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 calibration, considering gear ratios and the centrifugal effect on tyres at speed.
| Identifier | ExFiles\Box 59\2\ Scan131 | |
| Date | 26th March 1931 guessed | |
| -11- Now suppose that with normally inflated normal size tyres the number of dead slow revolutions per mile of the road wheels is R.{Sir Henry Royce} The speedometer drive gear ratio being G the dead slow revolutions per mile of a speedometer drive will be GR.{George Ratcliffe} In order to correct the mile counter for the centrifugal effect of the tyres it will be seen (referring to the 3% contraction table) that the revolutions of the instrument spindle per mile indicated by its counter must be smaller than GR{George Ratcliffe} in the proportion of 44.27 45.00 N.B. This relation is, of course, obtainable either by altering the gear in the instrument or the gear ratio G.{Mr Griffiths - Chief Accountant / Mr Gnapp} Having secured this relation and supposing that we already have a contracted speed indicating scale on the basis of an approximate 5% contraction at 90 miles per hour, it remains to adjust the instrument, by the adjustment of the relative position of cup and magnet, until it is correct in itself internally when reading 45 miles per hour. This condition is secured when the speed at which it must be driven in order to read 45 miles per hour = 45/60 x 44.27/45.00 x GR{George Ratcliffe} revolutions per minute the 45/60 being miles per minute and the remainder being revolutions per mile, the product of the two quantities therefore being revolutions per minute. An instrument arranged in the above way with, of course, the correct drive ratio from the wheels must necessarily function very satisfactorily as regards both the mileage indications being correct and the speed indications being correct but with a reasonable tendency to slight over-reading at the very high speeds. It is clearly the correct way in principle to arrange such an instrument, the basis upon which it is done resulting in an instrument which, though not now having a universally internally correct relation, is nevertheless actually more correct on principle for the circumstances in which it is used. E.F.C. | ||