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 detailing the effective or rolling circumference of various tyre sizes under specific load and inflation pressures.
| Identifier | ExFiles\Box 90a\4\ Scan154 | |
| Date | 5th May 1936 | |
| Mr. Robotham. X262 GRY.{Shadwell Grylls} JY/HDE. 5th May, 1936. E.{Mr Elliott - Chief Engineer} R.{Sir Henry Royce} Hall Esq., 53 Berkeley Court, Baker Street, LONDON, N.W.1. Dear Mr. Hall, After talking with you this morning regarding the effective or rolling circumference of different size tyres, I have had various sizes measured up all on the same width rim and with the same inflation pressure and load. The figures I gave you this morning were overall diameters without load as the figures which I had in front of me for effective radius were with loads and pressures which would not be applicable to your car. The following figures were obtained on the 4.00 rim with a load of 9 cwts and inflation of 38 lbs. Tyre Size. Effective Circ. (Standing) 5.50/19 93.38" 6.00/19 95.58" 6.50/19 98.91" 6.00/18 92.13" 6.50/18 95.14" At 100 m.p.h. the effective radius increases by about .25" which means the circumference will increase about 1.60" allowing for slippage. When the tread pattern has worn off the circumference will be reduced by about 2.5". I am sending a copy of this to Mr. Robotham as they/ - Continued - | ||