What Is the Correct Bicycle Frame Size for My Body?

Riding a bicycle that's the wrong size can be an uncomfortable experience.

Bicycling is a great way to get around, providing fun and exercise while cutting down on pollution, and for short errands it can be more efficient than driving. Without a properly fitted bicycle, though, rides can be uncomfortable, leading to excessive arm, leg and neck fatigue, back pain and potentially serious knee injuries. The single most important component of bike fit is frame size, but this can be a tricky thing to understand, as different bicycle manufacturers use different measurements or different numbering schemes.

Traditional Frame Sizing

For the better part of the 20th century, finding the right size bicycle frame was easy. If, when standing over the fully equipped bicycle, there was 1 to 2 inches of clearance between the top tube and the rider's crotch, the bike would be said to fit. The length of the seat tube was the main measurement, and this and the head tube length were all that varied.

The Switch to Proportional-sized Frames

Changes in frame geometry have made the question of what fits more difficult to answer. Starting in the early 1980s, bicycle frame builders started building frames with different geometries. When a seat tube is longer, indicating a larger size, the top tube and down tube are also proportionally longer, providing a more comfortable ride for most cyclists.

Various Measurement Conventions

The use of proportionally sized frames has led to many different ways to measure bicycle size. Generally, the size provided is a measurement of the seat tube length, either in inches or centimeters. However, the points measured can vary significantly. While most use the center of the bottom bracket as one end, the other end can be the tip of the seat tube, the center of the top tube or, if the top tube is slanting, where it would intersect the seat tube if it were straight. The size number can seem quite arbitrary, or even downright dishonest, as a result.

The Importance of Overall Fit

While the current by-the-book method of frame-sizing still involves the rider standing over the outfitted bicycle to make sure there is enough clearance, there are several other things to consider. First, the way the bike is used must be considered; a racer will adopt a lower, less comfortable position than someone who just runs occasional errands. Once this is determined, the length of the frame's top tube, the height of the handlebars and the seat positioning and angle all play a large role in the cyclist's comfort.

Get Fitted

The best way to get the right frame size is to head to a bike shop with an idea of how you want to ride and let the bike shop fit you to a bike. They will take various measurements and apply some technique, such as the New England Cycling Academy FitKit, then suggest a frame size and geometry. This will reduce a lot of discomfort if you plan to do a lot of riding.

Conventional Wisdom

While there is no agreed-upon fitting system, there is a rule of thumb that works for most everyone. Measure your inseam, from your crotch to the floor, then multiply it by 0.65 for a road bike, or 0.59 for a mountain bike, and you will have a good estimate of the size bicycle frame that fits you. For example, a 6-foot-tall man with a 34-inch inseam would probably fit on a 56-cm road bike, and a 5-foot-7-inch-tall woman with a 32-inch inseam would be reasonably comfortable on a 17-inch mountain bike.

References

• Sheldon Brown: Revisionist Theory of Bicycle Sizing
• eBay: Bicycles Buying Guide