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).
Letter from Ferodo Limited detailing findings on brake noise, friction materials, and testing procedures.
| Identifier | ExFiles\Box 26\4\ Scan186 | |
| Date | 29th October 1923 | |
| From FERODO LIMITED. 29/10/23 Sheet No. -2- Messrs Rolls-Royce Limited, drums used were of very substantial character and no other material previously used made any sound at all. There was nothing special about the particular piece of lining, its coefficient of friction was 0.365, which is normal. A spot of oil on the drum instantly produced silence - for a short period only - and graphite had a similar effect, but both reduced the coefficient of friction to 0.3. However, without recourse to any lubricant, the noise was permanently stopped by inserting a piece of thin asbestos cloth between the back of the lining and the shoe which carried it, from which it would seem to be indicated that in this case, the lining itself and not the drum was primarily responsible. We have also noted that noise is prevented by taking out one shoe from the brake, i.e. the one which opens out from cam end with the direction of rotation of the drum, and doing all the braking on the remaining shoe. Reversing the arrangement, the noise again arises. We refer here to shoes that actually hinge on their fulcrum pin - as you know, in examples of this kind one shoe does very much more work than the other, if the applied pressure through the cam is equally divided between the two shoes. We suggest that reduced pressure offers an explanation for the groaning set up on occasions when a car is nearly at rest, when it may be assumed that the coefficient of friction is highest when slipping has nearly reached zero, and we, of course, know that an increase in the coefficient of friction increases the braking effort in much greater proportion than the increase in the coefficient itself. We further find that drums of heavy section such as those that permit of ribs being machined in them for radiation are rarely noisy, which also may be said of drums mounted on wood wheels and drums lined with cast iron. We are shortly mounting one of your standard drums in our Test Machine for the purpose of a thorough exploration into the whole matter, which as earlier remarked, has come on with remarkable suddenness, fortunately, only a few cars are implicated, but it makes the solution rather more difficult when we find that one car out of many hundreds of the same make is noisy and all the others quiet. We will conclude for the present by stating that we quite agree that the difficulty can be overcome as described in your letter by methods (1) and (3), but since the majority of brakes have very little margin it is rather open to question to employ a lubricant, particularly as we have found that the drums can be damped by a very simple method. One further note, we find that brakes normally silent will occasionally squeak for the moment on first application in wet weather, especially after a car has stood all night in a cold garage. Yours faithfully, FERODO LIMITED Chief Engineer | ||