The bike stem is an essential component because it connects the handlebars to the vertical tube of the front fork. A stem that's too large for the bike can cause the handlebars to become misaligned with the front wheel, resulting in a painful crash or slideout. Choosing the proper stem length allows you to maintain control of the handlebars during high-speed maneuvers. According to the Mountain Bike Buzz website, the quill stem featured on traditional mountain bikes comes in a 1-inch and 1 1/8-inch size. Most threadless mountain bike stems are measured in millimeters.
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Lift your leg over the bike to straddle the frame. Lower yourself onto the seat, and place your feet flat on the floor. Extend your arms toward the handlebars, and take hold of the grips.
Examine the curvature of your arms to determine if the stem corresponds with your physical dimensions. Choose a stem that's 20mm to 30mm smaller if your arms must fully extend to reach the handlebar grips. Upgrade the size of the stem if your arms bend at less than a 45-degree angle.
Select a stem according to your type of biking. Use a 80mm to 120mm stem to ensure an aerodynamic riding stance on cross-country, mountain-biking terrain. Select a stem with a shorter length for increased handling and stability on a downhill MTB course. Pick a model that features a series of spacers if you need to adjust the length of your bike stem easily and quickly.