Dynamics

Main article: Bicycle and motorcycle dynamics

A bicycle stays upright by being steered so as to keep its centre of gravity over its wheels. This steering is usually provided by the rider, but under certain conditions may be provided by the bicycle itself.

A bicycle must lean in order to turn.

This lean is induced by a method known as countersteering, which can be performed by the rider turning the handlebars directly with the hands or indirectly by leaning the bicycle.

Short-wheelbase or tall bicycles, when braking, can generate enough stopping force at the front wheel in order to flip longitudinally.

This action, especially if performed on purpose, is known as a stoppie, endo or front wheelie.

Bicycle and motorcycle dynamics is the science of the motion of bicycles and motorcycles, in entirety or in parts, due to the forces acting on them during balancing, steering, braking, and suspension. Experimentation and mathematical analysis have shown that a bike stays upright when it is steered to keep its center of mass over its wheels. This steering is usually supplied by a rider, or in certain circumstances, by the bike itself. Long-standing hypotheses and claims that gyroscopic effect is the main stabilizing force have been refuted.[1][2] While remaining upright may be the primary goal of beginning riders, a bike must lean in order to turn. The higher the speed or smaller the turn radius, the more lean is required. This is necessary in order to balance centrifugal forces due to the turn with gravitational forces due to the lean. When braking, depending on the location of the combined center of mass of the bike and rider with respect to the point where the front wheel contacts the ground, bikes can either skid the front wheel or flip the bike and rider over the front wheel.

Forces

The forces that act on a bike and its components include gravity, ground reaction (usually at the front and rear wheel contact patches with both horizontal and vertical components), centrifugal (in the accelerating reference frame of the bike and rider; in a stationary, inertial reference frame this is simply inertia and not a force at all), gyroscopic (due to rotating parts such as wheels, engine, transmission, etc.), propulsive (usually at the rear wheel contact patch), torques applied between the steering mechanism (front fork, handlebars, front wheel, etc.) and rear frame, and between the rider and the rear frame, friction (between any parts that move against each other: at the tire contact patches, as mentioned above, in the drive train, between the steering mechanism and the rear frame, etc.), and aerodynamic (mostly in the form of drag, but also possibly from a crosswind).

A bike remains upright when it is steered so that the ground reaction forces exactly balance all the other forces it experiences such as gravitational, inertial or centrifugal if in a turn, and aerodynamic if in a crosswind.[3] Steering may be supplied by a rider or, under certain circumstances, by the bike itself. This self-stability is generated by a combination of several effects that depend on the geometry, mass distribution, and forward speed of the bike. Tires, suspension, steering damping, and frame flex can also influence it, especially in motorcycles.

If the steering of a bike is locked, it becomes virtually impossible to ride. Instead, if just the gyroscopic effect of rotating bike wheels is cancelled by adding counter-rotating wheels, it can still be easily ridden.

Steering inputs counter lean to maintain balance. At high speeds, the input required to return the bike to upright only needs to be small; a much greater input is required to maintain balance at low speed. As such, it is easier to maintain balance at high speeds.