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(1) In motor vehicles, an intense vibrational process that occurs in a vehicle’s steered wheels and front suspension. Steered wheels that are connected to a motor vehicle’s frame or body by an elastic suspension and a steering linkage are components of a complex vibrating system. Such wheels vibrate when a vehicle travels on a bumpy road, when the vehicle’s wheels are improperly aligned, or when the vehicle’s suspension and steering gear are kinematically mismatched.
Shimmy impairs road safety, increases the rolling resistance at the tires, and intensifies the wear of the parts of the steering gear and suspension. It also increases tire wear.
To reduce the harmful effects of shimmy, independent front suspension is widely used in present-day passenger cars. In other motor vehicles, the kinematic design of the steering gear and suspension may be improved by moving the centers of wheel vibration closer to the drag links and the hinge points of the springs. Other measures to reduce the effects of shimmy include dynamic wheel balancing and stabilization of the direction in which the wheels move when they encounter road irregularities.
(2) In airplanes, a free vibration of the nose wheel of a tricycle landing gear that occurs during taxiing, takeoff runs, and landing runs. The free vibration is manifested as a yawing of the nose wheel. It may result in the collapse of the shock strut or in an airplane crash.
Shimmy is eliminated by means of shimmy dampers. They prevent the occurrence of free vibration but do not prevent the nose wheel from being steered.