Chassis stability

Chassis stability
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. Motorcyclists on a Honda CB600F Hornet.SuspensionModern designs have the two wheels of a motorcycle connected to the chassis by a suspension arrangement, however 'chopper' style motorcycles often elect to forego rear suspension ("rigid frame").
The front suspension generally consists of sliding steel tubes with long springs inside called forks which use hydraulic fluid for damping shock absorbers. The front fork is the most critical part of a motorcycle. The angle of rake determines how controllable the steering is. The rake should be chosen so that precessive force from countersteer and body steering slightly overbalance the leaning forces from the weight of the bike, at a speed near the running speed of a person. This is the speed at which the rider's feet can no longer be safely used to balance the bike.
The rear suspension supports the swingarm, which is attached via the swingarm pivot bolt to the frame and holds the axle of the rear wheel. The rear suspension can consist of several shock arrangements:
dual shocks, which are placed at the far ends of the swingarm traditional monoshock, which is placed at the front of the swingarm, above the swingarm pivot bolt 'Softail' style monoshock, which is mounted horizontally in front of the swingarm, below the swingarm pivot bolt
BrakesThere are generally two independent brakes on a motorcycle, one set on the front wheel, controlled by the right hand lever, and one on the rear controlled by the right foot. In older motorcycles the rear may be on the left foot. However, several models have "linked brakes" which apply both at the same time, although one more than the other. The front brake is generally much more powerful than the rear as roughly 2/3rds of stopping power can come from the front brake when properly applied and in some cases 100% depending on the model of motorcycle and operator; rear wheels can generally lock and skid much more easily than the front due to weight distribution dynamics. Brakes can either be drum or disc based, with disc brakes being more common on large, modern or expensive motorcycles for their far superior stopping power, particularly in wet conditions. There are many brake performance enhancing aftermarket parts available for most motorcycles including brake pads of varying compounds and steel braided brake lines. Some manufacturers have created Antilock braking systems (ABS).
In virtually all cases, 70% to 90% of total braking force should be applied by the front brake when operated on a hard surface such as tarmac, with the remainder being simultaneously applied to the rear brake. Riders fear that aggressive use of the front brake will stop rotation of the tire and cause loss of control, or a skid, and therefore often fail to use the front brake to its full potential. Another common misconception is that application of the rear brake will cause motorcycle instability. The phenomenon known as a "stoppie" may only be achieved if the front brake is used aggressively with no application of the rear brake; if sufficient force is applied to the front brake, the rear of the motorcycle chassis will lift off the roadway, while the bike continues to move forward on the still-rotating front wheel. This is a highly skilled (and generally illegal) maneuver which requires practice to perfect. Trailbraking is a term used to describe carrying the braking action of a vehicle past the turn entry, allowing the rider to adjust speed all the way through a turn to the apex. Another variation of brake use can be seen at top level motorcycle roadracing and motorcross events. The technique of steering the motorcycle in a high speed turn (or lower speeds on a dirt course) using the rear brake is called "backing it in" (or "turning" on dirt). Racers while hard on the front brake will feather the rear brake just enough to start a controlled rear slide, thus rendering a sharper turn angle. Note: This technique is not recommended for public road use.