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 tyre tread pattern, irregular wear, and inflation pressure.
| Identifier | ExFiles\Box 158\1\ scan0011 | |
| Date | 14th January 1927 guessed | |
| contd :- -3- (III) Tyre tread pattern. High speed wobbles depend for their maintenance upon the grip between the tyre and the road. Thus on a road where a skid is likely to occur we find that 'shimmy' is very much less violent. It follows that a tyre with a very effective non-skid tread is worse than one with a plain tread for setting up high speed wobbles. (c) Irregular tyre wear. If a tyre starts its existence with a tendency to high speed wobbles, even though perfectly balanced, it may be said that it will undoubtedly get worse and worse in this respect throughout its life. The reason for this is as follows :- When the wheel reaches the critical speed at which it tends to shimmy, the pressure between the tyre and the road alternately increases and XXXXXXXXXXXX decreases once per wheel revolution, even if the wheel does not actually leave the road. This is due to the criss cross motion of the axle (See wobbles, under steering). This means that the tyre wears unevenly. In a recent case we had a pair of tyres in France which consis- tently wore 1/4 lb. out of balance per 500 miles running. It was quite useless balancing these tyres because the tread was out-of-truth and gave the effect of an elleptical wheel. This fact should be noted when a case of a perfectly balanced wheel that wobbles badly, is encountered. (d) Rear tyre inflation pressure and car control. There is a continual tendency to run the rear tyres under-inflated with a view to obtaining the maximum contd :- | ||