Keep on Tracking

Text: Robert Smith • Photography: Robert Smith

RoadRUNNER reader Rick Starling recently wrote to ask if we would enlighten our readers with an explanation of Rake/Trail.

Good question. A motorcycle's steering geometry is one of the least understood aspects of its specification, so the goal of this article is to satisfy Rick and others with a clear understanding of what rake and trail are, and how they affect motorcycle steering. First…

How Does a
Motorcycle Steer?

Though slight changes of direction can be achieved by shifting your body weight, the most effective way (and some, including riding guru Keith Code would say the only way) to steer a motorcycle at road speeds is by countersteering. That's the action of pushing on the handlebars to initiate a turn, and it works counter-intuitively: push left and you go left; push right and you go right.

The reasons for this apparently contradictory motion lie in what's called "out-tracking." When you push on, say, the left handlebar, you're twisting the front wheel spindle to the right, and the front wheel tries to turn to the right. But because of the bike's momentum, it can't immediately follow the front wheel, and as the front wheel is trying to move to the right, the bike has to compensate by leaning to the left. That means the front tire contact patch is no longer in the center of the front tire, but off to the left as well, so the front wheel acts as a cone instead of a cylinder and steers left.

So the front wheel moves right, which means the motorcycle must now be leaning to the left. The front wheel is therefore no longer acting as a cylinder in contact with the road, but as a cone, and consequently steers the motorcycle left. So now the motorcycle is both tracking and leaning to the left, and fortunately, the centripetal force of the turn acts to balance out the lean, and the rubber side stays down. In practice, it's a lot more difficult to explain than to do.

How Do Rake and Trail Come into it?

Let's consider rake first. Rake (or caster) is the same as the steering angle or steering axis - that is, the angle of the motorcycle's steering head from the vertical. (European specifications may give the angle from the horizontal, so subtract from 90 degrees to get the number used in the U.S.) That's fine, but why does the steering head need to be angled at all? (Note: rake is not the fork angle: sometimes the forks are at a different angle from the steering axis.)

Dual-track vehicles (cars) can get away with a zero caster angle, because they steer (mostly) without leaning. But it turns out that to optimize steering for a number of factors (straight-line stability, speed of turning, braking stability and handling), the front wheel turning axis of a single-track vehicle needs to be leaned forward. Generally, the greater the lean angle (the rake) the more stable the bike is in a straight line - but it requires more effort to turn. That's why sportbikes typically have much smaller caster angles (rake) than cruisers.

There is another factor at play here, and that's trail. It's defined as the distance between an imaginary point, the point where a straight line drawn through the steering head would hit the road, and a point on the road directly under the front wheel axle. In other words, it's the distance in front of the tire contact patch that the steering pivot axis is at road level. In practice, the front fork needs to be moved ahead of the steering head to reduce the trail to the optimum value. Trail is affected by steering head angle, fork offset, steering angle offset (if the forks are not parallel to the steering head) and tire diameter.

Rake and trail work together to affect stability and handling characteristics, and it's important to consider the impact of any change to either rake or trail on the other. On many race bikes, the rake and trail can be adjusted to suit the characteristics of different tires, or for rider preference. Arriving at the optimum settings is usually a matter of trial and error based on rider feedback.

Steering and Braking

One of the main criticisms of the ubiquitous telescopic fork used on modern motorcycles is that the steering geometry changes as the fork compresses and extends during riding. This is most noticeable when braking in a turn. With your bike leaned over in a bend, applying the front brake compresses the fork (known as "dive") which also changes the rake and trail. This causes the bike to want to "stand up," reducing the lean angle and changing the turning radius. It can be very disconcerting for less experienced riders. It's one reason why many older riders lamented the demise of the "girder" style fork, which could be set up to minimize the effects of suspension movement on geometry, and which was, in any case, less prone to dive.

It's worth noting that suspension guru Philip Vincent, builder of the most iconic motorcycles of all time, never adopted the telescopic fork for his bikes, but instead designed his own advanced version of the girder fork.

Over the years, many types of front suspension have been tried with varying levels of success. The Earles-type fork used on pre-1970 BMWs, for example, has the advantage of being far less prone to dive (the front brake acts to limit suspension compression under braking) but was heavier and more complicated to manufacture; in the 1980s, many Japanese motorcycles were fitted with an anti-dive telescopic fork; and more recently, BMW has offered anti-dive Duolever and Telelever front suspension.

One other factor affected by rake and trail is braking performance. To avoid locking the front brake in an emergency stop, front suspension needs to be fully loaded, maximizing traction, before applying full braking pressure. Rake, in particular, has a big influence on stopping performance. A bigger rake angle generally means that the front wheel is further from the motorcycle's center of gravity, resulting in less downforce on the front tire, and, all other things being equal, less traction under braking.

(For more on this and related subjects, see Proficient Motorcycling by David L Hough, Bowtie Press 2000, $ 24.95)