Speed wobble

Wobble, shimmy, tank-slapper,speed wobble, and even death wobble are all words and phrases used to describe a quick (4–10 Hz) oscillation of primarily just the steerable wheel(s) of a vehicle. Initially, the rest of the vehicle remains mostly unaffected, until translated into a vehicle yaw oscillation of increasing amplitude producing loss of control. Vehicles that can experience this oscillation include motorcycles and bicycles, skateboards, and in theory any vehicle with a single steering pivot point and a sufficient amount of freedom of the steered wheel, including that which exists on some light aircraft with tricycle gear where instability can occur at speeds of less than 50 mph; this does not include most automobiles. However, coil-sprung vehicles with a track bar setup such as the Jeep WJ, XJ, ZJ, TJ, and JK with both stock and after-market suspension lifts may also have this problem. The initial instability occurs mostly at high speed and is similar to that experienced by shopping cart wheels and aircraft landing gear.

Sustained oscillation has two necessary components: An underdamped second order or higher system and a positive feedback mechanism. An example of an underdamped second order system is a spring and mass system where the mass can bob up and down (oscillate) when hanging from a spring.

If shimmy can not be designed out of the system, a device known as a steering damper may be used which is essentially a notch filter designed to damp the shimmy at its known natural frequency.

Shimmy is usually associated with the deformation of (rubber) tires. However, it can also be observed in nondeformable (e.g. steel) wheels. The phenomenon can be comprehensively explained by introducing multicomponent dry friction forces, apart from the usual forces considered in the literature.

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