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).
Servo brake thumps in the 20/25 HP car, comparing dashpot and friction damping methods.
| Identifier | ExFiles\Box 102\3\ scan0064 | |
| Date | 17th March 1932 | |
| To R.{Sir Henry Royce} from Hs{Lord Ernest Hives - Chair}/Lid.{A. J. Lidsey} c.c. to Sg.{Arthur F. Sidgreaves - MD} Wor.{Arthur Wormald - General Works Manager} c.c. to R.{Sir Henry Royce} Da.{Bernard Day - Chassis Design} c.c. to By.{R.W. Bailey - Chief Engineer} 20/25 HP. CAR - SERVO BRAKE THUMPS. We attach herewith sketch showing roughly the manner in which the dash pot was rigged up to act as a damper for brake thumps. In our previous report, (Hs{Lord Ernest Hives - Chair}/Lid.{A. J. Lidsey}2/MA. 26.5.32) it was not stated that a considerable amount of work had been carried out on the standard system before it was decided to try and use the dash pot connected up to the front brake system. There are two factors that have made thumps more difficult to cope with since the adoption of the hinged tip brake shoe, - increased brake clearance necessary for the hinged tip, and the fact that the tip of the shoe itself always comes into contact with the drums first. We have found previously that it is possible to eliminate thumps by filing away the tip of the leading shoes. The more efficient the brakes are, the greater is the tendency to thump. We think this latter factor is more the cause of increased thumping than the larger clearances. If the movement at either end of the balance lever is the same, i.e. even when the friction arm does not move, violent thumps can be produced by smart brake application. A further scheme tried was a friction disc at the balance lever pivot, but even with this, in conjunction with the standard friction arm, it was not possible to cure thumps. The friction loads at these two places were put up until the front brake pull off springs were not strong enough to overcome the friction and return the brakes to the 'off' position, but the brakes still thumped. Dashpot damping is superior to friction, because the damping increases with the speed of application, whereas friction damping is at its greatest at the commencement of braking and decreases immediately movement of the friction surfaces takes place. | ||