Two Wheels and an Engine - Part 1

Those who have followed my travels through the blogosphere, will perhaps remember a post of the same name on my other blog. That one was a tribute to the freedom and individuality that the bikes provide to their riders. This one here, in tradition of this blog of mine will contain a few technical details of the bikes.

The "two wheels and an engine" signify the prime construction of the wonderful iron beasts that are motorcycles, but here we will concentrate upon some lesser known things about the motorbike.

Motorbike Steering Geometry
Ever wondered why your eliminator is superbly stable on the highway, but can be a handful in the city traffic, whereas your pulsar can handle both with aplomb, albeit the highway stability is a wee bit inferior to the eliminator.

It all comes down to the steering geometry of the two bikes, which are built for different purpose. But before we can comprehend to understand how, we must first know the nomenclature and understand the technicalities of the key specifications of the motorcycle steering system.

Rake, or Steering Axis Angle is the angle in degrees of the steering axis from the vertical. The larger the angle, the more 'rakish' is the motorbike design, and these designs with large angles are the characteristic features of the custom choppers.

Offset, or Fork-Offset is the perpendicular distance between the steering axis and the centre of the front wheel (usually this coincides with the axis through the forks, as in most modern applications, the front wheel is mounted directly on the fork bottom).

Trail, perhaps the most important of all, is the horizontal distance between the point where the steering axis intersect the ground, and the actual point of contact of the front wheel on the ground. It is said to be positive if the front wheel contact point is behind the intersection point of the steering axis and ground, and negative otherwise. A related term is mechanical trail or normal trail, which is the perpendicular distance between the wheel contact point and the steering axis.

Trial has a fair role in determining the handling characteristics of the bike. The longer the trail (in the positive direction), the higher is the high speed straight line stability of the bike, but it also sacrifices the steering response and low speed manoeuvrability. Too long a trail makes a bike unmanageably slugguish. A short, or even negative trail will make the bike extremely responsive to steering inputs at slow speeds, but the drawback is a serious lack of stability at higher speeds, including twitchy behaviour and wobbles, which can be deadly.

Choppers have a characteristic raked look, and that would usually induce a very large trail, good for cruising down the long interstates, but impossible to turn anywhere. A number of tricks are used to make the trail manageable, such as the fork offset, and the fork rake angle, where the fork axis differs from the steering axis, making the steering axis intersect the ground well before the fork axis does, controlling the trail to manageable limits.

So, the next time you have a tough time on an eliminator / avenger on the crowded streets of New Delhi, try something with less rake and trail!