A bike remains upright when it is steered so that the ground reaction forces exactly balance all the other internal and external forces it experiences, such as gravitational if leaning, inertial or centrifugal if in a turn, gyroscopic if being steered, and aerodynamic if in a crosswind.[26] 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. Even when staying relatively motionless, a rider can balance a bike by the same principle. While performing a track stand, the rider can keep the line between the two contact patches under the combined center of mass by steering the front wheel to one side or the other and then moving forward and backward slightly to move the front contact patch from side to side as necessary. Forward motion can be generated simply by pedaling. Backwards motion can be generated the same way on a fixed-gear bicycle. Otherwise, the rider can take advantage of an opportune slope of the pavement or lurch the upper body backwards while the brakes are momentarily engaged.[28] If the steering of a bike is locked, it becomes virtually impossible to balance while riding. On the other hand, if the gyroscopic effect of rotating bike wheels is cancelled by adding counter-rotating wheels, it is still easy to balance while riding.[5][6] One other way that a bike can be balanced, with or without locked steering, is by applying appropriate torques between the bike and rider similar to the way a gymnast can swing up from hanging straight down on uneven parallel bars, a person can start swinging on a swing from rest by pumping their legs, or

a double inverted pendulum can be controlled with an actuator only at the elbow. The track stand[1] or standstill[2] is a technique that bicycle riders can use to maintain balance while their bicycle remains stationary or moves only minimal distances. The technique originated in track cycling and is now used by other types of cyclists wishing to stop for a short time without putting a foot on the ground, such as bike commuters at stop signs. To perform a track stand, a cyclist holds the cranks in an approximately horizontal position with the front wheel steered to the left or right, and pedals forward, and back in the case of a fixed-gear bicycle, which the steered front wheel converts into a side-to-side motion. A fixed-gear bicycle (or fixed-wheel bicycle, popularly known as a fixie) is a bicycle that has a drivetrain with no freewheel mechanism. The freewheel was developed early in the history of bicycle design but the fixed-gear bicycle remained the standard track racing design. More recently the 'fixie' has become a popular alternative among mainly urban cyclists, offering the advantages of simplicity compared with the standard multi-geared bicycle. Most bicycles incorporate a freewheel to allow the pedals to remain stationary while the bicycle is in motion, so that the rider can coast, i.e., ride without pedalling using the forward or downhill momentum of bike and rider. A fixed-gear drivetrain has the drive sprocket (or cog) threaded or bolted directly to the hub of the back wheel, so that the rider cannot stop pedalling. When the rear wheel turns, the pedals turn in the same direction.[1] This allows a cyclist to apply a weak braking force without using a brake, by resisting the rotation of the cranks. It also makes it possible to ride backwards although learning to do so is much more difficult than riding forwards.