Riding a bike is good exercise, and it is a lot more enjoyable if you have a bike that fits your body. A bicycle that is too large for you is more difficult to mount and to balance when you stop because your feet won't reach the ground easily. If the bike is too small you won't be able to pedal efficiently, so you will end up working harder than necessary.
Components
Determining the size and height of a bicycle frame involves several parts of a bicycle. Frame size for a bicycle is based on the length of the set tube, which is the vertical tube extending from the bottom bracket to the seat post. The bottom bracket is the cylindrical component that the pedals attach to. The seat post is the part the seat, or saddle, that is attached to and is used to adjust the seat height. The top tube is the part of the bicycle frame that connects the upper part of the seat tube and the handlebar assembly at the front of the bike.
Frame Size
If you want to measure the frame size of a bike, use a tape measure to find the distance from the center of the bottom bracket to the point where the seat post enters the seat tube. A 16-inch bike frame size is appropriate for most people who are 5 feet to 5 feet 8 inches tall. An 18-inch frame will fit people ranging from 5 feet 7 inches to about 6 feet 1 inch in height. If you are 6 feet or taller, you probably will want a 20- or 21-inch bicycle frame. Keep in mind these are estimates. Individual proportions vary and so does the right bike frame size for a particular person.
Stand-Over Height
The stand-over height is the distance from the ground to the upper side of the top tube. If you straddle the bike, standing just in front of the seat, you must have some clearance between the top of your inseam and the top tube, both for safety and convenience in mounting and dismounting the bike. Sheldon Brown recommends one to three inches clearance for road bikes. Allow additional clearance for off-road bikes
Seat Height
Unlike bike frame size and stand-over height, the seat height on most bikes is adjustable. To find the correct seat height, first sit on the bicycle. Make the following tests to see if the seat height is right for you. When your foot is on a pedal in its lowest position, your leg should be just slightly bent. If you take your foot off of the pedal, you should be able to touch the ground with your toe tip. When your foot is on a pedal at its highest point, your upper leg should not rise far enough to be parallel with the ground. To adjust the seat height, loosen the pinch bolt located near the point where the seat post enters the seat tube. Move the seat up or down as needed and tighten the bolt.
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