Fitting a road bike to the rider has become more of a science than ever, yet there is still plenty of art in a good fit. It may take some time, but once a rider has a good fit, maintaining the same fit on a new bike is critical. How do you tell what size new bike can achieve the desired fit? For example, what size stem and how many spacers are needed?
One way to think of the task is to divide the dimensions into two categories: frame dimensions, and rider dimensions. The frame dimensions describe how long the seat tube and top tube are, etc. and the rider dimensions describe where the rider contacts the bike such as seat height and setback, reach and drop to the bars, etc. The rider’s fit is correctly maintained on any frame if other components such as seat post, stem, etc. properly bridge from the frame dimensions to correctly meet the rider dimensions.
If you know your rider dimensions and have frame dimensions to compare, then you can choose between different frame sizes and predict what size components are needed (seat post, stem and spacers, etc.). If the component sizes are reasonable then the rider’s position can be duplicated on the new frame. Unfortunately, making the comparison isn’t trivial, since some of the dimensions aren’t purely vertical or horizontal, and it also seems more difficult these days now that head tubes are extended sometimes, top tubes sometimes slope and different brands measure their frames in different ways. However, if you have a CAD package and enough dimensions you can draw in the frame and rider dimensions and find out the component sizes that might complete the fit.
Below is an example from Franck, who had his fit professionally determined to establish his rider dimensions. I used these dimensions to find out what size Madone might work well for Franck.
Based first on his desired seat height (845 mm), and by refering to the Madone’s seat height table, Franck could consider either a 62 or 64 cm Madone. He’ll need the optional tall seat cap. (The green line is for a different rider, not Franck.)
For the handlebar position, I used SolidWorks to make a sketch of Franck’s bike and rider dimensions.As you can see from the drawings, I’ve used the bottom bracket as “home base” and positioned Franck’s saddle and handlebar clamp according to the rider dimensions from his fitter. I also used the Madone’s geometry table to sketch in a few frame dimensions.
Looks like Franck could comfortably ride a 64 cm Madone in a Performance fit. He could try a 130 or 140 mm stem angled down, using about 25 mm of spacers (including the head set top cap).
I also tried the frame dimensions of a 62 cm Madone, which could fit Franck with 44 mm of spacers (including headset top cap) if the stem also became longer: 145 mm – not a commonly available length! So in a Madone at least, a 64 looks best for Franck.
As always, questions and comments are welcome!
Damon Rinard