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).
Technical report on steering geometry, wheel fight, and the effects of wishbone suspension.
Identifier | ExFiles\Box 170\3\ img019 | |
Date | 25th February 1936 guessed | |
-7- (9) Steering Geometry - Up & Down. When we first saw the peculiar arrangement of the two unequal tie rods on the Chrysler we assumed that the intention was to get minimum change of toe-in from normal load to maximum bump. We find on testing the toe-in variations that this is not so. The center ball for the long tie rod is placed well for this result, but the inner end of the short L.H. tie rod is an inch too low. We have never known how much "up and down" geometry affected wheel fight, so, to get at the root of the matter, we produced for the T.W. job a steering linkage in which there was no toe-in error. In doing this we probably made the linkage very rigid. Our earlier impressions were confirmed. This was by far the worst set up for high frequency fight that we have ever put on the bump rig, and putting error into the geometry by lowering the inner centers of the tie rods made the wheel fight neither better nor worse. Substituting the original linkage which has very definitely worse geometry, but is undoubtedly more flexible, gave a wheel fight picture which by comparison was quite good. CONCLUSIONS. (1) Up and down geometry errors of the type which exist in wishbone suspensions have no effect on wheel fight. (2) Wishbone suspensions in which the masses which swing about the kingpins are small, have an inherent tendency to high frequency wheel fight at about 550 a minute, which is associated with wheel hop, and under some conditions may become very violent and lead to true "shimmy" on smooth roads. (3) The only basic cure we have found is to "tune it out". This consists in so arranging the flexibility of the steering gear and linkage, that the natural frequency at which the wheels "flap" about the kingpins is 400 a minute or less. (4) The practical limit to the softening of the steering is the sensation of rubbery steering. | ||