From TRW

TRW has provided the following information about brake judder, defined as rotational vibrations of the steering wheel that can occur during braking, usually accompanied by a booming noise.
Depending on the cause, it can be divided into thermal judder, which occurs during deceleration from high speeds, and cold judder, which can occur at any speed.

Hot/thermal judder
‘Thermal judder’ is a booming noise in a frequency range between 100 and 250 Hz. The intensity of the booming can vary during deceleration but does not affect braking. It is normally accompanied by vibrations in the steering wheel and a pulsation in the brake pedal.
The effects of hot judder can usually be identified by a circular arrangement of spots on the friction surfaces of the brake disc.
This martensite formation is harder than the basic structure of the disc material and can be removed only by machining. When repairing a disc with martensitic spots, make sure to completely remove the hardened areas without reaching or falling short of the minimum thickness of the brake disc.

Cold judder
‘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.
Cold judder can occur virtually every time the brake is applied, regardless of temperature, and has a much lower frequency range (about five to 50 HZ) than thermal judder. These noises can be louder or quieter depending on the speed.
The main cause of cold judder is the thickness variation in the brake discs. Defective bearing parts and wheel imbalances can also increase the effect, as with thermal juddering.

In most cases, judder is caused by several factors, making it difficult to clearly identify the root cause. Below are some general steps to follow:
• Determine whether the judder is coming from the front or rear axle.
• Check brake components (brake pedal, brake booster, master cylinder, brake lines, brake hoses, wheel brakes). If replacing components, make sure to replace brake pads and brake discs, brake drums and brake shoes in complete axle sets.
• Check the wheels for balance, axial runout, radial runout and the condition of the tyre tread.
• Check the axle settings, wheel suspension, steering components and adjust/repair as necessary.

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