Definitions

over acceleration

Highsider

A highsider or highside is a type of motorcycle accident which is caused by a rear wheel gaining traction when it is not facing in the direction of travel. For a highsider to take place, the rear wheel must first lose traction and drift out of the line travel of the motorcycle. The initial traction loss may be caused by a rear locked wheel due to excessive braking or by applying too much throttle when exiting a corner.

When the slipping rear tire suddenly regains traction it is not moving in the same plane as the rest of the bike. This causes the motorcycle to straighten up so quickly the rider is almost always thrown off, even at low speed.

Highside accidents are typically caused by over-acceleration or braking in corners, but are also a result of locking the rear brake in an emergency stop while traveling straight at high speeds, such as on a highway. Chopper style motorcycles with the front brakes removed are especially prone to highsider accidents.

Avoiding a highsider

When braking, use both front and rear brakes on the motorcycle, and avoid locking the rear brake.

When exiting a corner toward a straight direction of travel, do not apply sudden acceleration inputs until the motorcycle returns to a near upright position. Lower gear selection and sharp acceleration inputs increase the likelihood of a rear tire abruptly losing traction on acceleration. Instead smoothly apply throttle as the bike becomes more upright and traction increases. If a riders experiences a rear tire slide under acceleration the best thing to do is slowly roll-on the throttle. If one reduces throttle while the bike is sliding you may regain traction suddenly resulting in the highsider. Tire warmers can increase tire grip for track days and racing. Riding conservatively until the silica in rubber compound and friction generate heat within the tire can also improve grip. Many highside incidents result from aggressive riding on tires that have not yet achieved a temperature suitable for optimal grip. Many tire manufactures add silica to the rubber compound to generate heat as the tire flexes. Temperatures of are common for race ready tires.

Technical explanation

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 rider 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 and 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 rider is already leaning to counteract the centrifugal force and lead to a lowsider.

The usual maneuver 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 side of the motorcycle that the rider will separate from. If forcibly thrown over the bike, the rider is said to have dismounted on the high side.

Riders are usually advised to do a lowsider rather than a highsider if neither can be avoided. The highsider has the additional disadvantage of the rider often being catapulted into the air by the sudden jerking motion of the motorbike and the increased possibility of the motorbike sliding behind the rider and threatening to crush him.

Because highsider accidents are so much more deadly than lowside accidents, the Motorcycle Safety Foundation recommends that if a rider locks the rear brake, it should be kept locked until the motorcycle comes to a stop. If necessary, locking the front brake to deliberately cause a lowsider is recommended.

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