JUNE 1st, 2018
Trail is defined as the lever arm between the point of tire/ground contact and the point of contact of the steering axis intersection with the ground.
It can assume positive, negative or null values. It is defined as positive when the intersection of the steering axis with the ground is in front of the tire-ground contact point, negative when it is behind, zero when the two points are coincident. After an external perturbation (obstacles on the asphalt, side wind...), a force parallel to the direction of travel arises. This force is decomposed into two components: one parallel to the wheel axis (Cp), the other orthogonal to it (Co). The orthogonal component, multiplied by the value of the trail, generates a moment. This moment tends to straighten the front wheel if the trail is positive, while it favors the rotation of the steering if the trail is negative.
With the same external perturbation, therefore, the higher the value of the trail, the higher the moment generated. It is clear that the trail should always be positive. However, due to the conformation of the road, it is possible that the trail becomes negative, causing momentary loss of control of the vehicle. This can happen, for example, when going over a step. For this reason, off-road motorcycles generally have a high trail combined with a large diameter front wheel.
Steering geometry is described by three parameters:
The value of the trail is calculated:
a = Rf tan(∈) - d/cos(∈)
It can be seen from the formula that the trail:
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The interaction between bike and ground can be represented by 3 points: the two points of contact tire/ground, and the axis of rotation of the steering. In addition to the "front" trail (just defined), there is also a "rear trail", which can be obtained by adding the wheelbase of the bike to the value of the front trail. You can evaluate the handling of a motorcycle by comparing the front and rear trail. We will call this ratio "Rn". If it is true that, following a perturbation, as the front trail increases, the moment generated around the steering axis increases, the opposite is also true, that is, as the front trail increases, it is necessary to generate a higher moment in order to impart a perturbation to the motorcycle. However, this is true with the same wheelbase. Two motorcycles with the same front trail value, but different wheelbases, will have different handling characteristics.
As you increase Rn:
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