The length of the seat tube determines a bike’s frame size. This portion of the bike extends from the point where the pedal cranks attach and the seat post enters the frame. A common technique for measuring a bike frame with a horizontal top tube is to straddle the bike with your feet flat on the ground. There should be one to two inches of space between your crotch and the top of the horizontal bar.
Since women’s and many mountain bike styles do not have this horizontal bar, you will have to use another technique for determining the correct frame size. The proper seat height should allow you to extend your leg until it is almost completely straight when you are sitting on the seat and the pedal is in the bottom-most position. The slight bend in your knee helps to maximize power and minimize fatigue. Your feet should not touch the ground when you are sitting on the seat.
Step One
Use a tape measure to take measurements of your body to determine the bike that fits your individual physique.
Stand on the floor with your feet shoulder width apart and then measure from the floor to your crotch to determine the length of your inseam.
Determine the length of your torso by measuring from your crotch to the base of your neck.
To measure the length of your arm, hold a pencil or pen in your hand and make a fist. This simulates the grip required to hold the handlebars. Determine the distance from the tip of the pen or pencil to the base of your collarbone.
Step Two
Once you have your body measurements, you must convert these figures to determine the correct size bike frame. Multiply your inseam by 0.66 to determine the correct length of the seat tube.
Add the length of your arm and the length of your torso. Divide this number by two and then subtract six. The result is the optimum length between the center of the seat and the handlebar attaching point.
This measuring technique will enable you to select the correct size frame when purchasing or renting any type of bicycle.