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.
“Thermal judder” can be described as follows:
- A booming judder in a frequency range between 100 and 250 Hz. The intensity of the booming can vary during deceleration but does not affect braking.
- Torque fluctuations can often be felt as a vibration in the steering wheel, pulsation in the brake pedal and vibrating chassis components.
The occurrence of brake judder depends on the pedal force. Thermal judder can usually be identifi ed by a circular arrangement of spots on the brake disc’s friction surfaces. These are caused during braking by local overheating, which results either in a transfer of material from the brake pad to the brake disc and/or a permanent change in the structure of the brake disc casting material. Transferred material is usually removed when braking normally, but structural changes – also referred to as martensite formation – which
are harder than the disc material’s basic structure and can be removed only by machining. When repairing a disc with martensitic spots, it is important to completely remove the hardened areas. To prevent risks, the disc should ideally be replaced.
Spot formation through local overheating is caused by several factors:
- The disc can under certain conditions distort when heavy braking takes place. This flexing of the disc can result in permanent distortion.
- The brake disc has worn below the minimum thickness (see manufacturer’s recommendation), which reduces the disc’s capacity to dissipate heat.
- The disc brake pads are excessively worn and have an insufficlient braking effect.
- The brake disc casting and tolerances do not conform to the manufacturer’s specification.
- The disc brake pads fitted are unsuitable for the application and/or do not comply with the original equipment or any comparable quality standard.
- The brake system is not working correctly or some of its components are not dimensionally correct.
In addition to the causes of “thermal judder” due to local overheating described above, other factors which can cause or increase the likelihood of thermal judder include poorly balanced wheels, worn bearing components in
wheel suspension system, steering and a misaligned front axle.
In most cases, judder is caused by several factors, making it diffi cult to clearly identify the root cause.
Thorough and careful investigation of the cause and remedying of the fault is, therefore, necessary. This work should be carried out by a specialist repair shop with a high level of experience.
The investigation essentially consists of the following test procedures:
- First determine whether the judder is coming from the front or the rear axle.
- Perform a visual inspection to determine which functional components are worn and to what extent. Heavily worn or grooved discs or pads must always be replaced as an axle-set.
- Verify that the brake pads fitted have been approved for the application in accordance with the manufacturer´s recommendation.
- Check the operating condition of the disc brake, paying special attention to caliper components, and repair, if necessary. Repair should be carried out by a specialist workshop.
- Check whether the wheel is out of balance and rebalance if necessary.
- Check the operating condition of the suspension and the steering components and replace any faulty parts.
- Check the individual components of each wheel bearing for defects (bearing clearance) and replace if necessary.
- Check and, if necessary, correct the axle-geometry according to the vehicle manufacturer’s guide values.
Thermal judder can usually be reduced by choosing suitable friction materials, provided that the other vehicle components referred to are in a fault-free condition. When an optimisation of this kind is performed, it must be kept in mind that it complies with all the requirements set for the braking system.