Have you watched road racing on television? Then perhaps you have noticed that rider less machines sometimes continue on their own, stable as projectiles, straight ahead after the rider has been thrown off. The reason they can do this is mainly to be found in the steering geometry.
The motorcycle’s fork is oblique, so that an extension of its centreline hits the ground a bit ahead of the front tyre’s contact point with the ground. The distance between these two points is called trail.
The geometry of the front fork is a prerequisite for the bike’s inherent balance and directional stability. If you want to see for yourself how this works, straddle the bike with both feet solidly planted on the ground.Lean the bike a bit to one side. Make sure you support it firmly with your thighs so it doesn’t fall. Let go of the handlebar and watch what happens.
If the friction between tyre and ground isn’t too much, you will see the front wheel steer to the same side that you lean the bike. The effect of this, at speed, is that the bike on its own, without aid from the rider, will try to steer under the centre of gravity when it cants over to one side or the other, and in this way manages to keep balance and directional stability, thus ‘trail’ provides stability.
Sports bikes with less trail have less stability but a faster turn in, while custom bikes are the opposite end of the scale with greater stability but with a correspondingly slower turn in.