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 development and performance of Bentley Aluminium Composite Brake Drums.
| Identifier | ExFiles\Box 85\1\ scan0030 | |
| Date | 4th March 1936 | |
| Bentley Aluminium Composite Brake Drums. In view of the general tendency to increase the power/weight ratio of our cars, we started some time ago to develop ways of keeping brake drums relatively cool, because the heat the latter are required to dissipate seems to vary as the weight of the car and some power of the acceleration, the index probably being greater than unity. The reasons for desiring cool-running brake drums can be explained by quoting from the I.A.E. report No.8200 class 381,42,45, the findings of which we endorse from our experience - "Column 10 in table 5 (p.12) indicates, on comparison with column 1, the effect of previous high temperature (360°C) operation on the subsequent wearing properties at 100°C. In many cases, the high temperature run largely destroyed the subsequent wear resistance at the lower temperature, the increase in rate of wear being, in some cases, as much as ten-fold" - (Columns quoted reproduced and attached hereto). We have also reproduced from the same report a graph showing the variation of co-efficient of friction (μ) with temperature increase. While the "fade" which occurs from 220°C. onwards is dangerous, the subsequent recovery may be more so. For both these reasons, wear and variation of μ, the aluminium drum, which is the best of the 3 ways we have considered for reducing brake temperatures, is desirable. We would like to amplify the statement in the preceeding paragraph, that the recovery of μ may be more dangerous than its fade. In the course of a run to the Red Gate in a 4 1/4 litre Bentley with standard brake drums in which were thermocouples, the brakes, which had seen some service, became so fierce that the greatest care had to be exercised in their application, otherwise judders occurred. The third judder we experienced twisted the axle 9° and of course the car became uncontrollable at a high speed. The peculiarity of this judder was that the brake toggles were not buckled, which proves conclusively that no excessive brake load was applied. | ||