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).
Technical memorandum discussing brake pedal 'sponge', fading issues, and proposed solutions.
| Identifier | ExFiles\Box 92\4\ scan0114 | |
| Date | 21th November 1935 | |
| -2- The above figures do not account for the whole of the pedal travel; but on the road heavier pedal pressures will be used than we were able to apply in the garage; and moreover when a car is in service it is seldom the brakes are fully adjusted. The trouble appears to be a combination of two things: fading of the brake shoe linings, and a pedal so spongy that the maximum available output from servo and direct braking cannot be used, as the pedal reaches the boards. It seems likely, from the study of the graph of pedal movement against load, that a small reduction in "sponge" would mean a big increase of braking power. The "sponge" in the servo is due to the compression of the anti-jagger cruciform springs, and is no worse than that of the Ph.II servo. A small amount of slack in the 3 brake cables is inevitable. All the rest of the sponge is in the rear brakes. We have already asked the Design Dept. for a lengthened brake pedal stem, so as to make allowance for the thick noise-proof carpets we now place under the brake pedal, and we have also asked for more rigid brake drums. These are at present far too weak, and sparks may be observed flying from the rear water excluder when the brakes are applied lightly, owing to the distortion. We hope these two alterations will be the solution to our trouble. We have instructed RHC.{R. H. Coverley - Production Engineer} to make 4 sets of drums with deeper ribs, one for each of our cars. Once the shoes have attained the same temperature as the drums, the movement of the pedal under a given load appears to be much the same at all temperatures. The feeling of the brake pedal retreating towards the boards as the brakes get hot we think must be due, partly to the unequal rate of heating up of drum and shoes, partly to the fading of the brake linings which necessitates a bigger pedal pressure to have the same stopping effect on the car. The servo is powerful, and so the increase of pedal pressure is not easily noticeable. | ||