Motorcycle Shimmy Phenomenon - Expert Article

In this article, Motorcycle Engineer Roland Hoover provides an overview of the motorcycle “shimmy” condition and discusses the factors that contribute to motorcycle crashes.

He examines the high-speed wobble, shimmy, tank slapper, and the tail-wagging phenomenon in motorcycles while addressing the standard of care for motorcycle manufacturers, service dealers, part suppliers, mechanics, and motorcycle owners within the context of rider safety.

Motorcycle Shimmy Phenomenon - Expert Article

Motorcycles are designed and constructed to be inherently stable on two inline wheels. The gyroscopic effect of the wheels and engine creates this stability. All tires, even motorcycle tires, will deflect laterally in response to the variations in road surfaces that can vary from cracks and seams to large rolling hills. When this lateral deflection occurs, the tire will typically respond in a controlled or damped motion. There are times when the tire will begin to vibrate or oscillate. This unwanted lateral or rotational oscillation condition of the front or rear tire can develop into a “shimmy” like vibration with grave consequences. Bicycles, motorcycles, cars, trucks, buses, and even airplanes can exhibit a shimmy vibration; however, the dynamics of motorcycles make them more susceptible to a loss of control and collision due to shimmy.

“High-speed wobble,” “shimmy,” “tank slapper,” and “tail wagging” are all terms that describe the same phenomenon happening where the tire and pavement meet. In this discussion, the focus will be on the shimmy phenomenon that occurs while riding in a straight or nearly straight path with no heavy braking or acceleration. Shimmy is a term that originates from a dance move, and much like the dance, when a shimmy occurs on a motorcycle the tire no longer rolls along a straight line and the contact path of the tire moves from side to side while rolling causing the motorcycle to shake. Typically, this vibration initiates only within a specific speed range and will die out as the speed is increased or decreased.

Rarely do you find a perfectly glass-like road to ride on; motorcycles are designed to absorb bumps from the road in a controlled manner. As illustrated by the diagram below, the motorcycle takes inputs from the ground as points of excitation, then quickly absorbs and dampens out the motion as seen in the blue arrows. In this example, the unstable region represents the area where a motorcycle can develop a large shimmy oscillation. In motorcycles that have good ride characteristics, the unstable region is very small to non-existent. However, when the motorcycle possesses a condition that triggers the self-excitation of shimmy, this unstable region is large, noticeable, and can cause a loss of control. When a motorcycle rolls over a bump, hill, or wave motion in the pavement while within the critical speed range, the minor external excitation can initiate the shimmy oscillation, which will grow in magnitude and remain until the motorcycle is sped up or slowed down, as shown by the red lines on the diagram.

External & Internal Factors Contributing to Shimmy

For shimmy to occur in a motorcycle, it must be initiated by an external factor (environmental or road conditions), and then allowed to worsen by internal factors.

EXTERNAL FACTORS

Road Surface: At critical speeds, multiple evenly spaced roadway seams can impart a harmonic frequency into the motorcycle that, when combined with a motorcycle defect, can initiate the shimmy. Other instantaneous objects experienced in the roadway such as potholes, road debris, and even longitudinal grooves or paving unevenness that may visually appear as minor can be enough to initiate a shimmy if other defect conditions are present.

Environment: A sudden crosswind or wind from larger vehicles can cause the motorcycle to drift and possibly cause a sudden change to the contact patch, resulting in lateral forces exerted on the tire that may be enough to trigger a shimmy.

INTERNAL FACTORS

Tires: Defective tires can influence the triggering of a shimmy, and can be the source of the external and internal factors that cause a shimmy. Tire construction, storage, age, pressure, and mounting are all considered when evaluating shimmy.

Wheels: Many motorcycles are equipped with wire spoke wheels, improperly assembled or maintained wire spoke wheels can be out of round or centered improperly. Both wire wheels and aluminum wheels can be deformed allowing a shimmy to develop.

Improper wheel and tire balance, or lack of wheel weights to balance a tire will excite a low amplitude vibration found at all travel speeds, (unlike the shimmy), and is typically an annoying harmless vibration.

Vehicle Compliance: Compliance is the relative movement between the physical components that make up all motorcycles. Manufacturers design in space or tolerance between parts to ease assembly and design some flexibility between components such as using bushings to reduce small amplitude vibration that improves rider comfort. The compliance between components on a motorcycle act as a free sliding or spring-like motion that can directly contribute to the shimmy motion, provided that it is initiated by an external factor.

Vehicle Assembly: Manufacturing or service assembly of several areas of the motorcycle can become an internal factor in the shimmy phenomenon. Component systems such as the steering head bearings, which if not properly assembled and tightened, can be too tight or too loose and become the root cause of the shimmy.

Vehicle Design: The geometry and kinematics of the motorcycle can be a non-physical defect in the motorcycle’s resistance to shimmy. One of the key components is the trail measurement of the front wheel, which is the distance between the axle and steering axis projected on the ground. Many changes can influence trail such as raising or lowing the suspension or installing a different wheel size for cosmetic reasons. Even changing the position of the forks within the triple trees to achieve an improvement to the turning ability of the motorcycle can cause the trail to change into an unstable region.

Aerodynamics: while traveling at higher speeds or into a headwind, the motorcycle can experience aerodynamic lift, lightening the forces through the suspension and contact patch, changing the effective trail of the steering, which allows the motorcycle handling to become unstable and shimmy.

The shimmy requires an input and a condition or defect within the motorcycle to initiate a large magnitude vibration. The external and internal conditions that are required to initiate the shimmy motion that occurs within a range of speed can sometimes be difficult to replicate because the unstable region of shimmy motion is unique to each individual motorcycle.

EXAMPLE

Imagine a motorcyclist riding their bike on the smooth sweeping turns of a mountain road. After getting their bike tuned up by a well-known and reputable dealer, the motorcyclist notices the steering movement is a little heavier than normal, but assumes that nothing is wrong because the dealer replaced the steering head bearings and should have properly installed them. While leaning through sweeping turns, the bike responds in a rather jerky or notchy motion to the rider’s counter steering. As they exit the corner and begin to travel straight upright, they drive over a few gravel pebbles in the road. On this occasion, the pebbles cause a slight sideslip, (an instantaneous change in lateral force). This then triggers the front wheel to begin whipping side to side, the rider’s hands hitting the tank on both sides, the force and speed of the steering oscillation causes the rider to lose their grip on the handlebars. The back and forth motion renders the rider unable to grab back ahold of the handlebars before reaching the next turn in the road. The only option is to jab the rear brake and hope the motorcycle settles down before the turn, but instead, the front tire gives out and both the rider and the bike are sliding off the road.

This is the case of over tightened steering bearings. The pebbles in the road were not an uncommon condition, but caused an excitation, which when combined with the specific speed, and having an internal condition that allows the shimmy to escalate, resulted in an unexpected major crash. Smooth, fluid-like motion is required for the motorcycle to steer properly. The over tightened steering bearings were found to have caused roughness and sticky spots in the steering movement, the lack of fluidity allowed the tire to spring laterally causing an unstable condition. Ultimately, the dealer technician did not follow the specified service procedure for tightening the steering head bearings after replacement.

VEHICLE ENGINEERING INVESTIGATIONS

When a crash incident has occurred, an expert specializing in motorcycle dynamics and tire performance may be required to discern if the motorcycle exhibited a condition that would initiate a shimmy, and if the shimmy vibration was a contributing cause to the crash. The vehicle experts at Robson Forensic examine crash sites, use field measurements, scrutinize crash scene photos, and review vehicle design documents to determine if vehicle or environmental factors or road conditions contributed to the cause of motorcycle crashes.

For more information, contact the author of this article or submit an inquiry through our website.

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