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 and mileage recorder calibration to account for the effective increase in tyre diameter with speed.
| Identifier | ExFiles\Box 59\2\ Scan059 | |
| Date | 25th March 1930 | |
| X4308 PN.{Mr Northey} From Hs{Lord Ernest Hives - Chair}/Rn.{Mr Robinson} Hs{Lord Ernest Hives - Chair}/Rn.{Mr Robinson}5/MJ.25.3.30. With ref: to PN{Mr Northey}3/WT21.8.30. Personally we consider that we ought to take into account the effective increase in tyre diameter with speed, and ask the makers to calibrate the miles per hour of the speedometers a further 2% fast to allow for this feature. At the same time it would probably be desirable from your point of view to reduce the reading of the mileage recorder 2% to ensure absolute accuracy. However, we have had so many conflicting reports as to the opinion of sales on the speedometers that are at present being sent out, that we should like you to give us your views. We suggest that you send a copy of your observations to E.F.C. as he is now looking after the speedometers and it would give him some practical information as to what is actually required. As a matter of interest we send you a curve which was supplied to us by The Dunlop Rubber Co. some time ago, taken on high pressure tyres blown up to 70 lbs/sq.in. Naturally, the balloon tyre responds much more readily to centrifugal force. You will note that even on these tyres the increase in effective radius was of the order of 2% at 30 miles per hour. Hs{Lord Ernest Hives - Chair}/Rn.{Mr Robinson} | ||