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 tyre 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 travelling 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 tyre abruptly losing traction on acceleration. Instead smoothly apply throttle as the bike becomes more upright and traction increases. If a rider experiences a rear tyre 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. Tyre warmers can increase tyre grip for track days and racing. Riding conservatively until the silica in rubber compound and friction generate heat within the tyre can also improve grip. Many highside incidents result from aggressive riding on tyres that have not yet achieved a temperature suitable for optimal grip. Many tyre manufactures add silica to the rubber compound to generate heat as the tyre flexes. Temperatures of 170 °F (77 °C) are common for race ready tyres.

Technical explanation

Forces occurring between the motorcycle and the road (such as accelerating, decelerating and turning) are transmitted by friction occurring at the contact patch. There is a limited amount of force that the contact patch can transmit before the tyre begins to lose traction, and therefore slide/ skid.

When going through a curve on a motorcycle, centripetal force (added to the other lateral forces such as acceleration or deceleration) 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 during the curve (or taking a curve too fast) would increase the centripetal force, thereby increasing the total net force through the contact patch. If the net force is greater than the static friction coefficient of the tire multiplied by the normal force of the motorcycle through the tyre, the tyre will skid outwards from the direction of the curve. By locking a tyre during a curve using the brakes, the friction giving the tyre its traction would be overcome by the previous forces discussed.

Once a tyre slips in a curve, it will move outwards under the motorcycle and cause the motorcycle to lay down in the direction the rider is already leaning to counteract the moment applied by the centripetal force and lead to a lowsider.

If the tyre regains traction after the rider starts to skid while the motorcycle is moving sideways, the tyre 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.

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, threatening to crush them.

Because highsider accidents are so much more deadly than lowside accidents, the Motorcycle Safety Foundation recommends that if a rider locks the rear brake at higher speeds and the traction is good the brake should not be released.

Article released under CC-BY-SA license agreement. http://creativecommons.org/by-sa/3.0/