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Brake Judder is usually perceived by the driver as vibration, barely noticeable to moderate, around 60 Hz, transferred through the vehicle chassis components during braking. 

The Judder phenomenon can be classified into two distinct subgroups: thermal Judder and cold Judder. 

Thermal Judder usually originates as the result of long, moderate braking at high speed, when the vehicle does not  fully stop (only constant reduction of speed under constant pressure). It usually occurs when the vehicle decelerates from 120 Km/h to 60 Km/h, resulting in severe vibration transmitted through the vehicle subsystems, which is felt by the driver. That vibration is a consequence of uneven thermal distribution caused by what is commumly denominated Hot Spots.

The latter being classified as concentrated thermal areas that alternate between both sides of a braking surface area of the disc, distorting it in such a way as to produce sinusoidal waves around its overall diameter.  Once the brake pads get in touch with the sinusoidal surface during the breaking process, severe vibration is induced as a consequence and may produce a dangerous driving condition .

Brake Disc with Hot Spots

Cold Judder, on the other hand, is the result of uneven brake disc wear or DTV (Disc Thickness Variation). The variation on the surface of a disc usually is the result of extensive use of a vehicle.  The Disc Thickness Variation is generally attributed to the following causes: uneven braking surface of the rotor; excessive runout of the brake disc; elastic deflection; thermal distortion of the brake disc; brake pad wear and transference of friction material to disc.

Brake disc with friction material deposit 

Souce: Pedro A. Diulgheroglo, on Linkedin Pulse. Read the original article here (portuguese).