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The lowsider or lowside is a type of motorcycle crash usually occurring in a turn and caused by a loss of grip between the tires and the road surface. It is most often caused by either locking a wheel due to excessive braking or application of excessive power out of or through the turn. It may also be caused by slippery or loose material (such as oil, water, dirt or gravel) on the road surface.
Behaviour leading to a lowsider and physical explanation 
All forces occurring between the motorcycle and the road (such as accelerating, decelerating and steering) are transmitted by friction occurring in the contact patch. There is a limited amount of force the contact patch can transmit before the tire begins to slide. Typically, the forces reduce slightly upon sliding.
When travelling in a curve, the tires provide the centripetal force needed for the acceleration towards the center of the curve. The capsizing moment provided by the motorcycle lean into the corner is in moment equilibrium with the centripetal forces at the ground when all is working correctly. If, having reached a given lean angle, the centripetal forces are reduced then the motorcycle increases its angle of lean until it touches the road surface, usually unseating the rider in the process.
Lowsides may be caused by exceeding the lateral friction limit - by leaning too far - or by exceeding the combined lateral/longitudinal friction limit - by braking too heavily in the curve, typically on the front wheel but not exclusively.
The name derives from the fact that it is usually the inward side the motorcycle will fall on (or the side that points downward in a curve, the low side).
Riders are usually advised to do a lowsider rather than a highsider if neither can be avoided. The lowsider has the advantage of the motorcycle sliding before the rider, thus not threatening to crush them. Also, a lowsider tends to send the rider sliding across the road rather than slamming them into the road, thus reducing the chances of serious trauma.
See also 
- "Tire and Vehicle Dynamics", Hans B Pacejka