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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).
List of conclusions drawn from slow motion road tests about the causes and conditions of wheel shimmy.

Identifier  ExFiles\Box 170\3\  img035
Date  14th July 1933 guessed
  
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This gives a fairly complete general picture of true shimmy as we have observed it on the drums and in Goodyear's slow motion pictures of road tests.

Certain obvious conclusions can be drawn :-

1. If the wheels can be fastened to the car in such a way that any possible rotary oscillation of the wheel pair in a transverse vertical plane is limited to low frequencies (for instance of the order of 60 to 70 cycles a minute ) then :-

first : wheel hop frequencies of the order of 600 a minute will not excite such oscillations;

second : a self-energised coupling of these rotary oscillations with a wobble frequency of about 400 a minute is practically impossible. This is one of the chief arguments behind independent suspension.

2. But note that although in independent suspension the car as a whole may rock at 60 a minute it is quite probable that if the frame is not stiff enough the body frame assembly may have a torsional frequency within its own structure of 400 to 600 a minute. In such a case bad shimmy is certain.

3. Cambered front wheels promote shimmy. Soft front tires promote shimmy (because of greater precession torque due to camber and greater torsionally rigidity of the tire when under inflated).

4. Greater caster promotes shimmy (because of greater precession torque due to slip angle).

5. Increased toe-in promotes shimmy (because of greater precession due to slip angle in the earlier half of the wobble than in the later half).

6. Unbalanced wheels excite shimmy, but if conditions are right shimmy may be 'detonated' by a wave in the road, even when the wheels are in perfect balance.

7. True shimmy is impossible in a rear axle in which the wheels cannot 'precess'.

8. Shimmy does not depend on the coupling of two wheels when there is any form of flexible connection between
  
  


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