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 effect of different tyre sizes on the braking performance of a Bentley.
| Identifier | ExFiles\Box 85\1\ scan0331 | |
| Date | 3rd May 1937 | |
| File Only. gp6 Rm{William Robotham - Chief Engineer}/AFM.{Anthony F. Martindale}8/AP.3.5.37 EFFECT OF TYRES ON BRAKES. BENTLEY The brake characteristics of Dunlop 17" X 6.50 and 18" X 5.50 tyres have been investigated. With a standard brake system on 5.B.lV., the wide tyre gave an average minimum stopping distance of 33 feet, the narrow 37 1/4 feet. This we explain by saying that the absolute co-efficient of friction between each tyre and the road is the same but the wider tyre has superimposed on this a greater 'spragging' effect than the narrower tyre. With wheels locked the stopping distance in each case was 42 - 44 feet. The effect on brake temperatures was also investigated as it seemed reasonable to suppose that if there was less slip between tyre and road with a certain tyre, then more of the K.E. destroyed in a stop would be converted into heat in the brake drum. This was found to be the case. On Donnington track the temperature rise of the rear drums was 13.5% greater with the wider tyres, measured by stopping the car from 90 m.p.h. to rest repeatedly, under constant conditions. This was confirmed by two runs to the Royal Red Gate. On the first run N.S.F. and O.S.R. had the wider tyres O.S.F. and N.S.R. narrower ones, and this was reversed on the second run. Allowing for the fact that the N.S.R. brake was fierce the attached chart bears out the previous results. We suggest that to improve our brakes we should fit 17 X 6.50 tyres and the drums with bigger ribs now on 3.B.lV. in France. Further, it seems that the poor stopping distances of our cars are partly due to the fact that they are 'under tyred', i.e. for their weight they have less contact with the road than other makes of car. If we arrange a Bentley so that all its' wheels lock simultaneously we cannot stop it in less than 35 1/2 feet. Rm{William Robotham - Chief Engineer}/AFM.{Anthony F. Martindale} | ||