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).
Tyre and wheel specifications for the 25 HP model, focusing on different sizes and their impact on gear ratios.
| Identifier | ExFiles\Box 106\4\ scan0130 | |
| Date | 23th October 1931 | |
| To Ev.{Ivan Evernden - coachwork} from Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} c. EV.{Ivan Evernden - coachwork} Sg.{Arthur F. Sidgreaves - MD} Handwritten: X 5810 Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}5/AM23.10.31. 25 HP. TYRES AND WHEELS. X.5820. X.5810. With reference to De/Ev.{Ivan Evernden - coachwork}2/M20.10.31, we regret that you did not realise we were using Fort tyres on the 19" rim on the 25 HP. As a matter of fact we have never supplied anything but Fort tyres with this rim unless by a special request. We should certainly like to try some 18" wheels on 20-GIV and will get them as soon as possible. The only difficulty we can foresee is that the inflated diameter of the existing standard tyre is 32.1" and the tyre we propose to order which is 30 x 6" Fort is 31.1". Will these tyres look unsightly with the wings you have provided for this car? If so you will perhaps take the matter up with the coach-builders. The effective running radius of the 30 x 6" tyre is only 14 3/4" against the standard of 15". This means that the gear ratio will be lowered by 3 1/2% but we think this will be an advantage as we propose to use a 12-toothed axle which without this reduction is perhaps a little too high geared. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||