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 judder, comparing linings and detailing the mechanics of servo-induced judder with proposed solutions.
| Identifier | ExFiles\Box 108\3\ scan0119 | |
| Date | 3rd March 1938 | |
| -2- in use on this car are superior to the V.M.30 linings in use previously. 3L is running in France and costs less than half as much as Ferodo. The only way permanently to prevent this type of judder is to develop a lining having the stable characteristic, i.e. a value of µ which is a maximum when speed is also a maximum. We are giving as much time as possible to the development of such a lining, for the purpose of preventing brake squeaks, and have succeeded in getting Ferodo Ltd. interested. (2) Mechanical servo judders have so far proved to be due to one cause only. In our servos the direct braking and servo braking pulls unite at some point in the linkage. Assuming that the servo is less than 100% efficient, the following judder cycle can occur. The pedal progressively pulls on the rear brakes a certain distance, during which time the servo does nothing. The servo then takes hold and having nothing to resist its motion (the brakes being partially on) gains speed and impulsively pulls the brakes on further than the pedal pressure applied actually warrants. This relieves the servo cams of load and the servo output falls accordingly. The brakes thus being released, load is reapplied to the servo cams and the servo once again jerks the brakes on too far. Such judders can be maintained for as long as the servo continues to rotate. Although the judders on Wraith are of a higher frequency than usual (due probably to the rigid brake system) they are beneficially affected by the usual cures for judders, thus showing that it is the same type of judder as we have encountered previously. Star springs, when working properly, entirely prevent judders on the car we have been experimenting with, and so does the addition of a 'pull on' spring to the front end of the servo to rear rod, where it joins the I lever, provided it is strong enough. The 'pull off' springs have to be stronger, to keep the brakes off. Star springs effect a cure for judders by ensuring that small movements of the servo cams do not materially alter the pressure on the servo discs. The 'pull on' spring ensures that the servo does not lag behind the direct braking rods, and thus prevents impulse loads. This is a superior cure to star springs as it does not introduce 'sponge' in the brake system. | ||