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).
Comparison of different tyre types, their construction, and performance characteristics for 40/50 HP and 20 HP models.
| Identifier | ExFiles\Box 42\1\ Scan047 | |
| Date | 15th March 1926 guessed | |
| contd :- -2- it was apparent that the triple stud is a flatter tread, and so the edges get worn almost as quickly as the centre. The traction tread edges, however, do not normally do very much work and are therefore left behind as the centre wears. The actual thickness of rubber at the centre of each tread is identical. It is therefore questionable whether the triple stud really does have a much longer life as a smooth tyre. COMFORT. For the 40/50 HP. two sizes of tyres are recommended - 33 X 6.75 and 6" for 21" rims. These are both 6-ply tyres, the main difference is that the 6" tyre is a very much stronger tyre owing to its being the same number of plies but a smaller section. It therefore gives less riding comfort and must be run at a higher inflation pressure than the 6.75 tyre. For this reason the 6.75 tyre may be called a 'semi-balloon' whereas the 6" is more of an over-size medium pressure tyre. In the next smaller size i.e. 20 HP. tyre, the same procedure is followed - the 6" is a semi-balloon and is called a 33 X 6 in spite of the fact that it is also a 6-ply tyre. It is however a slightly lighter tyre than the 6" for 21" rim previously mentioned and can be run at lower inflation pressures giving relatively greater riding comfort. The oversize medium pressure tyre for the 20 HP. is called 5¼" for 21" rims. Though the nomenclature of these particular tyres is hardly ideal, their respective functions will be less obscure when tabulated thus :- contd :- | ||