Chassis

The chassis of a motorcycle is typically made from welded aluminium or steel (or an alloy) struts, with the rear suspension being an integral component in the design.

Some motorcycles include the engine as a load-bearing (or stressed) member; this has been used all through bike history but is now becoming more common.

The fuel tank is usually mounted above the engine. This tank is generally made of stamped, brazed or welded sheet metal, or blow molded high-density polyethylene. At least one motorcycle manufacturer (Buell) offer models that use a hollow frame as the fuel tank, and various manufacturers offer designs which use part of the frame as an oil reservoir. The wheel rims are usually steel (generally with steel spokes and an aluminium hub) or 'mag' type sandcast aluminum. Performance racing motorcycles often use carbon-fiber wheels, but the expense of these wheels is prohibitively high for general usage.

A plastic or fiberglass shell, known as a fairing, is placed over the frame in some models to shield the rider from the wind. Drag is the major factor that limits motorcycle speed, as it increases at the cube of the velocity. Despite the streamlined appearance of new performance motorcycles, there is still virtually no aerodynamic technology included in the design, and motorcycles still effectively push their way through the atmosphere with brute force. This is generally due to the fact that no designs have been discovered that can improve aerodynamic performance without unacceptably compromising the rider's ability to control the machine. In the absence of a fairing or windshield, a phenomenon known as the windsock effect occurs at speeds above 100 km/h, where the rider becomes a major source of drag and is pushed back from the handlebars, tiring the rider.

Chassis stability

A good motorcycle chassis has no stability problems.

There could be three kinds of stability problems with motorbikes:

Capsizing is well known in low speeds, and easy to overcome by going a bit faster.
Wobbling (AVI movie) is a high frequency (7-9 Hz) oscillation of the front wheel. It is often relatively harmless but annoying (and quite frightening if previously inexperienced). It can appear at moderate speeds.
Weaving (AVI movie) is a low frequency (2-3 Hz) oscillation of the whole vehicle. It can become unstable at higher speeds with fatal results.
Based on The control and stability analysis of two-wheeled road vehicles:

The weave oscillations damp out once the rider reduced the roll angle.
Tyre (tire) characteristics and inflation pressures are important variables in the behaviour of the motorcycle at high speeds.
From a stability point of view it is desirable to make the lateral stiffness as large as possible, with the possibility of an optimum value for the torsional stiffness of the rear frame.
Common levels of lateral stiffness at the wheel spindle deteriorates the wobble mode damping substantially with significant changes in the wobble frequency as well, and slight reduction in the weave mode damping at high speeds.
Lateral distortion should be opposed as much as possible by locating the front fork torsional axis as low as possible.
The largest contribution to the weave damping came from the cornering and camber stiffnesses and relaxation length of the rear tyre and not so much from the same parameters of the front tyre.
Amongst others, stiff frames, a long wheelbase, a long trail and a flat steering head angle were found to increase weave mode damping.
Degraded damping of the rear suspension, rear loading and increased speed amplifies cornering weave tendencies.
Rear load assemblies with appropriate stiffness and damping were successful in damping out weave and wobble oscillations.

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