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Bicycles come in frame sizes for almost everyone, and custom bikes can improve fit even further.
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Bikes come in a variety of sizes to match the different proportions of riders that are different heights. The most common bicycle frame styles -- road and mountain bikes -- come in standardized sizes that correspond to the inseam of the rider, making bikes easy to size based on a single measurement. The small inconsistencies in arm and torso length are compensated for with seat posts and stems of different lengths.
Importance of Fit
Getting a correctly sized bike is crucial for your comfort, power transferal to the drivetrain and aerodynamics. When you lean forward to reach the handlebars, you form a triangle, with the top tube at the bottom and your head at the apex. This allows you to absorb shock and vibration from the road using your arms, distributing your weight more evenly between your palms and sit bones. Over long rides, this position can make a substantial difference in mitigating soreness. An incorrectly sized bicycle can cause lower-back soreness, burning in the knees, and a much less efficient pedal stroke.
Mountain Bikes vs. Road Bikes
The sizing in the United States for road and mountain bikes is separated along the U.S. standards of measurement and the metric system of measurement. Road bikes come in measurements of centimeters, and most frames fall between 48 and 63 cm for the seat tube height. Mountain bikes are measured in inches, with seat tube heights usually varying between 13 and 23 inches. Mountain bike frames generally have a shorter standover height than road bikes, since it minimizes the chance of a rider getting injured by the bike in the event of a crash.
Finding Your Pubic Bone Height
The inseam is known in cycling as the pubic bone height and is a basic measurement of the distance from the base of your pubic bone to the pedal. Start by measuring the distance from your pubic bone to your heel along the inside of your leg. Then, compare this measurement to a manufacturer's bicycle-sizing chart to determine which frame has the optimal seat tube length to match your inseam. For the majority of riders, this will correspond to an ideal top tube length.
Fitting the Bike
Once you have a frame that is correctly sized for your inseam, your top tube length should approximately correspond to your torso and arm length. However, different seat tube lengths and different stem lengths can customize the fit to put you as low as you're comfortable with. Check that your seat and handlebar height are approximately equal. This will put you in a good position to remain aerodynamic without straining your back. Lower-back pain is an indication that your handlebars may be too far forward or too low for comfortable riding.