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).
Letter to The A.T. Speedometer Co., Ltd. regarding the effect of tyre radius on speedometer accuracy.
| Identifier | ExFiles\Box 165\2\ img333 | |
| Date | 1st August 1941 | |
| 6065 -8- RHC{R. H. Coverley - Production Engineer} 1st August, 1941. Rm{William Robotham - Chief Engineer}/EC.1/RH.{R. Hollingworth} Your ref: HRB/LAH. The A.T. Speedometer Co., Ltd., Chronos Works, North Circular Road, L O N D O N. N.W.2. Attention of Mr. H.R.Buckland. Dear Sirs, We have now fully considered the matter in your letter of the 14th July, concerning the effect of increase of tyre radius on the accuracy of your speedometers. We are glad to note that you can meet with our special requirements, in that, it is possible to arrange for special gearing to meet the mileage question, and, at the same time, special calibration to meet the speed requirements. Thus, we can treat each function separately, and arrange for the speed indications to be accurate at 30 m.p.h., and the mileage recorder to be accurate at 40 m.p.h.. In the past, it has been our practice to make an allowance for 1% increase in tyre radius when calculating the calibration value of the instrument, since this figure agrees with the latest figures supplied by the tyre manufacturer for a speed of 30 m.p.h., we have always been correct at 30 m.p.h. It has also been our practice to provide a flatter of m.p.h., per each 10 m.p.h., after the "30" mark. This is intended as a flatter and not a tyre correction. The latest tyre figures show us that the greater part of this flatter has been lost in the tyres, which loss increases from 1% at 30 m.p.h. to 4% at 90 m.p.h. We are also informed by the tyre manufacturers that a further increase of 0.1 inch in radius occurs when the tyre becomes hot under normal running conditions. Following is a table showing wheel revs/mile and speedo drive revs/mile for both 4 ply and 6 ply Dunlop "Fort" -Over- | ||