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).
Speedometer accuracy issues due to the increase in effective tyre radius at speed.
| Identifier | ExFiles\Box 165\2\ img320 | |
| Date | 11th April 1941 | |
| To: Hs{Lord Ernest Hives - Chair}/Wym.{G. Harold Whyman - Experimental Manager} From Rm.{William Robotham - Chief Engineer} c. RA/RC.{R. Childs} Ev.{Ivan Evernden - coachwork} Rm{William Robotham - Chief Engineer}/GB. SPEEDOMETER ACCURACY If the scale of a speedometer is divided into equal segments for speed ranges of 10 m.p.h., it is impossible to get a reading on the road which is correct at both 10 m.p.h. and 90 m.p.h. The reason for this is that the effective radius of the tyre increases with speed. We have always known of this, but it is many years since we got Dunlops to get any figures for us. Recently they checked our latest standard 6.50 - 16 tyres for us. It will be observed that without the temper- ature effects, at 90 m.p.h. the effects of radius of the tyre is increased 5%. This means that speed- ometers which we think are 5% fast are really accurate at 90 m.p.h. It also explains why, when we put an accurate speedometer on a car like 9.B. we have the greatest difficulty in getting up 90 m.p.h. on the road, though we know that on Brookland's track, the car will do almost 90.m.p.h. in still air. We shall be glad if Childs will take up with the makers the question of correcting the scale of the standard speedometer to make allowance for increase in effective tyre radius. Such a correction will be particularly important in the case of the Corniche, which is expected to do 110 m.p.h. Rm.{William Robotham - Chief Engineer} | ||