Finding the right size bike frame for your body is essential for speed, comfort and safety. You will feel cramped, experience more wind resistance and won’t handle the bike as effectively on a frame that is too small. A bike that is too big will also be harder to steer, can cause discomfort in your neck, back and shoulders and can be dangerous to mount and dismount. The frame’s size depends on how the tubes that make up the frame fit together.
Measurement
A bike’s size describes the distance between the axis on which the pedals spin, or the center of the "bottom bracket," to the top of the seat tube -- usually. According to cycling guru Sheldon Brown, “anarchy reigns” in measuring frames because many manufacturers measure to different sites on the frame. Since each bike frame has a unique shape, no two bike sizes are alike, and a size that fits you with one model may not fit with another.
Quick Fit
On most road, mountain and hybrid bikes, you can guess whether a certain frame is right for you by straddling the frame and lifting the bike snugly into your crotch. Used Bicycle Guide recommends choosing a bike that gives you 1 to 3 inches between both wheels and the floor. When your hands are on the handlebars, your elbows should be slightly bent and your shoulders away from your ears. The only way to truly know if a bike fits you is to take it on a test ride.
Road Bikes
Road bike frame sizes are usually measured in centimeters. The smallest women’s frames are about 46 cm, while the largest frames can exceed 61 cm. On a road bike, you only want 1 to 2 inches of clearance between your crotch and the top tube. Many new riders are tempted to buy a smaller frame with a shorter reach to the handlebars that allows them to sit more upright. However a road bike’s aggressive crouch is what allows roadies to duck out of the wind and travel so fast.
Mountain Bikes
Mountain bike sizes are measured in inches and can range from about 13 to more than 21 inches. Mountain bike frames fit a bit smaller, with a shorter reach to the handlebars and 2 to 3 inches of stand-over clearance. The shorter reach affords more control on tight, technical trails, and the lower top tube ensures that your foot will always reach the uneven ground before your soft tissues crash into the top tube.
Women's Bikes
Women’s frames run smaller and have slightly different angles than men’s frames. Most women have proportionally longer legs and shorter torsos than men, so women’s bikes have a shorter reach to the handlebars relative to the stand-over height. However, many women do not have “typical” proportions and may fit better on a men’s frame. In a few cases, “leggy” men fit better on women’s frames.
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