Steering Geometry: What is Trail?

Trail is the distance the contact patch of the front tire lags behind the point where the steering axis intercepts the ground. Consider the casters on a shopping cart: the wheels are offset behind the steering axis, and the drag of the wheel tries to center the wheel on the direction of travel. The greater the distance between the steering axis intercept and the contact patch, the stronger the centering force becomes.

For a given steering angle, offsetting the hub forward reduces trail, while offestting the hub backward increases trail. Most road bikes have the hub offset ahead of the steering axis. This can be done either by bending the fork blades or by having the blades leave the fork crown at an angle to the steering axis. For a given steering angle and fork offset, the trail is the same regardless of how the offset is obtained.

The image below diagrams the rake of a 1930s racing bike. You can do the same for any bike. It's easiest with a scale drawing or a good photograph. Draw a line down the center of the steering tube and extend it to the ground. Drop another line from the center of the front hub perpendicular to the ground. The distance between the contact patch and the steering axis/ground plane intercept is the trail.

As shown in the formulas on the left, trail can be calculated given the head tube angle, the radius of the inflated tire, and the amount of fork offset. For someone designing a fork, it is more useful to start with the desired trail and calculate the needed fork offset, which can be determined from the tire radius, head tube angle, and trail.


This page written by Joshua Putnam. Please feel free to email questions, comments, corrections, suggestions, etc. 


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