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).
Selection of appropriate wheels and tyres for the 20HP model.
| Identifier | WestWitteringFiles\Q\April1927-June1927\ 33 | |
| Date | 25th April 1927 | |
| TO HS.{Lord Ernest Hives - Chair} FROM R.{Sir Henry Royce} C. to BL, BY, DA.{Bernard Day - Chassis Design} ORIGINAL R2/M25.4.27. X5810 X4565 X3725 X5460 WHEELS FOR 20HP. We should not let what Dunlop's say retard us from our endeavours to select a suitable dia. of wheel, and tyre, for our chassis. My suggestion is in no way in the nature of a final command to use a smaller wheel than our old standard, but I understood that the 21" wellbase rim with the 6" low pressure tyres, gave us a wheel larger than our old standard, whereas I was wishing that the external dia. of the tyre should be slightly smaller than our old standard. Firstly, we do not want the frame of the 20HP. to be higher off the ground than we can help, and secondly, we do not want the 20HP. to look as big as the 40/50. So what we are really aiming at is a tyre where the weight, etc, should be proportionate to the weight of the two complete cars, which we look upon as a maximum relatively of 3 tons and 2 tons, and since the 20HP. does not go as fast as the 40/50., there will be this in favour of it, so we want a wheel and tyre of about two thirds the weight, cost, and load carrying capacity of the 40/50., and the life if possible should be the same in car miles. Regarding the life, judging from the 40/50 just returned from France, which has been 10,000 miles on the rough French roads on the single set of tyres, it would look as if we obtained the same life from the 20HP. it would not be worth while to handicap the car with larger and heavier wheels than is proportionate to give the same life. I am extremely pleased with the 21" wellbase rims and the 7¼" low pressure tyres on the Phantom, and it would appear that the 19" wellbase rim with the 6" low pressure tyre would be about proportionate for the 20HP. considering it is two thirds of the weight, and from 10% to 20% slower. If you agree that there is hope of this size being suitable, will you kindly get a set as soon as possible. You will notice that the smaller dia. of wheel makes it better for our back axle, and also better for our steering, and I think a better general appearance, and should be lighter, and less first cost. It would therefore only be necessary to consider the running costs and the road clearance. Regarding the tank hitting the road this should be more the fault of the tank than the road wheel. We ought to be able to make our lighter chassis run on a wheel of about 32" dia. The 6" tyres on the 21" rims resulted in a wheel so large that it would not go on the back carrier. I saw the spare wheel being carried in a very unsatisfactory manner on the side carrier. This refers to the car at present running in France. It will be remembered that for a long time the rear tank has been altered in the sump part to give more road clearance (I believe nearly 1½"). If this is not enough we should alter the tank. Am I to understand that the tank bottom was lower than the centre of the axle (i.e. bevel gear-box.)? R.{Sir Henry Royce} | ||