Читать книгу Proficient Motorcycling - David L. Hough - Страница 39

When test riders refer to heavy steering, they are talking about a machine that is so stable in a straight-ahead situation that it requires a lot of muscle to get it leaned over and held into a turn. What they mean by a flickable machine is one that is relatively unstable, that can be easily leaned over or straightened up with very little effort on the grips. This is a delicate balance, and sometimes the engineers have to walk a tightrope between low-effort (flickable) cornering and bad manners, such as the front wheel suddenly steering itself toward the curve (tucking), uncontrollable oscillations (speed wobbles), or falling into turns at slower speeds.

The behavior of a bike is related to its steering geometry. If you stand off to one side of your motorcycle and observe the angle of the front forks, you’ll notice that the top of the fork tubes are angled (raked) back. And if you look closer, you’ll see that the fork tubes aren’t exactly in line with the steering head on the frame. While you are standing off to one side of the bike, imagine a laser beam passing through the steering head until it strikes the ground. The laser beam represents the pivot center, or steering axis of the whole front end. When we talk about rake angle, we’re talking about how far the steering head is angled back from vertical. Today’s cruisers typically have around 30 degrees of rake, while flickable sportbikes have rake of 24 degrees or less. In general, greater rake produces greater straight-ahead stability at speed, and steeper rake produces low-effort steering. But rake is only part of the equation.

The steering axis intersects the ground somewhere ahead of the contact patch where the front tire is sitting on the surface. The distance between the contact patch and the steering axis is called trail (the contact patch trails behind the steering axis). Typically, street bikes have trail somewhere in the three- to six-inch range. In general, longer trail results in a machine that resists leaning into corners, and shorter trail results in quicker, easier steering, or perhaps even a machine that wants to fall into corners. Since rake and trail are interdependent, the figures in bike reviews are usually given as rake/trail.

A motorcycle like this one with greater rake and trail typically has very stable straight-ahead steering but requires more effort to roll into or out of turns.

A motorcycle like this with steep rake and short trail has very easy steering, making it very flickable from side to side.

Trail is the distance from which the front wheel contact patch follows behind the steering axis.

Because of rake, the steering head actually rises and falls when the front end is steered.