Bike Geometry Explained

Cut through the clutter to really understand the bike frame fit.

On a bike maker’s website you’ll find a schematic called a geometry chart for each of its models. These cluttered numeric diagrams outline all the dimensions of a frame—the length of the tubes and angles between them. Every piece of data is critical for building the bike, and many impact handling, but only a select few describe the bike’s fit characteristics. The rest are red herrings in the search for a well-fitting frame. After finding your desired position—whether it’s on your current bike or through a new fit—a few simple numbers can describe the frames that will match your dimensions. Here’s how to understand which coordinates to pay attention to and which to ignore.

Frame fit

Ignore: Seat tube length, top tube length, head tube length and frame sizes.

The lengths of these tubes only tell part of the story, and frame sizes are even less helpful. “Like buying a pair of shoes, different brands are going to fit very differently,” says J.T. Lyons, F.I.S.T. fit instructor and owner of Moment Cycle Sport in San Diego. “We have bikes with a size small that is the same length as another bike’s size large.” Three critical dimensions solve all these problems and are becoming the focus of a growing number of bike makers.

Heed:

1.  Stack height, the vertical distance between the bottom bracket and the top of the head tube.

2.  Reach, the horizontal distance between the bottom bracket and head tube.

3.  Seat tube angle, which determines saddle position.

These three simple measurements define a frame’s fit characteristics and should guide your bike choice. Compare your own measurements to different frames and sizes to track down a list of bikes that can accommodate your fit. Ignore the rest. Stack and reach dimensions within about 5mm of your ideal will work perfectly because components can be used to micro-adjust for these differences.

Component conundrum

Aerobar selection is the other big piece of the puzzle. “You’re not riding just a frame,” says Lyons. “You’re riding a frame plus a stem plus the bars plus the saddle.” The impact of those components is murkier than the frame because they are less easily quantified and are often adjustable. “Navigating aerobar fit is the biggest disconnecting point in bike fit right now,” Lyons says. Many adjustable bars can change fit even more than swapping frames. Using the same bar and stem is the best way to ensure your bike, not just your frame, matches your fit—although several options can match the same position. As for finding the right one, well, “to some extent, that’s what good fitters are for,” says Lyons.

Autopilot

Armed with bike fit data, you can make an educated decision on your next purchase by paying attention to the measurements listed here. If you want to take it a step further, both Retül and Cannondale offer systems where you simply plug in your fit numbers to compare bike frames and parts before your purchase.