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How do I get the proper fit for my new frame?
We'll talk primarily about fitting a road bike, and make notes about fitting a mountain bike where appropriate. Think of the bike frame along two dimensions, vertical and horizontal. The best frame size for a cyclist is as small vertically as possible, with enough length horizontally to allow a stretched out, relaxed upper body. This frame will be lighter and stiffer than a larger one, and will handle better and be more comfortable than a smaller one.
To determine your proper frame size, you'll first need to get an accurate inseam measurement. On a hard floor, stand with your bare feet 6"apart and your back against a wall, looking ahead. Place a large book or carpenter's square between your legs with one edge against the wall, and pull it up firmly into your crotch, simulating the pressure of your saddle while riding. Have a helper measure from the top edge of the book to the floor, in centimeters; if measured in inches, convert to centimeters by multiplying inches by 2.54. Repeat two or three times, for consistency, and average the results to get your inseam length.
Frame size refers to the frame's seat tube length. Pro frames are measured along the seat tube in two ways, center-to-top(C-T) and center-to-center (C-C). C-T measures the distance from the center of the bottom bracket to the top of the top tube or seat lug. C-C measures from the center of the bottom bracket to the center of the top tube at the seat lug. Since C-C measures to a point lower on the frame, a frame measuring 55 cm. C-C also measures roughly 56.0-56.5 cm. C-T, a difference of 1.0-1.5 cm.
To establish your proper C-T road frame size, we use a guideline of .67 x inseam length. For example, if you have an 84 cm. inseam, your C-T frame size would be
.67 x 84 cm., or 56.3 cm.
LeMond's formula, from his former coach, Cyrille Guimard, establishes C-Csize by the formula .65 x inseam length, which yields virtually the same frame size when you add the 1.0-1.5 cm. difference
between C-C and C-T.
Larger riders (6'0" and up) may find that this C-T formula puts them on a too small, and uncomfortable, road frame. A taller cyclist who wants a more comfortable frame may be better off by selecting a frame 27-28cm. less than inseam length, C-T.
For a mountain bike, we start by recommending a frame in the rangeof 10-12 cm. smaller than you take in a road frame. For example, if you ride a 56 cm. C-T road frame, look for a 44-46cm. (17-18") C-T mountain frame.
In many ways, though, it is more important to fit a mountain frame by the top tube length needed, rather than by the seat tube length. For instance, you might be able to get to the proper frame clearance, saddle height and neutral knee position (see below) on either a 17" frame or a 19" frame. Yet the 19" frame will likely have a top tube 1" longer than the 17" frame, which changes your stem length accordingly. Or, one manufacturer's 17" frame may give you a 22" top tube, while the next one's 17" gives you a 22.8".
