effect of torsion movement. When this happens the vehicle will no longer brake! Whenever pads are replaced it is important to ensure that the disc will not fall below minimum thickness during the life of the new pads.

Heavily worn disc and pads.

Pad wear / disc wear: (Min. Th.)

An important consequence of wear -and, therefore, of the reduction in the disc's thickness- is the temperature increase reached in two identical instances of braking from the point of view of the pressure applied. Above all the volume of cast iron is less in as much as the disc's thickness is minimal and, given an equal energy transmission, the temperature of the braking surfaces is higher.

Distortion temperature.

Under equal test conditions it is interesting to compare respective wear measured for a new disc/pad combination and wear found when the pads are new but the disc has reached minimum thickness. It can be noted that with the worn disc the pads show wear after a 30% shorter distance when compared to a disc of original thickness. This phenomenon is even more serious as far as the disc is concerned.
It is also possible to compare the temperatures reached in a brake equipped with a new disc and, for the same brake, with a used disc. The test consists of performing a series of braking actions in rather rapid succession so that the temperature increases smoothly. The lower volume for the braking surface areas causes quite a rapid increase in temperature. Due to dilation the disc distorts and begins to run out. As this happens more often in the case of used discs, brake pad wear is uneven and comfort decreases.

Fading.

Such distortion may become permanent and compromise the correct functioning of the braking system.
Again in tests under the above conditions, a more rapid onset of fading - namely, the reduction of friction coefficient at high temperatures- can be noted. In this case too, the test consists of a series of braking actions performed at constant deceleration and at equally spaced time intervals. It can be observed that a progressively higher pressure is required to reach a given deceleration level. The same can be said for the force that needs to be applied to the brake pedal.
In a similar test, but involving a greater number of braking actions, it can be observed that there is a considerable increase in pedal slack at the end of the test.
In fact the fluid temperature exceeds 200°C leading ta the early appearance af vapour-lock.

Vapour-lock.
 

WEAR AND COMFORT
Apart from instances of wear that directly affect safety , those affecting comfort must also be considered. For example, not only the presence of scores and grooves but also cases of uneven wear including oblique pad wear or any other type of uneven wear. Generally speaking, such instances are not the result of normal wear. When they arise their origin must be analysed very carefully. There may also be uneven disc wear: For instance, exarnination of the braking surface profile may indicate curving and a considerable variance in thickness (DTV). The disc vibrates and the effect of this can be felt at the brake pedal and steering wheel levels. There may even be acoustic ernissions. In order to analyse such instances in amore technical manner, the torque can be measured during braking at constant deceleration: very heavy vibrations can be noted in the case of a worn disc.

New pads fitted with heavely scored disc.

By the same token, if new pads are fitted and the disc is heavily scared (0.5 mm) the pedal will be "spongy" and running-in difficult. There is also every possibility af both vibrations and whistling noises.

Uneven wear and vibrations.
 

REPLACING A DISC
In the previous sections we have reviewed problems arising out of disc deterioration. We have also emphasised the fact that the majority of such problems must lead us to reflect on the state of the braking system, its origin and its use... Certain of these questions concern brake maintenance and, in particular, the care taken when replacing pacis and discs.
The first observation regards the personnel who perform such maintenance operations. Clearly they must be skilled and have undergone specific training on the subject of brake component and, above all, disc replacement. Their knowledge needs to be refreshed and, from time to time, should be completely updated in training courses.
The second important, indeed mandatory requirement is that the installation instructions must be available, read and followed.
The third observation, already mentioned before, is that the pads and discs that are removed are an important source of information. For instance, the state of the disc must be observed carefulIy (state of braking surfaces, colour, profile). This examination can highlight faults in the functioning of one or more components (calipers, pads, bearings, etc.). It is important to resolve such problems before replacing the disc.
When do brake discs need to be replaced? We will summarise the various reasons already mentioned:
-When, during the course of a normal check, it is found that the disc's thickness is less than or has reached the minimum thickness indicated on the outer rim of the disc itself (MINimum THickness).
-When, while checking or replacing pads, cracks longer than 30 mm are found.
-When circular score marks are observed, deeper than 0.3-0.4 mm.
-When dark patches are found on the disc's surface.
-When, after a check, measurements reveal distortion or noticeable variations in height between a number of points over the disc's braking surfaces.

Lastly, before moving on to practical issues, we should bear in mind the following general rules:
-Instructions for the replacement of components shou1d be read and scrupulously followed.
-Both discs on the same axle must be replaced on the same occasion.
-Make sure that the disc reference number corresponds to the vehicle on which it is to be installed. The same goes for the pads.
-Install two discs from the same pack (from the same production batch).
-Pads must always be replaced when discs are replaced.

For Maintenance and Deterioration see:
MAINTAINANCE by Brembo
DETERIORATION by Brembo