How to Custom Fit Your Bike (Part 1 of 3)

MarketBoy is here to share some insights about custom fitting your bike. Forget the in’s and out’s of carbon vs. aluminum frames, Specialized vs. Pinaraello, the one thing matters more than anything else when you select your new bike — that it fits you!

When a cyclist fits her bike well, she rides more comfortably and efficiently. She rides with more power, and yet the effort comes easier to her. Her body is relaxed, her bike handling skills almost second nature.

True, setting up your ideal position on a bike takes some time, and can be difficult. Why? We roadies hear conflicting advice, and conflicting formulas, on sizing and positioning. Variances in body sizes and proportions, in levels of fitness and flexibility, in technique and style, all affect a rider’s best position on the bike. Truthfully, though, the best resource you have is your own experience, especially when teamed with feedback from knowledgeable cycling kakis.

So, here we go…


Your Inseam and Frame Size

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.


Determine Your Proper Frame Size

To determine your proper frame size, you’ll first need to get an accurate inseam measurement. Stand with your back against a wall, your bare feet 6" (15cm) apart on a hard floor, looking straight ahead. Place a book 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. Repeat two or three times, for consistency, and average the results to get your inseam length.

Frame Size

Frame size refers to the length of the frame’s seat tube. Pro 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 bellow). 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 0.67 x inseam length. For example, if you have an 85cm inseam, your C-T frame size would be 0.67 x 85cm, or 57cm.

Greg 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 1–1.5cm difference between C-C and C-T.

Larger riders (>1.8m) 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.



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)