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).
The investigation into front brake dry judders, with specific reference to Bentley vehicles.
| Identifier | ExFiles\Box 85\1\ scan0141 | |
| Date | 27th July 1936 | |
| Hs{Lord Ernest Hives - Chair}/AFW.2/KW.27.7.36. Front Brake Dry Judder with especial reference to Bentley. Since the introduction of front wheel brakes we have been troubled with judders. On the earliest cars it was found that fitting axle controls and weak drums overcame this complaint, but since that time the trouble has slowly become worse until on the 4¼ litre Bentley we have reached a highly dangerous condition. We have therefore been making a considerable number of experiments, most of which are described hereinafter, with the object of finding out how judders can occur and although there remain certain inconsistencies yet to be explained, we have now got two practical schemes which are judder-free under all conditions. A judder is a torsional oscillation of the carrier plate. Its frequency generally seems to be the same as that of a wet judder, which is about 100 cycles/sec. Although it is a forced vibration this frequency must be closely the same as the natural frequency of the carrier plate, which is determined by the stiffness factor of the axle beam and its fixings and the moment of inertia of the oscillating parts. We think that if this natural period was sufficiently changed from what it is now, judders would not occur. This has been done in the following cases :- (1) Bentley with cycle type wings. Low frequency. (2) All rear axles. Frequency probably much higher. (3) Phantom III. Frequency higher. Judders have not occurred in these cases. Although judders can be self-starting, usually they occur through an initial displacement being given to the carrier plate from its stable position under steady braking torque. This can be done by :- (1) A punch application of the brakes, which produces a thump. (2) Tramping. | ||