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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.

The frame size is measured from the seat lug at the top to the center of the bottom bracket. To calculate your correct frame size, divide your height by three, or subtract 9 inches from your inseam length (measured from crotch to floor, in bare feet).

Regardless of the calculations, the frame should be easily straddled with both feet flat, with perhaps an inch of clearance. While a smaller frame can be compensated with a higher seat and headset, of course, a frame which is too large for adequate groin clearance should be avoided at all costs, for the inevitable is inevitable. If you can adjust the seat and bars properly with two different sized frames, the smaller one will be stiffer and absorb less of your pedalling power through flexing. As men have proportionately shorter legs than do women, your frame and seat will usually be higher than a man of the same height.

Most bikes have a 72 degree seat tube and head tube angle. This provides an excellent combination of road holding, shock absorption, and power transmission, and is great for bumpy streets. Racing frames are steeper or "tighter," at 73, 73.5, or 74 degrees, or a combination like 73 head and 74 seat, for a stiffer ride and better power transmission. The main great thing about suspension and shock-absorbent materials like titanium and carbon fibre is that more efficient setups like steeper frames or higher tire pressures become tolerably comfortable.

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-C size 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 27-28 cm. less than inseam length, C-T.

For a mountain bike, we start by recommending a frame in the range of 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 44-46 cm. (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". More on this below just make sure that you'll be able to work out your top tube and stem length for a given frame.
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1 comments posted so far.
Posted By: Aloysius Wee on August 7th, 2008
Will this measurement of inseam X 0.67 work for a compact geometry bike or one with a sloping top tube? I assume it won't because the seat tube length of a conventional bike and a sloping geometry bike cannot be the same though they may both fit the same person ideally.