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Costly misconception

Over the past 13 years I have written many articles on the subject of balancing tires and complete wheel assemblies. What is also important to talk about now, is the shock absorber.

There is a false belief that replacing shock absorbers will solve vibrations cause by out-of-balance wheel assemblies. The shock absorber's function is to stabilize the spring suspension and dampen shock. After hitting a bump on the road the spring suspension would continue for four or five re-bounces before gradually settling down. A properly working shock absorber will stabilize the spring in one or two re-bounces. This is why it is important to have good working shocks on trucks for the overall control and safety of the vehicle.

Out-of-balance wheel assemblies produce the same type of effect at every revolution as a bump on the road, rendering the shock absorbers almost useless to prevent the out-of-balance wheel assemblies' (vibrations) axle hop.

What most people have problems understanding are the multiplying forces at play in an out-of-balance rotating wheel assembly. For example, 6oz out of balance in a truck tire at 60mph multiplies to 60 lbs through centrifugal force alone. Those 60 lbs will multiply again when the wheel assembly revolutions reach the suspension resonant frequency. For example, a wheel turning at 600rpm and a 10Hz (600 rebounds per minute) resonant suspension frequency will multiply that 60 lbs to a force great enough to cause an axle hop at every revolution of the wheel assembly in a 10,000 lbs axle. This is felt as a vibration.

This out-of-balance wheel assembly axle hop can be amplified after hitting a bump. This happens when the heavy spot of the wheel assembly is at the top end of the turn and turns down to hits the pavement, at the same time as the suspension and the vehicle gravity force drive it down. This creates a rebound at the resonated (rebound) frequency of the suspension. The heavy spot will keep that axle hop going forever, unless the vehicle speed (wheel rpm) is reduced or increased below or past the resonate (rebound) frequency of the suspension. The shock absorber can dampen that shock wave but can’t stop it. As a result the oil in the shock heats up from being overworked and the oil viscosity drops, allowing for freer movement of the oil, or less shock dampening. This is a design function of shock absorbers to prevent overheating.

There is only one solution to this problem, which is to balance all the complete wheel assemblies. Balancing all complete wheel assemblies has been proven to improve fuel economy (Auburn University (PAVE)), increase tire life expectancy as much as doubling, and also improving the life expectancy of the casing (Brazilian Federal University). There is also good solid reason to believe that out-of-balance truck tires are shortening the life of pavement and bridges in the same manner that they shorten tire life.

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