How to Fit a Mountain Bike

by Robert L. Kronisch, MD from THE PHYSICIAN AND SPORTSMEDICINE - VOL 26 - NO. 3 - MARCH 98


Having a mountain bike that fits you well is important not only for comfort but also for helping to prevent injuries. A bike that's the wrong size or poorly adjusted increases your chance of developing an overuse injury. The best way to adjust your bike so that it correctly fits your body is to follow these guidelines with the assistance of someone who understands the relevant anatomy and has experience working with cyclists. It is very difficult to perform some of the measurements on yourself. (These guidelines also apply to fitting a road bike, except where indicated.)

Frame Size

Correct frame size is determined by straddling the bike in a standing position. (For basic mountain bike terminology, see figure 1 (not shown); for frame sizing, see figure 2.) Lift the entire bike off the floor until the top tube is pressing against your crotch. The distance between the bottom of the mountain bike's tires and the floor should be 3 to 6 inches (1). (For a road bike, a 1- to 2-inch clearance is usually adequate.) Some newer mountain bikes do not have a traditional top tube; for them you have to rely more on comfort and balance to select the right frame size. If you are buying a new bicycle, test-ride as many bikes as possible and select the one that is the most comfortable.

[FIGURE 2]

Foot Placement

This varies, depending on what type of pedal system you use. If your bike has simple platform pedals or pedals with toe clips and straps, all you need to know is that the widest part of the foot should be centered over the pedal axle. If you use clipless pedals, which firmly attach to cleats bolted to your cycling shoes, you must also make sure that your cleats are adjusted so that your foot placement on the pedals is the same as your natural foot position while standing. For example, if one of your feet toes out slightly when you stand, then your cleats should be adjusted so that your foot toes out slightly when you ride (2). This is usually accomplished by trial and error, but in difficult cases, a good bike shop should be able to help you find the right cleat position.

Most modern pedal systems offer a choice of cleats; some fix the foot to the pedal in a single position, and others allow some internal and external rotation (floating) of the foot while pedaling. The latter type may be less likely to cause overuse injuries.

Saddle Position

Correctly positioning the saddle will help maximize power and stability while minimizing the risk of knee pain. To make adjustments, you will need a goniometer or another device for measuring the knee angle, a carpenter's plumb line and level, and an allen wrench for moving the saddle. Note that after the saddle adjustments are made, you may need to fine-tune them in 1/4-inch increments every few rides until you find the optimal position.

Height. To set the saddle height, sit on the bike in a normal riding position with the crankarms straight up and down (figure 3). With your foot at bottom dead center (6 o'clock position), your knee should be bent at an angle of 25 to 30 (1,2). To measure knee angle precisely, center the goniometer at the lateral femoral condyle, and point the ends toward the greater trochanter at the hip and the lateral malleolus at the ankle. Some mountain bikers prefer a lower seat to improve off-road stability and maneuverability.

[FIGURE 3]

Fore and aft. To set the saddle fore and aft position, drop a plumb line from the front of your patella, or kneecap, while sitting on the bike with the crankarms horizontal (figure 4). Adjust the saddle forward or backward until the kneecap of your front leg is directly over the pedal axle (1,2). Some mountain bikers prefer to have the saddle about 1/2 inch back from this position to improve rear wheel traction while climbing. The knee angle (saddle height) should be rechecked after changing the fore and aft position, and vice versa.

[FIGURE 4]

Tilt. Use a carpenter's level (figure 5) to ensure that the saddle is level or tilted slightly upward so that you remain seated on the widest part of the saddle. Women may prefer to have the front of the saddle tilted down slightly.

[FIGURE 5]

Upper-Body Position

Determining upper-body position or reach is less exact than other aspects of bike fit and is influenced by your experience, flexibility, and comfort (2). A comfortable reach will allow you to maneuver the bike easily without shifting your center of gravity too far forward.

Factors that determine upper-body position include the top tube length, stem length, and saddle fore and aft position. Proper frame selection is important, since changing upper-body position usually involves buying a longer, shorter, or more or less angled stem.

A good upper-body position will allow you to sit comfortably on the bike with your arms relaxed and your elbows slightly bent. You should not feel cramped, and your elbows should not interfere with knee motions. Beginners usually prefer the stability provided by a more upright position, while advanced cyclists often prefer to have their upper body lean forward in a more aerodynamic position. Handlebar height is also a matter of preference, but as a general rule the handlebars should be 1 to 2 inches below the top of the saddle

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