Highsider
From Wikipedia, the free encyclopedia
The highsider is a type of motorcycle accident usually occurring in a curve which may be caused by a locked wheel due to excessive braking or more commonly by applying too much throttle when exiting a corner causing the rear tire to lose traction.
[edit] Behaviour leading to a highsider 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 lose contact and slide.
When going through a curve on a motorcycle, the driver has to apply force on the motorcycle in order to make it alter its direction of travel. This centripetal force is transferred from the road to the motorcycle through the contact patch and is directed at a right angle to the path of travel. Applying too much throttle will increase the stress in the contact patch, because now there is an additional force which also has to be transmitted through the contact patch, this time parallel to the direction of travel. This additional amount of force may cause the tire to slide. It is during this slide that reflex will cause the rider to rapidly shut the throttle & cause the rear tire to lock. This same condition can be caused by applying too much rear brake while cornering. If braking is applied equally to both tires, the rear tire will begin to slide first because braking causes a weight shift towards the front tire, improving its contact with the road while lessening the rear tire's grip.
Other reasons for the tire losing grip may be too little tire profile and/or bad road conditions like gravel, water, snow etc.
Once a tire slips in a curve, it will move outwards under the motorcycle and cause the cycle to lay down in the direction the driver is already leaning to counteract the centrifugal force and lead to a lowsider.
The usual manoeuvre to get a locked tire to stop slipping is to make it turn again by reducing the amount of force applied by the brakes (thus reducing the amount of force acting on the contact patch and reestablishing the grip on the road). However, this is critical at this stage. If the tire suddenly regains traction while the motorcycle is moving sideways, this is similar to hitting an obstacle in the road: the tire will stop its sideways movement causing the motorcycle to suddenly jerk into an upright position (and beyond). This movement can easily cause the rider to be thrown off.
The name derives from the fact that it is usually the outward side the motorcycle will fall on (or the side that points upward in a curve, the high side).
Drivers are usually advised to do a lowsider rather than a highsider if neither can be avoided. The highsider has the additional disadvantage of the driver often being catapulted into the air by the sudden jerking motion of the motorbike & the increased possibility of the motorbike sliding behind the driver and threatening to crush him.
External link: http://www.robspeed.com/highside.htm
