Selecting the appropriate size mountain bike frame is important for your comfort, safety from overuse injuries, and your ability to control your bike in adverse terrain. Unfortunately, there are no standard sizes for mountain bike frames, or even standard ways with which to measure the size. Sizes vary between manufacturers, and also between different lines of bikes built by the same manufacturer.
How MTB Frames are Measured
The most common method in determining frame size is measuring from the center of the crank to the top of the top tube. However, while manufacturers often size their frames based on the length of the seat tube, they may measure from the center of the bracket to the center of the top tube, or the top of the top tube, or they may use a seat tube that extends a greater height from the top tube.
Standover Height
When you have the opportunity to physically examine a number of different bikes, the simplest way to determine fit is by the standover height. To test standover height, straddle the bike forward of the seat. Mountain bikes generally have a top tube that slopes downwards toward the seat, so it will be lower closer to the seat tube. You want two inches of clearance between the bike and your body.
Top Tube Length
Early in the history of mountain biking, frame length was a constant. The only variable in bike frame size was the seat tube; the length of the top tube from seat tube to headset would remain the same. Modern mountain bikes now have proportionate sizes, so bikes with shorter seat tubes will have shorter top tubes. The most important factor in sizing a bike's top tube length is your torso and arm length. You want the handlebars close enough to reach comfortably, but not so close that you support too much of your weight on your hands.
Bottom Bracket Clearance
One final factor in sizing your mountain bike frame is the distance between the bottom bracket and the ground. This is important because, while two bikes might both be listed with a size of 19 inches, this refers only to the seat tube length. One bike may actually have a higher standover height even though tube lengths are the same, simply because the geometry of the frame gives the bottom bracket a greater ground clearance height.
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