G
Guest
·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 con- ducted 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 in- stead 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.). From the theoretical standpoint there is not much differen:ce 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 dif- ferent. 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 accompa- nied 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 synchro- nous. The frequency varies from around one hundred or more Hertz when brak- ing 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 fust 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 vibra- tions can be identified: Cold Judder, linked to dimensional anomalies indepen- dent of use of the brakes (anomalies that often occur when the brakes are not used) and Hot Judder that occurs af ter use at high temperature. Cold judder vi- brations are doe to both geometrical imperfections of the disc itself and to de- fects 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 D1V (Disc Thickness Variation) or a wobble or planarity defect which in turn causes a DTV. Instances of this occur very infrequently thanks to the pro- duction 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 m?vements (right pad, left pad) cause vibration iO ~en this phenomenon arises, possi- bly a few hundred or thousand kilo- metres after installation, the vibration ,,~-~~|EQU||EQU|::|EQU||EQU|;:;;;;; start~ gradually but then becomes more iii:teflse as the dïstance trav- elled increases. In certain cases the pads oscillate and vibrate in the absence of brak- ing action, for instance when travel- ling 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 installa- tion will cause a similar type of pad wear and after a few thousand kilo- metres 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 aresult of a significant temperature increase. When it cools down it does not re- turn to its original shape and the wobble this causes will lead to ir- regular wear and, after a certain distance, the appearance of cold judder-type vi~ons.
Surface deviation during one rotation
Internal surface
External surface
Disc thjckness varjation during one rotation
If the vibrations appear af ter or during exposure to high tempera- ture then the phenomenon is known as hot judder. In an attempt to cause such vibration a series of average level deceleration braking ac- tions are performed while speed is increased up to 90% of top speed. Por in- stance 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 r~ IQ~~lised~rature increase. VibratiGns ap'.pea and amp;: SimlJ!!afleOusty at the hot points, which are normally distributed in a uniform manner over the disc, and these, on cooling, create more or l~ss vis- ible dark patches, or blue points. This transformation mechanism, the cause of which can be attributed t() pad material (rapid friction coefficient variation, encrustation, etc.) is extremely detri- menta! and the disc is dahlaged be- yond 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 tem- ! perature and is non-reversible: as the blue points wilrbe IBs subject to wear than the rest of the surface, the phe- namenon 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 de- velopment of pads for a .new vehicle the possibility that a certain composition may cause hot judder is sufficient reason to discard it. As aresult, pads ho- mologated by the original equipment manufacturer do not have this defect. It should be noted however that use of discs wom 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 some- times 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..
regards
andre
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 con- ducted 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 in- stead 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.). From the theoretical standpoint there is not much differen:ce 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 dif- ferent. 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 accompa- nied 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 synchro- nous. The frequency varies from around one hundred or more Hertz when brak- ing 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 fust 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 vibra- tions can be identified: Cold Judder, linked to dimensional anomalies indepen- dent of use of the brakes (anomalies that often occur when the brakes are not used) and Hot Judder that occurs af ter use at high temperature. Cold judder vi- brations are doe to both geometrical imperfections of the disc itself and to de- fects 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 D1V (Disc Thickness Variation) or a wobble or planarity defect which in turn causes a DTV. Instances of this occur very infrequently thanks to the pro- duction 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 m?vements (right pad, left pad) cause vibration iO ~en this phenomenon arises, possi- bly a few hundred or thousand kilo- metres after installation, the vibration ,,~-~~|EQU||EQU|::|EQU||EQU|;:;;;;; start~ gradually but then becomes more iii:teflse as the dïstance trav- elled increases. In certain cases the pads oscillate and vibrate in the absence of brak- ing action, for instance when travel- ling 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 installa- tion will cause a similar type of pad wear and after a few thousand kilo- metres 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 aresult of a significant temperature increase. When it cools down it does not re- turn to its original shape and the wobble this causes will lead to ir- regular wear and, after a certain distance, the appearance of cold judder-type vi~ons.
Surface deviation during one rotation
Internal surface
External surface
Disc thjckness varjation during one rotation
If the vibrations appear af ter or during exposure to high tempera- ture then the phenomenon is known as hot judder. In an attempt to cause such vibration a series of average level deceleration braking ac- tions are performed while speed is increased up to 90% of top speed. Por in- stance 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 r~ IQ~~lised~rature increase. VibratiGns ap'.pea and amp;: SimlJ!!afleOusty at the hot points, which are normally distributed in a uniform manner over the disc, and these, on cooling, create more or l~ss vis- ible dark patches, or blue points. This transformation mechanism, the cause of which can be attributed t() pad material (rapid friction coefficient variation, encrustation, etc.) is extremely detri- menta! and the disc is dahlaged be- yond 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 tem- ! perature and is non-reversible: as the blue points wilrbe IBs subject to wear than the rest of the surface, the phe- namenon 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 de- velopment of pads for a .new vehicle the possibility that a certain composition may cause hot judder is sufficient reason to discard it. As aresult, pads ho- mologated by the original equipment manufacturer do not have this defect. It should be noted however that use of discs wom 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 some- times 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..
regards
andre
