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).
Implications of fitting low-pressure tyres to the 20 HP chassis.
| Identifier | ExFiles\Box 58\1\ Scan044 | |
| Date | 15th May 1928 | |
| X5820 BY/Cghl/G.15.5.28. re TYRES FOR 20 HP. CHASSIS. With reference to C7/C9.5.28. the position in regard to 33 x 6 low pressure tyres for the 20 HP. chassis is that although standardisation sheets were circulated and completely signed up in connection with the proposal to supply and fit these tyres when customers specially call for them, it was afterwards proposed to adopt them as standard, but BJ. turned this proposal down owing to the fact that these tyres would have the effect of increasing the rear axle gear ratio. He pointed out that he would prefer that we adopt these tyres at the same time as we adopt the lower ratio rear axle bevels. The new ratio referred to is 10/47 rear axle bevels, which would make the reduction ratio when used with the larger section tyres the same as at present with the medium pressure tyres. In addition to the above we pointed out that there is not sufficient room between the petrol tank and the rear cross member to accommodate the increased section of tyre, and consequently it would be necessary to raise the position of the spare wheel 2.325" higher than the wheel with our present standard 5 1/4" section medium pressure tyres. Sales pointed out that the spare wheel at its present height already touches the hood of touring cars and on some Cabriolets. Another difficulty would be that having to raise the centre bracket which carries the wheel, the central stay which is bolted to the channel section cross member would demand the coachwork cutting away considerably more than at present. To enable these points to be appreciated we send you herewith blueprint F.54972 showing general arrangement of the rear spare wheel carrier of the modified design to take a wheel with a 33" x 6" low pressure tyre on which we have indicated in red the position of the present standard medium pressure tyre. BY/Cgh.{W Clough} | ||