In many publications dealing with the ECE-R90 test, the word compressibility appears.
But what does this really mean?
The compressibility of a brake lining means the resistance shown at a specific pressure applied. In other words, tests are run to see how much the brake lining thickness changes when subjected to pressure. This is measured in μm. A μm is one thousandth of a mm. Such a tiny unit illustrates very clearly how little a brake lining can actually be pressed together. Nevertheless, due to the compressibility, there are quite considerable differences that a driver can experience in various braking scenarios. The brake lining’s compressibility can also affect the noise level produced by the brake system.
A high level of compressibility signifies a “soft” lining. However, if the linings are too soft, the driver might experience long pedal travel when applying the brakes. This means that he has to press the pedal further down in order to achieve the desired braking effect. This does not procure a safe braking sensation.
Low compressibility, on the other hand, means that the brake lining is “hard”. Logically, the disadvantages caused by linings which are too soft, as described above, are not present in such a case. However, with increasing lining hardness there is a growing risk that brakes start to squeal or rub. For driver and passengers, this manifests in the form of steering wheel vibration and a humming noise.
The answer is to find the right degree of hardness for the brake lining to achieve the ideal balance between a good braking sensation on the one hand and a good noise level together with good comfort features on the other. To find this balance, comprehensive test drives are required in addition to the lab compressibility tests.
Illustration of a testing procedure
The brake lining (2 on the diagram) is laid, friction side down, on a solid, polished and heated steel plate (1). By means of a piston adapter (3) which corresponds to the type of piston used in the brake system itself, the measuring piston of the testing device (4) presses the brake lining together. The force with which the lining is pressed together corresponds to a braking pressure of 160 bar. A distance sensor (5) measures how much i.e. how far, the lining can be pressed together. The measurements are carried out at room temperature and when the steel plate is heated to 400°C. The difference to the thickness of the lining can amount to a maximum of 2% at room temperature and a maximum of 5% at a temperature of 400°C.