Mountain bikes are designed to be used on off-road terrain such as rocky or dirt trails and logging roads. One of the key factors to safe and successful riding is having a bike frame that fits you properly. As a result, mountain bikes are available in a variety of frame sizes, along with additional adjustments to the seat and handlebars to fit your body size, type and riding style. Mountain bike fames are generally measured in inches from the distance between the center of the crank to the top of the frame at the seat tube.
Initial Measurements
Mountain bike frame size is determined by a variety of factors, including height and leg length. The first step in determining the recommended mountain bike frame size is to measure your inseam, or leg length, in inches as you stand barefoot. Multiplying your inseam by 0.65 will give you a recommended road bike frame size. Subtract another 4 inches to convert the measurement to a mountain bike frame. For example, a 32-inch inseam has a recommended frame size of about 17 inches -- 32 x 0.65 - 4 = 16.8.
Frame Size Chart
Frame size categories may differ according to different mountain bike brands, models and styles. A general rule of thumb, however, recommends extra small frames for 14 to 15 inches while small frames fit 16 to 17 inches. Medium frames fit 18 to 19 inches, large frames fit 20 to 21 inches and extra-large frames fit 22 to 23 inches.
Positioning
When you ride a mountain bike, you have three contact points -- handlebars, pedals and seat. Each contact point can be adjusted to match your frame size and overall fit of the mountain bike. Moving the handlebar position forward, for example, can improve power, aerodynamics and speed control, while moving the handlebar closer can improve comfort during recreational riding. Changing the saddle height can also change the recommended size of the frame. Proper saddle, or seat, placement should be set where your heel can barely touch the pedal with a straight leg.
Considerations
By following the mountain bike frame size chart and changing the contact points, you can adjust the frame size up or down to maximize comfort, fit and performance. For example, a 6-foot tall male cyclist with a 32-inch inseam has a recommended frame size of 18 inches, but should consider frame sizes of 17.5, 18, 18.5 and 19 inches. Choosing a larger frame can reduce safety, but a smaller frame can affect performance.
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