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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Analysis of brake/clutch judder, featuring a friction characteristic graph for Ferodo MR against Millenite.

Identifier  ExFiles\Box 85\1\  scan0142
Date  20th July 1936
  
-2-
Hs{Lord Ernest Hives - Chair}/AFM.{Anthony F. Martindale}2/KW.20.7.36.

If the judder is not to die out immediately, power must be fed into the cycle. The energy is derived from the K.E. of the car. This can be done in two ways, as follows :-

In a judder of 100 cycles/sec. frequency and amplitude 6°, maximum variation of rubbing speed between drum and Ferodo is about 1950 ft/min. in the case of a 12" drum and assuming simple harmonic motion, and from the curve below at 28 m.p.h. this means there is a maximum variation of 17% on the mean value of 'u'. When the drums are warm this characteristic is said by Ferodo Ltd. to be much more marked.

[Graph Text]
CO-EFFICIENT OF FRICTION, μ
CURVE SUPPLIED BY FERODO LTD
MR AGAINST MILLENITE
M.P.H. CAR SPEED
FT/MIN RUBBING SPEED
[End Graph Text]

Thus, there is a bigger friction force between shoes and drum when they are rotating in the same direction than when they are rotating in opposite directions, and hence power is supplied to the cycle. This source of power does not seem to be sufficient in itself to cause a judder.
  
  


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