Copyright © 2002
Prepared by
Warren Cay
bikeNOW P/L
Frame Size
Although manufacturers measure frame size by the distance from the frame’s Bottom Bracket centre to the top of the seat tube, I believe that the length of the frame is more critical. For a bike to feel natural to control, one should aim for a stem as close to 110mm. Longer or shorter by more than 25mm will have adverse affect on how the bike handles. Therefore it is recommended to measure a bike by the length of the top tube (B) and not the height (A), although for your safety it is important that you can stand over the top tube comfortably.
Manufactures measure two ways;
Center-to-top (C-T) C-T measures the distance from the centre of the bottom bracket to the top of the top tube or seat lug (see "A" and "A1" in picture below). Center-to-centre (C-C) C-C measures from the centre of the bottom bracket to the centre of the top tube. This is very important when measuring a bike because, C-T measures to a point higher on the frame than C-C. A frame measuring 55cm using C-T would measure roughly 53.5–57cm C-C, a huge difference!.
Greg LeMond’s formula, applied by his former coach, Cyrille Guimard, establishes C-C size by the formula inseam length x 0.65 = C-C. This C-C formula is preferred because manufactures vary how much tube extends beyond the top tube intersection.
For a mountain bike and/or newer Compact road frames, we start by recommending a frame in the range of 10–12cm smaller than the formula result above. For example, if you ride a 55cm C-C, look for a 43–45cm (17–18") Compact or MTB 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 virtually all cases, chose the size that gives the best top tube length. Seat posts are far easier to adjust than stems and we can adjust the height so almost anybody could fit 3 different sizes. We also don't want to have radically long or short stems as it affects how the bike handles (short stems for DH, Trials and Dual Slalom excluded) It is a bit silly that the whole industry sizes on height not length but it evolved this way for our groin safety.
We now know roughly what size frame you need for height however we will confirm the size after we measure your stem position later on.
