Riding a mountain bike with the proper frame size for your build is as essential as wearing shoes that fit, notes veteran cycling journalist Steve Worland in "The Mountain Bike Book." No matter how much money you spend on your mountain bike and what fancy features it sports, you'll be wasting money and diminishing your comfort, efficiency and enjoyment unless it fits well. The ideal mountain bike, according to the editors of Mountain Bike magazine, is 2 to 4 inches smaller than your road bike frame size.
Identification
If you are checking a fully assembled mountain bike at a bike shop for fit, stand over the bike's top tube and lift the bike straight up until it hits your crotch, recommends racer, writer and frame builder Lennard Zinn in "Mountain Bike Performance Handbook." The wheels should be a least 2 inches off the ground and can be more, up to 5 inches or so, as long as the top tube is long enough for your torso and the handlebar height can be adjusted for comfort.
Misconceptions
Because you can comfortably stand over one maker's frame does not translate into another manufacturer's frame size also fitting you correctly. Differences in top tube angles, bottom bracket heights and methods of measuring frame size affect the final stand-over height. Frames can be measured from the center of the bottom bracket to the center of the top tube, the top of the top tube or the top of the seat tube.
Considerations
If you are ordering a bike online or having one custom-made, you can fine-tune the determination of a proper frame size by taking your inseam measurement. Stand in your stocking feet with your feet about 2 inches apart. Measure from the floor to the top of a broomstick held firmly against your crotch. Multiple the result by 2.54 to convert inches to centimeters.
Application
Subtract 34 cm to 42 cm from your inseam length to find your frame size if the bike has a horizontal top tube. Subtract closer to 34 cm is you are smaller and 42 cm if you are taller. If it has a slanted top tube, project a horizontal line to the seat tube or seat post and measure from the center of the bottom bracket to this point. If the bottom bracket is higher than 29 cm, subtract the additional length over 29 cm from the seat-tube length, Zinn recommends.
Expert Insight
During your test ride, see whether the reach feels comfortable when you lean forward from the seat to hold the handlebars or the bar ends. Test the reach of the brake levers. Ensure that your toe doesn't hit the front tire and that your knees do not hit your elbows, Zinn advises.
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