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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).
Analysis of front axle wobble (shimmy), its relationship with wheel revolution speed, and the design challenges presented by modern components.

Identifier  ExFiles\Box 178\1\  img115
Date  13th July 1926 guessed
  
- 16 -

Every front axle when it oscillates in a criss-cross manner
wants to do so at a fixed rate. The number of criss-cross vibra-
tions or wobbles it wishes to make per minute varies with the
construction of the axle. If the following changes are made in
a front axle, the number of criss-cross vibrations which it wishes
to make per minute will be reduced.

Wt. at the ends of the axle increased.
Weak springs fitted close together.
Low pressure tyres fitted.
Balloon tyres fitted.

Whenever a car wobbles, we have found by experiment that the axle
makes one complete criss-cross vibration for every wheel revolution.
In other words, if a front axle wishes to make ten wobbles per sec.
it will wait until the car wheel is rotating at 10 revs. per sec.,
or on a 40/50 HP. car, doing 60 m.p.h., then it will start to shimmy.
This is because an out-of-balance wheel or out-of-truth tyre produces
the periodic force necessary to start the wobble off. It is clear
that if the maximum speed of the car is 60 m.p.h. and the criss-cross
period of the axle is 12 vibrations per second, the car can never
wobble because the wheel revolutions never reach 12 per second. This
is the solution from the designers point of view; to make the criss-
cross period of the axle so high that the critical wobbling speed
will be above that which the car can attain, the designer is handi-
capped by the present demand for front wheel brakes, balloon tyres,
and high speeds.

Generally speaking, the amount the front wheels turn in and
out alters inversely with the road speed at which the wobbles occur.

contd.
  
  


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