Bicycle Sizing Information
Sizing of a bicycle is probably the most important factor in the choice of a new bicycle. However, this seemingly simple statement opens the door to a whole world of information, misinformation, arguments and even a little divination. There is a wealth of information provided in books, magazines and on-line. However, the best method of determining the bike size for you is based on your experience coupled with the voice of a knowledgeable cyclist or coach, and possibly video or mirror analysis.
When a cyclist is well fit for the bicycle they ride, the rider is more comfortable, and more efficient. The rider then has more power with less effort, and is more relaxed, handling the bike with more skill. The challenge for finding the proper fit comes from the differences in body size, proportions, physical fitness levels, flexibility and riding technique and style.
Where does a cyclist start when trying to determine the proper bicycle frame size? The best frame size is as small as possible in the vertical sense, with plenty of length for the upper body to be relaxed and stretched out. The goal of this statement is to end up with a frame that is lighter and more stiff than a large frame, and more comfortable and handles better than a small frame.
Selecting Your Frame Size
An accurate inseam measurement is the starting place for determining the proper frame size for your bike. An easy way to find this measurement is, while standing flat against a wall, place a square cornered object (a large book, clip board, etc.) between your legs and pull it up to simulate the pressure of your bike saddle while riding. With help, measure from the top of the object to the floor. Be sure to use metric measurements (in centimeters), or measure in inches and multiply by 2.54. It is a good idea to repeat this process a few times and average the results.
The size of a bicycle frame refers to the length of the seat tube. However, this measurement can be determined two ways, each giving a slightly different length. One method is to measure from the center of the bottom bracket to the top of the top tube (C-T). The other method is to measure from the center of the bottom bracket to the center of the top tube (C-C).
To achieve your suggested frame size for a road bike, multiply your inseam measurement by .67 (for a C-T frame), or by .65 for (for a C-C frame). This formula works well for those under six feet tall. Taller riders may need to choose a frame by a different method.
For mountain bike frames, the suggested frame size is smaller than in a road frame, by about 10 to 12 centimeters. To determine this size, follow the instructions for road bike frames, then subtract 10 to 12cm. Just be sure that you pay attention to the C-T and C-C designations of the frames and use the corresponding sizing method. However, be sure that you can come up with the proper top tube length for the given frame. This will be covered further on.