An incorrectly sized or improperly adjusted bicycle is a very common cause of mountain biking injuries in children and adolescents. A properly fit bike will reduce stress on the neck, back, arms, hips, and knees while riding, helping to prevent overuse injuries.
Starting with the right frame size is the most important factor for proper bike fitting. Frame size is often determined by manufacturers guidelines for the specific frame, usually based on the rider’s height. Because a rider may fall between two frame sizes and because leg and torso ratio varies between individuals, it is always best to test ride a bike before purchasing. Adequate standover height (Fig 1, click images to view larger) and comfortable reach (Fig 2) are two factors to check for when testing a bike. You should be able to place your feet flat on the ground when you dismount the bike with about 2″ of clearance between the top tube and your perineum (or “crotch” in lay term) between your thighs. You should also be able to reach the handlebars while sitting on the saddle with your shoulders relaxed and your arms slightly bent. The reach can best be adjusted by changing the stem length. While making small adjustments forward or backwards to the seat (or saddle) position is an option, this can impact additional stress on the knees. You should not feel cramped and you should be able to turn the handlebars from side to side while pedaling without the handlebars touching your knees. Test ride several bikes and choose the frame that is most comfortable for you.
The next step is adjusting the saddle position. Improper positioning will cause strain on the knees and reduced power when pedaling. Start by adjusting the saddle height. Have someone support the bike so you can have both feet on the pedals. With the saddle level, sit on the bike and position the crank arms so that they are aligned with the seat tube of the frame (white dashed line, Fig 3). The knee of the extended leg should be bent 10-20 degrees. For better control on descents lower the saddle about 3 inches from the usual riding height. Next, adjust the fore/aft position by sitting on the bike with the crank arms in the 3 and 9 o’clock positions. The ball of your foot should be over the axis of the pedal. Adjust the saddle forward or backward until the knee cap of the front leg is directly over the pedal axis (Fig 4).
The correct handlebar position depends on the amount of suspension travel, style of riding, and personal preference. Generally, handlebars should be 1-2″ below saddle height. The minimum width between the hands should equal the width of the shoulders. Wider bars give greater steering control in challenging terrain, but excessive width will cause strain on the shoulders, elbows, and wrists.
Shifter and Brake Position
Position the shifters and brakes so they can be reached and operated without having to slide your hands inward or outward (Fig 5). The angle of the levers should be such that they fall in line with the natural angle of your hands as they rest on the bars. You should not have to rotate your wrists to reach the levers (Fig 6).
By Leslie Streeter & Jon Minor, MD
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REI. (2017). How to set up your new mountain bike. Retrieved from https://www.rei.com/learn/expert-advice/set-up-new-mountain-bike.html
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