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).
Page discussing the fundamental laws of vehicle braking, focusing on road surface friction and retardation.
| Identifier | ExFiles\Box 178\1\ img073 | |
| Date | 12th October 1925 | |
| - 11 - REF. Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}2/LG121025 (5) B R A K I N G . Fundamental laws. Weight of car. Gradients. Distance, and time to rest. Windage. Rotating parts, and pedal pressure. Max. rate of retardation. Personal comfort. Fundamental laws. (road surface). The rate at which a car can stop is not under the control of the designer. When the braking system has been arranged to give smooth and progressive action and to lock all four wheels (if required) with a reasonably light pedal pressure, the engineer has done everything possible, external conditions dictate the way in which the car behaves. The main deciding external factor is the co-efficient of friction between the tyre and the road. From practical experience we find that this can vary about .7 on a dry tar-macadam road to less than .1 on wet wooden setts. The retardation of a car is directly proportional to the co-efficient of friction between the tyre and the road once the wheels have been locked. If a co-efficient of friction of 1 was encountered, the car could pull up at the rate of 32 ft/sec2. If .7, at 22.4 ft/sec2, if .1, 3.2 ft/sec2. Put in other words, in the first case the car would come to rest from 40 m.p.h. in 54 ft. in the last, in ten times the distance or 540 ft. though in both cases the wheels would be locked. (contd). | ||