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 causes of and solutions for front brake thumps on Bentley, 20/25, and Phantom II models.
| Identifier | ExFiles\Box 102\3\ scan0174 | |
| Date | 11th March 1936 guessed | |
| -2- There is very little to admire in the brake linkages of the Bentley, 20/25 or Ph.II, and we are still hoping that the first two will be revised shortly. If it was not for the servo it would be impossible to use such linkages. In particular, we find fault with the shortness of the levers on the equaliser shaft. On a 20/25, they are not the lowest point of the car, and any gain in rigidity is far more than outweighed by the accompanying poor efficiency, which necessitates a low-geared set of levers between servo and brake shoes, causing the servo to reach its stop sooner than necessary. This shortness of the levers on the equaliser shaft is an aggravating factor in front brake thumps. As they are so short they have to move through a large angle, with the result that to start taking up the clearance a larger force is required of the servo than to hold it taken up. This is just the reverse of what it should be, and depends on the geometry. We have found that the setting of these levers vertical, instead of forward of the vertical, appreciably reduces thumps, although it is not an absolute or practical cure. The way to cure thumps is to reduce the force necessary to pull on the front brakes, and this we did by reducing the spring force on the torsion shafts in the front axle. It is not necessary to reduce it to zero. When this is done the usual poundage of 18 lbs. on the swinging arm of the servo is not necessary, 8 lbs. being adequate. This poundage is not critical. We have found that with the springs in the front axle slacked right off, and the swinging arm poundage 18 lbs., the links etc. being thoroughly dirty, the front brakes come off satisfactorily, and no thumps occur. Our recommendation to cure thumps on existing 20/25's is to reduce the spring tension in the front axle to such an extent that with an 8 lbs. swinging arm load, thumps do not occur. The swinging arm poundage may then be increased to 11 lbs. With this setting, the swinging arm does not move either when the brakes are applied or released, and only serves to compensate for unequal wear front and rear. The brakes come off perfectly satisfactorily as the balance lever rotates. Our first attempts to cure thumps consisted in fixing linkages to make up front brake clearance with the pedal. Some linkages were successful, but all introduced other complications. None of these linkages were able to give front braking without Servo. Hs{Lord Ernest Hives - Chair}/A.F.Martindale. | ||