The formula for determining the right size frame for a mountain bike comes from a method originally used for road bikes. Both kinds of bicycles measure frame size from the center of the bottom bracket to the center of the junction between the seat tube and the top tube, called the C-C length, or to the top of the seat tube itself, called the C-T length.
Step 1
Stand with your bare feet 6 inches apart and your back against the wall, recommends "Colorado Cyclist" magazine. Place a book between your legs, with one edge against the wall, pressed firmly against your crotch. Level the book horizontally. Have a helper mark where the top of the book hits the wall.
Step 2
Measure the distance from the floor to the mark on the wall in centimeters, or measure it in inches and multiply by 2.54. Multiply this centimeter figure, which represents your inseam length, by 0.67, and subtract 10 cm to 12 cm, to arrive at the correct frame size for a C-T mountain bike frame. Multiply your inseam length by 0.65 and subtract 10 cm to 12 cm to come up with the correct size for a C-C mountain bike frame.
Step 3
Straddle the top tube of the bike during a visit to a bike shop to determine the standover height--the distance between the top tube and your crotch. Lift the bike straight up while straddling it, and have a helper measure the distance between the wheels and the floor. Aim for 2 to 4 inches of standover height for a cross-country mountain bike and 6 to 8 inches of standover height for a downhill or free-ride mountain bike, recommends Mary Blomquist of the online site Mountain Bike Buzz.
Step 4
Test ride a mountain bike to check for a good fit. Pedal the bicycle outside to check whether the length of the top tube, almost more important for your comfort than the frame size, provides appropriate horizontal fit for your body. Glance down at the front hub while holding the dropped part of the handlebar; if the handlebar obstructs your view of the front hub, your horizontal fit is correct.
Step 5
Buy a bicycle with the frame size and top tube length that fits your build and is the most comfortable to ride. A custom-built bike or the closest size off the rack are both fine.
Things You'll Need
- Book
- Measuring tape
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