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).
Brake drum design, comparing the properties of aluminium and iron, and methods for improving cooling.
| Identifier | ExFiles\Box 85\1\ scan0271 | |
| Date | 2nd December 1936 | |
| -4- which are very heavy, but as the bulk of the Kinetic energy of the car is absorbed in the rear drums even these are hardly big enough. We are trying 14" dia. aluminium rear drums on this car. We have made them extremely light, we may even have overdone it slightly. We think that a 14" iron drum with more ribs would be better than the present 16" drum. Now that we have an anti-judder mechanism we can increase the cooling of Bentley front drums by increasing the rib height. We have already issued a standardisation sheet for a slight change in this direction. If we are not going to standardize the aluminium drums we shall have to consider increasing the rib height all round, as the present position of Bentley brakes is really not very satisfactory. They are not up to their job. Will you please note in connection with any future designs of drums that the rate of loss of heat (calories/min) is more or less independent of the drum diameter, but for a given gear rotational speed at a given temperature it is proportional to the surface area of the drum. We have many figures to show that this is so. Increasing the heat capacity of the drum (product of weight and specific heat) merely reduces the rise of temperature in a given stop, and on our cars this is at present only a small fraction of the temperatures the drums reach. In this connection we should like to add that as the ratio of the specific heat to coefficient of thermal expansion is larger in the case of aluminium than iron, in a given stop on aluminium drum of equal weight to an iron drum will expand less than the iron drum. The figures we usually take are:- | | Aluminium. | Iron. | |---|---|---| | Specific Heat | .22 | .1 | | Thermal Expansion | 22.4 x 10⁻⁶ | 11 x 10⁻⁶ | | Ratio. | 9.8 | 9.1 | Aluminium has a further advantage, namely, that its specific conductivity is 0.42 as against 0.12, and consequently, the entire aluminium shell acts as a heat reservoir. | ||