Home   Donate   Download   About   Contact   Italiano

© 2017 Mattia Piron. All rights reserved.

  1. Introduction
  2. Calculation
  3. Typical values
  4. Bibliography

Please help Mattia Piron

Immagine di copertina

JUNE 1st, 2018


Introduction       top

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.

Avancorsa Forza raddrizzante dovuta all'avancorsa Situazione in cui l'avancorsa puņ diventare negativa
Trail Straightening force due to the trail Situation in which the trail can become negative


Calculation       top

Steering geometry is described by three parameters:

  • Steering angle ∈
  • Fork offset d;
  • Wheel radius Rf;

The value of the trail is calculated:

a = Rf tan(∈) - d/cos(∈)

It can be seen from the formula that the trail:

  • It increases as the wheel radius Rf increases;
  • Increases as steering angle ∈ increases;
  • Decrease as fork offset d increase;


Typical values       top

Schema avancorsa

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:

  • The bike's handling decreases;
  • Steering responsiveness increases.
These are two almost opposite effects. If this ratio is too low, the steering will certainly be light, but the reactions of the motorcycle will be excessively slow. If it is too high, on the other hand, each action on the handlebars will correspond to a snappy reaction, but the steering could be excessively heavy, leading to a difficult ride. There are therefore some optimal values. Generally small displacement scooters, intended for city transport, have rather low values, around 4-5%. For racing, sport or road motorcycles, the ratio is around 6-6.5%, and can reach 8% on touring bikes. Particularly heavy motorcycles, such as cruisers, have ratios around 5%. Probably, to make the steering lighter, given the high weight. The weight distribution should also be considered in the calculation of Rn. Two motorcycles kinematically identical, but different in weight distribution, will have different behaviors. This is intuitive. The bike with a more advanced center of gravity, with the same steering angle, will move laterally the center of gravity of a greater value than the bike with a more backward center of gravity. Moreover, a greater weight on the front wheel will make the steering heavier. This means that, even if kinematically identical, the bike with a more advanced center of gravity will behave as if it had a greater Rn value than the bike with a more backward center of gravity.


Bibliography       top

  1. Effetto moto - Dinamica e tecnica della motocicletta, Gaetano Cocco, Giorgio Nada Editore, 2008
  2. Motorcycle Dynamics, Vittore Cossalter, Grafiche Gemma Borgoricco, 2008


Help me help you         top

You should know that I created this website out of passion and love for mechanics. If you would like to help me to maintain these pages, you can make a small donation as well. Thank you so much.

Please help Mattia Piron

Home   Donate   Download   About   Contact   Italiano