Determining Your Frame Size
Frame size refers to the length of the frame's seat tube. Frames are measured along the seat tube in one of two ways, center-to-top (C-T) or 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 (See image below).
C-C measures from the center of the bottom bracket to the center of the top tube. Since C-T measures to a point higher on the frame, a frame measuring 55cm C-T would also measure roughly 53.5–54cm C-C, a difference of 1–1.5cm.
To size your C-T road frame, we use a guideline of .67 x inseam length. For example, if you have an 85cm inseam, your C-T frame size would be .67 x 85cm, or 57cm.
LeMond's formula establishes C-C size by the formula .65 x inseam length, which yields virtually the same frame size when you add the 1–1.5cm difference between C-C and C-T.
Larger riders (6'0" and up) may find that this formula puts them on a too small, and uncomfortable, road frame. A taller cyclist who wants a more comfortable frame may be better off selecting a frame 27–28cm less than inseam length, C-T.
For a mountain bike, we start by recommending a frame in the range of 10–12cm smaller than you take in a road frame. For example, if you ride a 55cm C-T road frame, look for a 43–45cm (17–18") C-T mountain frame.
Frame Dimensions
A1 Seat Tube Length (C—T)
A2 Seat Tube Length (C—C)
B Top Tube length (C—C)
C Stem Length (C—C)
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 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.
Determining Your Seat Height
With the right frame size, you'll be able to set your correct seat height, which will be within a centimeter of .883 x inseam length, measured from the center of the bottom bracket to the low point of the top of your seat. This allows full leg extension, with a slight bend in the leg at the bottom of the pedal stroke.
LeMond recommends that you then shorten this length by 3mm when using clipless pedals. Also, you might consider a slightly taller seat height if you ride with your toes down and your heel raised. Most importantly, make any changes in seat height gradually, and give your body time to adapt to the new position.
Going by basic charts, I fell between sizes. Without seeking professional opinion, I chose the larger frame figuring it would do the trick. I spent two race seasons going from injury-free to frustrating bouts of swollen knees, back spasms, hip soreness and an adductor longus pull. Not all were necessarily created by the wrong bike fit, but you can bet this didn't help. It took a fellow athlete at the last championship event to convince me the frame was far too large.
Stand with your back against a wall, your bare feet 6" apart on a hard floor, looking straight ahead.
