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).
High-speed wobbles caused by low pressure balloon tyres on New Phantom cars.
| Identifier | ExFiles\Box 29\1\ Scan147 | |
| Date | 26th October 1925 | |
| R.R. 493a (50 H) (D.D. 31. 12-6-25) J.H.D. EXPERIMENTAL REPORT. Expl. No. 4457 REF: Hs{Lord Ernest Hives - Chair}1/LG26.10.25. To B.J. from Hs.{Lord Ernest Hives - Chair} BALLOON TYRES - HIGH SPEED WOBBLES. 44565 4457 We cannot agree to the fitting of low pressure tyres to the front of the New PhantomCodename for PHANTOM I cars. With low pressure the period is likely to come on at 50 m.p.h. We think it might be helpful if we gave technical reasons why we cannot recommend low pressure tyres on the front wheels. The reasons why low pressure tyres increase the tendency for steering wobbles are as follows :- (1) The criss cross or tramping period of the front axle dictates the speed at which high speed wobbles occur. Alteration to the springing, weight of the axle, tyre section or inflation pressure, will alter this criss cross period - with high pressure tyres the criss corss frequency is about 650/min. or a wheel speed of 65 to 70 m.p.h. With low pressure tyres the frequency is reduced to 500/min. or 50 m.p.h. This means that high speed wobbles are likely to be encountered at the most moderate speeds with balloon tyres. (2) The lateral instability of a low pressure tyre combined with its large area of contact on the road, seems to be active in increasing the tendency for a wobble to start. (3) The fewer number of plies in a low pressure tyre, reduces the damping which takes place when the tyre is deflate contd :- | ||