Bicycle frame geometry is a subject that can leave many people scratching their heads. With many variables effecting the riding dynamic of any given frame, a novice can easily find themselves in over their heads rather quickly. As someone who obsesses over every millimeter of frame geometry I am here to help.
My expertise in the subject comes from years working in shops, and a stint in the bicycle industry before I came to my senses. As someone always obsessed with what makes one bike ride different from another, I’ve spent far too long discussing geometry with people with far more knowledge and experience than myself. Thought this process, I’ve come to figure out a thing or two about what effects the way a bike handles.
The Front End
While there are many determining factors to how a bike handles, I’d like to start with the front of the frame. The major components to how a bike steers comes from the head tube angle, fork rake (or offset), and the resulting trail. The combination of these factors all contribute to how a bicycle will react to rider input. It’s all about finding that sweet spot between predictable steering and a bike’s ability to hold a straight line.
Head Angle/Steering Axis
The angle of the head tube determines the steering axis of any given frame. Head tube angles usually range from 71 degrees to 75 degrees depending on application. This angle, coupled with fork rake (or offset) determines the stability of any given frame This is done by controlling wheel flop, which is the tendency for the wheel to turn when leaned.
Generally speaking, the higher the head tube angle (approaching 90 degrees), the sharper the steering of the bicycle will be. A bike with a steep head tube, also known as aggressive geometry, will be more agile and require less effort to steer. An aggressive front end, while more nimble, will be more difficult to hold in a straight line. This type of geometry is good for track bikes, and criterium specific geometries.
The inverse is true for a bike with a more slack, or relaxed front end. While the bike may be nice and stable in a straight line, it will be harder to turn. Relaxed geometries are better suited for bikes used over long distances, such as touring and century rides.
Fork Rake (offset) and Trail
Trail is a key determining factor for how a bike handles. It is the difference between the fork rake (or offset) and the steering axis. This is what really influences how a bike will react to rider input. Where people get confused is with the inverse relationship between rake and trail. As rake increases, trail decreases. Another way to think of it: the more offset (rake) the closer the center of the wheel will be to intersecting the steering axis. Confused yet?
Visualizing this can be a bit tricky, but helps with understanding this inverse relationship. To establish trail, first imagine an imaginary line from the steering axis (center of the head tube) to the ground. Now imagine a straight line drawn to the ground from the center of the front wheel. This second line will be well behind the first. This distance between these two points is the bike’s trail. This is why more offset, which moves the front wheel forward, lessens the amount of trail.
The Sweet Spot
Depending on who you ask, the sweet spot for trail is anywhere between 5cm and 6cm. Different builders like their bikes to handle differently, but for the most part, most land within this range. Application also has an influence. A crit bike will tend to have less trail, while a century bike will benefit from more. It’s all in how you want the bike to react to steering input.
I hope this has been more enlightening than confusing. Steering angles are one of the more voodoo rich parts of frame geometry. It’s worth learning though, especially for more experienced riders looking for specific handling characteristics. I know I don’t consider a bike purchase without calculating trail. Stay safe out there, and as always, keep the rubber side down!