http://en.wikipedia.org/wiki/Countersteering
Countersteering is the name given to the counter-intuitive technique used by
cyclists and
motorcyclists to turn corners. It is the
only way a rider can cause the lean that a single-track
vehicle must have in order to negotiate a turn successfully.
It is important to distinguish between
countersteering as a physical phenomenon and
countersteering as a rider technique for initiating a lean (the usual interpretation of the term). The physical phenomenon always occurs, because there is no other way 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.
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 so a turn by shifting body weight.
[1] Documented physical experimentation shows that on heavy bikes (many
motorcycles) shifting body weight is ineffective at initiating leans and so turns.
[2]
It is also important to distinguish the
momentary steering torque and steering angle necessary to initiate the lean required for a given turn from the
sustained steering torque and steering angle necessary to maintain a constant radius and lean angle until it is time to exit the turn. The initial, momentary steer torque and angle are both opposite the desired turn direction. The sustained steer angle is in the same direction as the turn. The sustained steer torque required to maintain that steer angle is either with or opposite the turn direction depending on forward speed, bike geometry, and combined bike and rider mass distribution.