"Trail. What is it and what does it do?
Trail is defined as the distance between the centre of the front tyre’s contact patch and the point where the steering axis meets the ground. (See figure 3. 1)
Virtually every motorcycle and bicycle ever designed has a trail value, and this is what gives the front wheel a caster or self-centring effect when in motion. In other words, a machine being ridden in a straight line is designed to continue travelling in a straight line unless other forces are brought to bear upon it. These forces may be rider inputs such as steering or leaning, or external forces like crosswinds or road cambers.
Trail creates a certain level of steering stability, and so by altering trail, a chassis designer can ‘tune’ the handling characteristics of any motorcycle to suit the purpose for which the motorcycle has been designed.
Caster, and its role in agility and stability.
Caster (sometimes referred to as rake) is the angle between the steering axis and an imaginary perpendicular line running through the front wheel axle. (See figure 3:1) As we have already seen, a motorcycle’s trail measurement plays a crucial role in achieving chassis stability, and it is the caster angle (among others) that helps to determine trail."
