Countersteering is the technique used by single-track vehicle operators, such as cyclists and motorcyclists, to initiate a turn toward a given direction by momentarily steering counter to the desired direction ("steer left to turn right"). To negotiate a turn successfully, the combined center of mass of the rider and the single-track vehicle must first be leaned in the direction of the turn, and steering briefly in the opposite direction causes that lean.[1] This technique does not apply to conventional multiple-tracked vehicles such as trikes or sidecar equipped bicycles and motorcycles. The scientific literature does not provide a clear and comprehensive definition of countersteering. In fact, "a proper distinction between steer torque and steer angle ... is not always made."[2] It is important to distinguish the steering torque and steering angle necessary to establish the lean required for a given turn from the sustained steer torque and steer angle necessary to maintain a constant turn radius and lean angle until it is time to exit the turn. The initial steer torque and steer angle are both opposite the desired turn direction. The sustained steer angle is usually in the same direction as the turn, but may remain opposite to the direction of the turn, especially at high speeds.[3] The sustained steer torque required to maintain that steer angle is usually opposite the turn direction.[4] (See the graphs to the right.) The actual magnitude and orientation of both the sustained steer angle and sustained steer torque of a particular bike n a particular turn depend on forward speed, bike geometry, tire properties, and combined bike and rider mass distribution. It is also important to distinguish between countersteering as a physical phenomenon and countersteering as a conscious rider technique for initiating a lean (the usual interpretation of the term). The physical phenomenon usually occurs, because it is the easiest to cause the bike and rider to lean short of some outside influence such as an opportune side wind, although at low speeds it can be lost or hidden in the minute corrections made to maintain balance. Another way to cause the bike and rider to lean 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.[5] At the same time, the rider technique of applying pressure to the handlebars to initiate a lean is not always necessary, since, on a sufficiently light bike (especially a bicycle), the rider can initiate a lean and turn by shifting body weight, called counter-lean by some authors.[6][7][8] Documented physical experimentation shows that on heavy bikes (many motorcycles) shifting body weight is less effective at initiating leans.[9] This technique works by employing the natural tendency of most bikes to steer towards the direction they are leaned and is different from the acrobatic maneuver described above.