Fault assessment of brake discs
The term “brake judder” refers to non-uniform braking torques – and therefore to brake force fluctuations – which occur during braking or, more precisely, in the course of a full brake disc revolution. These phenomena, which have different causes, are divided into thermal judder, which occurs during deceleration from high speeds, and cold judder, which can occur at any speed.
“Cold judder” occurs during normal braking and can be identified by pulsation in the brake pedal, torque fluctuations in the steering wheel and/or vibration of axle and chassis components.
The characteristic differentiating this from “thermal judder”, is that cold judder can occur virtually every time the brake is applied, and has a much lower frequency range (about 5 to 50 Hz). The intensity of the judder may also vary with the vehicle’s speed.
Cold judder is caused mainly by variations in the thickness of the disc and similar to thermal judder, can be amplifi ed by faulty bearing components and out-of-balance wheels.
How does this thickness variation come about? Every brake disc contains radial run-outs caused by production and assembly tolerances. Disc brake pads will always make some contact with the brake disc during unbraked driving, and this contact will be most pronounced at certain points of the discs.
Although the contact forces are relatively low, they cause wear at these points of the brake disc. This results in a disc thickness variation, which eventually causes judder. Providing certain prerequisites are fulfi lled, this disc thickness variation can usually be reduced or eliminated again by normal braking, which keeps the alternating generation and reduction of thickness variations acceptably balanced. We will deal with these prerequisites later on.
The following factors infl uence disc thickness variation:
- radial run-outs of the brake discs in the fitted state
- brake pads rubbing onto the disc in the unbraked state
- the ability of the disc brake pads to reduce or remove disc thickness variations during normal braking
- correct caliper actuation that allows the release of the brake pads from the brake disc
- the road and environmental conditions and the driver’s driving style
The effects of existing and identical variations in disc thickness can vary significantly from one model to the next and depend on the force transmission factors and the damping capabilities of the axle, steering and chassis components.
When investigating the causes of judder, keep the following points in mind:
- Radial run-out of brake discs should be tested with the disc installed on the vehicle, ideally with a correctly fitted wheel. Radial run-out is tested with a dial gauge with a measurement accuracy of at least 0.01 mm and applied about 10 to 15 mm below the outer disc radius. A measurement on the mean friction radius is sufficient, however. The reading on newer vehicles should be taken over several revolutions and the run-out should not exceed 0.070 mm (Problem vehicles: < 0.040mm). Note that this test yields valid results only on new brake discs. Due to the component tolerances such low readings can not be achieved on older cars. An optimisation can still be achieved, however, by positioning the brake disc on the hub to the fastening bores so that the lowest measured value is achieved. But even on older cars, brake disc radial run-outs must not exceed 0.10 mm. If necessary, the component(s) causing the run-out (hub, brake disc, bearings) must be replaced. Take care that the contact surfaces is kept clean and free from defects.
- As mentioned above, the hub can also cause excessive radial run-out and therefore must be measured. In this context, a maximum value of 0.030 mm, referred to the outer measurable radius, can be taken as a guide. If the deviation exceeds this value, the hub should be replaced.
- A further factor affecting radial run-out is brake disc ripple. The disc’s parallelism should therefore also be checked and should not exceed 0.050 mm. Specialist equipment is necessary for this test.
- For a precise measurement of disc thickness variation, specialist equipment is required, although a precision micrometer gauge with a measuring accuracy of 0.001 mm will yield suffi ciently precise results. Measurements should be taken at 12 to 15 points around the disc’s circumference and about 10 to 15 mm below the disc’s outer friction radius. Depending on the vehicle type, thickness variations as low as 0.012 to 0.015 mm (Problem vehicles: < 0.008mm) can cause judder. On new discs these values must not be exceeded. They are also the absolute tolerance limit for Textar brake discs.
In addition to these tests, some of the test procedures described in the section about “thermal judder” should also be performed. These include checking the disc brake’s operating state, the wheel bearings, suspension and steering components, the front axle adjustment and the use of car manufacturer-approved disc brake pads.
As we have shown, the causes for excessive radial run-out and disc thickness variations can be difficult to identify. But by performing the possible measurements on the affected components and – if necessary – replacing them, these faults can largely be limited to acceptable levels.
As already mentioned, driving style as well as traffic and road conditions also play a part in causing brake disc thickness variations. Journeys of several thousand kilometres with little braking at low frictional power generation can result in sufficient disc thickness variations to cause judder. A subsequent driving phase containing a lot of braking can regenerate the discs again.