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).
Impact of a tyre change on car height and proposing alternative wheel and rim sizes.
| Identifier | ExFiles\Box 179\1\ img037 | |
| Date | 20th October 1931 | |
| FROM DA{Bernard Day - Chassis Design}/EV.{Ivan Evernden - coachwork} HS.{Lord Ernest Hives - Chair} ) Copy to BB. DA{Bernard Day - Chassis Design}/EV{Ivan Evernden - coachwork}2/20.10.31. Y5820. X5810. 25HP. TYRES & WHEELS. We have just discovered that last Sept. a change was made from Dunlop silent to Dunlop Fort silent tyres, and that this change raised the car .5". In setting out J.{Mr Johnson W.M.} 3, being unaware of the standardisation we have set the car out assuming the ordinary silent tread 6" tyres on 19" rims, giving a rolling radius of 14.52". When we changed from ordinary to Fort tyres on the 40/50 we recovered this increased height by a reduction in the rim size. Had we have realised the use of the larger wheel on the 20/25, we should have lowered the floor by increasing the set of the chassis frame and lowering the transmission so that the ground clearance would have remained unchanged. It is too late to do this now with convenience. It does look as though we might with advantage try 18" rims on the 20/25., and we suggest a very good car to try them on would be 20-G-4, the Sports Saloon for R.{Sir Henry Royce} No change will be necessitated in the bodywork and it does look as though no difficulty will be experienced with the brake drums or the cross steering tube. We shall supply discs to suit 19" rims as these will be the wheels most likely to be used for trials purposes. Sufficient paint will be supplied for the 18" rims to be sprayed the correct shade to match the car. Handwritten notes: RM{William Robotham - Chief Engineer} We ought to order a set of 18" wheels HS{Lord Ernest Hives - Chair} 6" x 18" FORT 18" Rim 30x6 Tyre Effective Radius 14.50 DA{Bernard Day - Chassis Design}/EV.{Ivan Evernden - coachwork} 32.1 18" 31.08 | ||