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Automobile Engineering

Static Wheel Balance Theory

Introduction:

When a tire-and-wheel assembly has proper static balance, it has the weight equally distributed around its axis of rotation, and gravity will not force it to rotate from its rest position.

 

Static Unbalance  

  • If a vehicle is raised off the floor and a wheel is rotated in 120° intervals, a statically balanced wheel will remain stationary at each interval.
  • When a wheel and tire are statically unbalanced, the tire has a heavy portion at one location.
  • The force of gravity acting on this heavy portion will cause the wheel to rotate until the heavy portion is located near the bottom of the tire

 

Results of Static Unbalance 

  • Centrifugal force may be defined as the force that tends to move a rotating mass away from its axis of rotation.
  • A tire and wheel are subjected to very strong acceleration and deceleration forces when a vehicle is in motion.
  • The heavy portion of a statically unbalanced wheel is influenced by centrifugal force. This influence attempts to move the heavy spot on a tangent line away from the wheel axis.
  • This action tends to lift the wheel assembly off the road surface. The wheel-lifting action caused by static unbalance may be referred to as wheel tramp.
  • When the wheel and tire move downward as the heavy spot decelerates, the tire strikes the road surface with a pounding action. This repeated slipping and pounding action causes severe tire scuffing and cupping.
  • The vertical wheel motion from static unbalance is transferred to the suspension system and then absorbed by the chassis and body. This action causes rapid wear on suspension and steering components. 
  • The wheel tramp action resulting from static unbalance is also transmitted to the passenger compartment, which causes passenger discomfort and driver fatigue.
  • When a vehicle is traveling at normal highway cruising speed, the average wheel speed is 850 revolutions per minute (rpm).

 

  • A statically unbalanced tire-and-wheel assembly is an uncontrolled mass of weight in motion.