Motorcycle Trail Explained

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

The vehicle dynamics people describe a motorcycle as two casters joined at a common pivot. The pivot is the steering head, and a caster is a wheel pivoted in such a way that the center of its footprint is some distance from the pivot axis. This distance from the center of the tire footprint to the projection of the steering axis on the pavement is called trail. Caster is what gives us self-centering steering, for if the pivoted wheel is at an angle to the direction of travel, a restoring force is generated, tending to swing the pivoted wheel until it aligns with the travel direction.

We can feel this restoring force whenever we deflect the vehicle’s steering from its present direction.

Trail gives a motorcycle stability by causing both front and rear wheels to align with the vehicle’s direction. Sporting motorcycles tend to have trails in the range from just over 3 inches to 3.75 or even 4 inches. Anything over 4 inches has generally been provided to give extra stability to large touring rigs. This is the geometric trail.

Race teams carry sets of fork crowns made with a range of offsets (distance from the central plane of the fork tubes to the steering axis) so that trail can be increased or decreased by small amounts. Back when Ducatis steered quite slowly, a race team might reduce trail to speed up the steering, at some cost in reduced stability. In first practice at Daytona, the high speed sometimes resulted in such bikes displaying instability. By second practice, those teams had added enough trail to restore stability.

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The distance from the point where the front tire contacts the ground to the point that the steering axis projects to the ground is defined as trail. <i>Andrew Trevit</i>

An interesting effect caused by the flexibility of the tire’s carcass is pneumatic trail. Imagine a front tire, rolling around a turn. In order to turn, both of a motorcycle’s tires must generate side-force. As an element of tread is about to enter the front edge of the tire footprint, it feels no stress at all. But as the tire rolls on, that element of tread feels an increasing side-force from the fact that the tire is slightly steered into the turn. Because that side-force peaks just before that tread element is lifted up at the trailing edge of the footprint, the generation of side-force is not uniform along the length of the footprint, but rises steadily from the front to the rear. The result of this force bias toward the rear of the footprint moves the effective center of side-force farther from the pivot, effectively increasing trail. The amount of this trail extension is called pneumatic trail.

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Experienced riders and drivers know that any time they feel a sudden reduction in steer restoring force, the reason is that friction between tire footprint and pavement has decreased. We’ve all had some scary moments of this kind on icy or dusty or gravel-strewn pavement. Racers, feeling the self-centering torque of their steering disappear, say that “I closed the front.” I have seen racers, accelerating out of a turn, try to keep the front—made lighter by acceleration—still steering by increasing the steer angle. This is called front-end push because increases in steer angle are no longer able to steer the accelerating bike, which has a less-than-normal load on its front tire. If front grip is lost, the rider’s muscular effort to steer can cause the front to hit its steering stop. Often at trackside or in the pit box you may see a rider pantomiming this loss of control to the crew chief by extending both hands to imaginary bars, turning them more and more…until suddenly steering angle increases because the restoring force has zeroed. The front end has closed.

What makes the front steer or not is the load on the front tire. Often you may see a rider, exiting a corner, use arm strength to move forward, to increase front tire load and thereby prevent push.

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