TRAFFIC ACCIDENT RECONSTRUCTION FOR MOTORCYCLES

Trail is a technical term for motorcycles that can be illustrated by observing the front casters on a grocery cart. The next time you go to your local grocery store to purchase food, get a grocery cart and observe the front casters. You will notice when the front caster's wheels roll across the floor say while you are pushing the cart in the forward walking direction, the wheel to floor contact patch is always behind the caster assembly's bearing axis. The bearing axis of the caster assembly is the steering axis of the caster. If you stop the grocery cart and then pull it backwards, the caster will become unstable where the wheel to floor contact patch will begin to rotate about its steering axis until the wheel to floor contact patch is again positioned behind the steering axis. At this point, the caster then beomes restabilized. No matter which way you spin the cart around, this will always occur. The distance between the caster's steering axis and the wheel's contact patch with the floor surface is called "trail". The term trail is derived from the fact that the caster's wheel to floor contact patch always trails behind its steering axis and is the most stable rolling location relative to its steering axis.

A motorcycle's front end fork assembly is called the triple tree assembly. It consists of an upper tree plate, a lower tree plate, a tree stem, a pair of roller bearings with bearing races, fasteners, two shock absorbers, a wheel which consists of a front rim and tire and the wheel's spindle shaft and bearings. The head lamp and signal lamps are also mounted on the front end triple tree assembly. The head and signal lamps are mounted on the lower tree plate in the photo below.

When an engineer designs a motorcycle, the tire's contact patch with the road surface must be located behind the front end assembly's steering axis which is located along the centerline of the goose neck of the frame. The perpendicular distance from the tire's contact patch with the road and the steering axis of the motorcycle's frame is called the "trail" of the motorcycle's front end. If the tire's contact patch with the road is located in front of the motorcycle's steering axis, the front end will become unstable as the motorcycle is riden forward (just like the grocery cart's caster discribed above), and the front end assembly will attempt to rotate about its steering axis creating an unstable condition causing the motorcycle to be uncontrollable. The trail measurement is established by the motorcycle manufacturer. See the drawing below.

If the trail is too short, the motorcycle will handle with ease at low speeds. However the steering will be unsafe at higher speeds due to high speed wobble which will cause the rider to loose control of the motorcycle. Too much trail will make the motorcycle handle sluggishly when riding on a curvy road. With a well designed and tested trail, the motorcycle will handle well at both low and high speeds.

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