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 cause and cure for front brake thumps on the 20/25 HP model.
| Identifier | ExFiles\Box 102\3\ scan0173 | |
| Date | 1st January 1936 | |
| FILE. x586 20/25 HP. Front Brake Thumps. We have spent several days going into this question, and have found a cure. Experiments were carried out on 19-G-IV, and the result has been applied successfully to the Vulture. With standard brake system, the cycle of operations on depressing brake pedal is as follows :- (1) Servo scissors are opened till servo just rubs. (2) Rear brake clearance is taken up by pedal directly; and at the same time the tension in the rear linkage to overcome the pull off springs is sufficient to operate the servo so that it follows up the rear brakes, but the servo during this period is unable to take up the front brake clearance. (3) Rear brake shoes come into contact with drum, and tension in rods increases considerable, so that the servo output also increases considerably and is therefore able to overcome the front pull off springs. Unless the brakes are applied extremely progressively, the servo output is increased with a jump and the front brakes are applied with a thud that makes itself felt. This is the explanation of brake thump. It is conceivable that this was intended to happen, so that the front brakes could not come on before the rear ones, but if so, it was a most mistaken attempt to introduce a safety factor, as owing to the extraordinary inefficiency of the front brake system, a thump application applies the brakes much harder than would a progressive one, and the tendency to cause a front wheel skid is thereby increased, not decreased. | ||