Motorcycle stability issues

What is a Motorcycle Stability Issue?

A Motorcycle Stability Issue can be defined as a discomfort of the rider and/or passenger while the vehicle is in normal use. This discomfort may appear randomly or constantly and can be a cyclic shaking, shimmy, or wobble and can affect the complete motorcycle or only parts of it.

Shimmy or Wobble describes a rapid (4-10 Hz) oscillation of primarily just the front end (front wheel, fork, and handlebars) at low speeds (60-25 mph). It occurs mostly during deceleration and the amplitude of the oscillation is usually decreasing with the speed of the motorcycle.

Shimmy usually doesn’t lead to a loss of control but can get worse if a firm grip is not maintained on the handlebars.

Often Shimmy / Wobble is described as a weave, but Shimmy only affects the front end and disappears while accelerating to higher speeds – this is the opposite to the so-called weave, see next topic.

Contributing factors for (Low Speed) Shimmy / Wobble:

• Inflation pressure
• Wear condition of tyre, especially of front tyre
• Imbalance / run-out of front tyre
• Different front/rear tyres
• Rim irregularities (esp. spoked rims)
• Worn or misadjusted steering head bearings
• Improperly maintained front fork
• Mass distribution of the load (passenger, luggage…)

With the so-called High-Speed Weave, the steering system (with fork and front wheel) and the motorcycle rear (with frame, engine and rear wheel) oscillates phase-shifted around the steering axle. At the same time, the machine tilts around the vehicle's longitudinal and vertical axle. It manifests itself as an undulating movement of the vehicle, between 2 and 4 cycles per second (2-4 Hz), followed by a "S" shaped trajectory on the road. 

The phenomenon usually appears from 75 mph depending on the type of motorcycle and environmental conditions (wind can cause High-Speed Weave). The weave can start after a prolonged period of riding at a constant speed and becomes more severe as the speed increases. It can only be stopped by reducing the throttle, otherwise the amplitude increases with increased speed.

Contributing factors at the motorcycle for High-Speed Weave:

• Inflation pressure of the tyres
• Wear condition of the tyres (especially worn rear tyre with new front tyre)
• Additional attachments (top case, side bags, superbike handlebar, wind shield...)
• Mass distribution of the load (passenger, luggage…)
• Rear wheel alignment (not aligned with front wheel)
• Imbalance / run-out of tyres (especially rear tyre)
• Loose swing arm and wheel bearings
• Too soft / damaged rear wheel dampers
• Environmental conditions (especially wind)
• Loose clothes of the rider
• Improperly maintained steering head bearings and fork 

Kick-back is, when the fork, including front wheel and handlebar, jolts suddenly around the steering axle. This effect is independent from riding speed and is triggered when the front tyre lifts off and then returns at an angle, such as when accelerating strongly from bends or riding over a bump in a curve. Depending on the speed and the angle of contact, the front tyre suddenly builds up lateral forces, which are noticeable in the handlebars as light twitch or a strong jolt. 

Usually, a kick-back includes one or two alternating movements of the handlebars around the steering axle and then the chassis calms down very quickly.

Contributing factors at the motorcycle for Kick-back:

• Very sporty motorcycle geometry
• Damping adjustment too hard
• Mass distribution of load (too little load on the front)
• Tyre pressure

A lack of directional stability describes various phenomena which do not usually repeat themselves cyclically or occur continuously, but which affect the riding characteristics of the motorcycle in certain riding situations, e.g.:

• Poor accuracy of steering movements
• Lack of cornering stability (especially in tight corners)
• High inertia when turning in / rolling over in curves
• Poor directional stability on straight roads

Contributing factors at the motorcycle for riding stability issues:

• Inflation pressure of the tyres
• New tyres with high tread depth / block profile (especially Dual Purpose/Cross tyres)
• Wear conditions of the tyres (especially spotty wear, heel and toe wear, centre wear)
• Imbalance / run-out of tyres
• Loose swing arm and wheel bearings
• Improperly maintained steering head bearings and fork
• Additional attachments (top case, side bags, superbike handlebar, wind shield...)
• Rear wheel alignment (not aligned with front wheel)

Vibration creates a discomfort while riding in various common riding conditions and can be noticed by a slightly shaking handlebar, seat or footrests. The movement direction caused by the vibration may be vertical, lateral or even mixed and can affect the front or rear part of the motorcycle.

Mostly, vibrations come from rotating or oscillating masses that are out of balance. It may also be caused by components and parts that are not fixed properly or have too much play. Therefore, poor maintenance of the motorcycle often leads to vibration problems.

Vibrations usually occurs at an unspecific speed - depending on the natural frequency of the affected system - and its amplitude remains stable or decreases over rising velocity.

There are various conditions that can cause vehicle vibrations. Typical sources are:

• Tyre wear shape (uneven wear)
• Static/dynamic Imbalance / Run-Out of tyres
• Tyre flat spot due to long vehicle parking period
• Tyre to rim mounting
• Rim irregularities (esp. spoked rims)
• Improperly maintained steering head bearings and fork
• Worn out chain or belt drive
• Worn wheel bearings
• Improperly maintained engine