Editing Disc brake (section)

===Judder or shimmy===
A brake judder is usually perceived by the driver as minor to severe vibrations transferred through the chassis during braking.<ref>Abdelahamid, M.K. (1997), "Brake judder analysis: Case studies", SAE, Technical Paper Series, no. 972027.</ref><ref>de Vries, A. et al. (1992), "The brake judder phenomenon", SAE Technical Paper Series, no. 920554.</ref><ref name=Engel>Engel, G.H. et al. (1994), "System approach to brake judder", SAE Technical Paper Series, no. 945041.</ref><ref>Gassmann, S. et al. (1993), "Excitation and transfer mechanism of brake judder", SAE Technical Paper Series, no. 931880.</ref><ref>Jacobsson, H. (1996), "High speed disc brake judder – the influence of passing through critical speed", In EuroMech – 2nd European Nonlinear Oscillations Conference, Prague, no. 2, pp. 75–78.</ref><ref>Jacobsson, H. (1997), "Wheel suspension related disc brake judder", ASME, no. DETC97/VIB-4165, pp. 1–10.</ref><ref>Jacobsson, H. (1998), "Frequency Sweep Approach to Brake Judder, Licentiate of engineering", Chalmers University of Technology Sweden.</ref><ref>Jacobsson, H. (1999), SAE Technical Paper Series, no. 1999-01-1779, pp. 1–14.</ref><ref>Stringham, W. et al. (1993), "Brake roughness – disc brake torque variation", disc distortion and vehicle response, SAE Technical Paper Series, no. 930803.</ref>{{Excessive citations inline|date=August 2024}}

The judder phenomenon can be classified into two distinct subgroups: ''hot'' (or ''thermal''), or ''cold'' judder.

Hot judder is usually produced as a result of longer, more moderate braking from high speed where the vehicle does not come to a complete stop.<ref>Thoms, E. (1988), "Disc brakes for heavy vehicles", ''IMechE'', pp. 133–137.</ref> It commonly occurs when a motorist decelerates from speeds of around {{convert|120|km/h|mph|1|abbr=on}} to about {{convert|60|km/h|mph|1|abbr=on}}, which results in severe vibrations being transmitted to the driver. These vibrations are the result of uneven thermal distributions, or ''hot spots''. Hot spots are classified as concentrated thermal regions that alternate between both sides of a disc that distort it in such a way that produces a [[sine wave|sinusoidal]] waviness around its edges. Once the brake pads (friction material/brake lining) come in contact with the sinusoidal surface during braking, severe vibrations are induced, and can produce hazardous conditions for the person driving the vehicle.<ref>Anderson, E., et al. (1990), "Hot spotting in automotive friction systems", ''Wear'', v. 135, pp. 319–337.</ref><ref>Barber, R., J. et al. (1985), "Implications of thermoelastic instabilities for the design of brakes", ''J. Tribology'', v. 107, pp. 206–210.</ref><ref>Inoue, H. (1986), Analysis of brake judder caused by thermal deformation of brake discs, SAE Technical Paper Series, no. 865131.</ref><ref name=Rhee>Rhee, K.S. et al. (1989), "Friction–induced noise and vibration of disc brakes", ''Wear'', v. 133, pp. 39–45.</ref>

Cold judder, on the other hand, is the result of uneven disc wear patterns or disc thickness variation (DTV). These variations in the disc surface are usually the result of extensive vehicle road usage. DTV is usually attributed to the following causes: waviness and roughness of disc surface,<ref>J. Slavič, M.D. Bryant and M. Boltežar (2007), "A new approach to roughness-induced vibrations on a slider.", ''J. Sound and Vibration'', Vol. 306, Issues 3–5, 9 October 2007, pp. 732–750.</ref> misalignment of axis [[run-out]], elastic deflection, wear and friction material transfers.<ref name=Engel/><ref name=Rhee/><ref>Kim, M.-G. et al. (1996), "Sensitivity analysis of chassis system to improve shimmy and brake judder vibration on the steering wheel", SAE Technical Paper Series, no. 960734.</ref>
Either type could potentially be fixed by ensuring a clean mounting surface on either side of the brake disc between the wheel hub and brake disc hub before usage and paying attention to imprinting after extended usage by leaving the brake pedal heavily depressed at the end of heavy usage. Sometimes a bed in procedure can clean and minimize DTV and lay a new even transfer layer between the pad and brake disc. However, it will not eliminate hot spots or excessive [[run-out]].