Even a variation in tire profile can change the relationship between contact patch and steering axis as the motorcycle leans, altering dynamic trail. Even a variation in tire profile can change the relationship between contact patch and steering axis as the motorcycle leans, altering dynamic trail. Photo courtesy of Dunlop

Static and Dynamic Trail: The Subtleties

Written by  on Thursday, 09 August 2012 15:18

In my last couple of blogs, I've talked about trail and how it changes as a motorcycle moves about on its suspension or leans into a corner. Trail gives a motorcycle its steering characteristics; in general, more trail is better for stability while less makes steering easier. Combining what we know about static and dynamic trail, and what changes affect each, it's possible to make subtle adjustments to improve rider feel and feedback in almost any given situation.

When a motorcycle is turned into a corner, there are a number of forces acting on the front end that are, in effect, trying to steer the motorcycle for us. Tony Foale's book "Motorcycle Handling and Chassis Design: The Art and Science" goes into the details of each of the forces, which include slip angle, camber thrust and longitudinal forces, but what we are concerned with here is the net result: oversteer, understeer or neutral steering. In an ideal world, these forces sum to zero and the rider never has to input a steering correction, no matter the speed, lean angle or pitch - the bike falls into a turn under braking, holds a perfect line in the middle of the corner with no input to the handlebar, and lifts from full lean of its own accord under acceleration. But the reality is that the rider has to intervene in many cases; the bike may have to be forced into a corner under braking, the rider may have to exert a constant pressure on the bar to hold a line or physically lift the bike out of the corner.

In many cases, it's rider preference that determines the goal regarding steering characteristics. Manufacturers generally design street bikes to give light, neutral steering and good stability in a wide variety of conditions. But some riders are willing to sacrifice that stability for low-effort steering quickness, while others prefer to use brute force and need more stability to do that. Additionally, deviating from stock components can quickly upset that balance the manufacturer worked so hard to attain - simply replacing stock tires with a grippy set of race tires that have a different profile changes how all the forces on the steering interact, in turn upsetting feel and feedback.

Here is where static and dynamic trail, and the differences between the two, becomes important: Small adjustments in trail can affect how those forces acting on the steering combine, altering feel and feedback to the rider and changing the steering/stability balance. And by making specific changes that affect only dynamic trail, those characteristics can be affected in very specific circumstances. For example, when a motorcycle is generally unstable or turns too easily, usually only static trail needs to be adjusted - a change accommodated by adjusting ride height or triple clamp offset. But if it's a problem only when the bike pitches on acceleration or braking, dynamic trail can be changed through fork springs, oil level height or the rear suspension linkage. Changes to how the bike acts in steady-state cornering can also be made by altering dynamic trail; this would be accomplished by changing tire profile, either through more or less pressure, a different tire, or a wider or narrower rim.

There are so many combinations and variables that it's easy to get lost when making adjustments that - intentionally or unintentionally - alter static or dynamic trail. But knowing exactly what those different adjustments actually change and how those changes affect handling is the foundation to understanding motorcycle setup.

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