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Q I have been advised that balancing the bogie wheels of
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my multi-wheelers is more important than balancing the front wheels. Do you agree with this advice?
/A It is debatable whether balancing the wheels of a bogie is more important than balancing the front wheels, but both can be recommended even if out-of-balance is not indicated by the way the vehicle handles on the road.
Front-wheel wobble is generally a function of both wheels being out of balance and occurs at a critical road speed, which may be higher than the normal maximum speed of the vehicle. At this critical speed, the frequency of vibration of the axle system caused by wheel wobble matches the flap frequency of the steering system and the resulting resonance amplifies the vibration of both systems.
At non-critical speeds the effect of outof-balance on the steering is slight or nonapparent to the driver although it may cause uneven tyre wear.
When the "heavy points" of diagonally opposite wheels of a tandem-axle bogie act in opposite directions, the resultant vibration will' tend to twist the chassis if the vehicle is unladen. This twisting may be severe if the vibration frequency matches the natural torsional frequency of the frame. This would normally occur at a relatively low speed and could lead to instability on the road. As with the front wheels, out-of-balance will tend to cause uneven tyre wear at any speed.
A heavy point at one section of a wheel/tyre assembly can be balanced by fitting a balance weight to the wheel rim on the opposite side. The wheel is than balanced statically but not dynamically and although the wheel will not tend to hop it may tend to rock on its bearings. This is not normally serious except in the case of a very high-speed vehicle and static balancing is usually adequate.
To rectify dynamic unbalance, balance weights have to be fitted to the wheel rim in such a way that any heavy point is balanced by a weight in the same plane. More specialized balancing equipment is required to measure dynamic out-of-balance forces.