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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).
Page discussing the preference for low frequency fight and engine tuning to reduce wheel wobble and shimmy.

Identifier  ExFiles\Box 170\3\  img017
Date  25th February 1936 guessed
  
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(6) Preference for low frequency fight.

The conclusion that it is preferable to seek the low frequency fight condition dates from at least twelve years ago. Rolls-Royce experimented with springs in drag links and found that the steadiest steering was given by reaching conditions where low speed wobble at 18 m.p.h. definitely occurred and then putting in enough friction at the kingpin and steering joints to just damp out the wobble.

The shimmy shackle was only another way of getting the same result with less additional flexibility in the steering, the masses which were permitted to oscillate by the added flexibility being greater.

Some of the difficulties met with in getting rid of wheel fight when straddle-mounted rocker shafts are fitted in the steering gear may be due to the same cause; the overall rigidity of the steering mechanism is increased, and the natural frequency of wheel wobble is therefore raised.

On Dubonnet suspensions it is apparent that for a given occurrence of wheel fight more swing axle effect may be produced by the bent arm than may be produced by "splaying" the arms on a wishbone suspension. Rolls-Royce is the only concern which uses on a wishbone suspension a swing axle effect comparable to our 15° bend on the experimental T.T. suspension, and they have had considerable trouble with wheel fight. The reason is probably the greater tendency of the Dubonnet suspensions to develop the low-frequency form of wheel fight which is not affected by gyroscopic action, and the correspondingly less tendency to develop the high frequency form.

(7) Engine Tuning.

(a) Low Frequency.

We have known for some time that it was possible to stop low speed wobble, and greatly reduce low frequency wheel fight by "tuning" the engine mount so that the engine movement on its rubber mounts acted as a harmonic balancer. In other words, the energy which would have caused the wobble is now dissipated in rocking the engine on its mounts. This was interesting because it was contrary to the generally accepted idea that softer engine mounts have necessarily increased wheel fight.
  
  


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