Comfort is an essential part of speed, and your fit on a bike contributes to your comfort. Cervélo frames have a carefully considered geometry that makes them “fit-system neutral”. We base our sizes on frame stack and reach.
As illustrated above, frame stack is the vertical length from the BB to the top of the headtube. Frame reach is the horizontal length from the BB to the center of the headtube. Same two points (bottom bracket and head tube), just vertical or horizontal distances.
But wait - how is this any better than the traditional ways we've sized bike all these years?
How We Used to Define Size
It's still true that your seat and handlebar locations relative to the bottom bracket define your position. We're not suggesting a different fit, we're suggesting a different way to look at frame size.
We used to define frame size by seat tube length.
Seat tube length doesn’t define fit. The grey and black frames above have the same fit, but different seat tube lengths. In other words, "B" can be shorter than "A" and you just raise the seat post to your saddle height.
Also take a look at the head tube length. Since handlebar height has a smaller window of adjustability than seat height, some recommend comparing head tube lengths. But as you can see (grey frame), if the fork is designed longer (or shorter), then for the same position, the head tube gets shorter (or longer) from "below." Fork crown height, BB drop, and internal/external bearings all affect head tube length. In other words, "C" can be shorter than "D" and your position is the same as long as the fork, etc. is correspondingly longer.
What about top tube length? Critical, no? Take a look at the image below. You can see that, as long as dimensions of your seat post and saddle rails cooperate, different top tube lengths can still put your handlebars and seat in the same position. This means that the top tube length you need depends on the seat tube angle you have.
So seat tube, head tube and top tube lengths are unreliable indicators of fit. That's why we helped develop frame stack and reach.
Plotting stack & reach
Plotting stack and reach numbers for a size run shows how well the various sizes serve a range of riders. Plotting frame stack & reach equals putting the bottom bracket at the origin (0,0) and noting where the top of the head tube lands.
Here's what it looks like when we zoom in on the area near the top of the head tube:
Plotting the stack and reach for several brands shows that not all geometries make sense. With some brands, sometimes going to a bigger frame actually moves the head tube CLOSER to the rider. See the green and grey lines above.
Why most small frames don't fit
Plotting stack and reach can explain why some small bikes sometimes don’t fit well. The typical size range below shows that the three smallest sizes do not bring the head tube any closer to the rider. Only the seat tube is steeper (hence the maker can publish a shorter top tube length), but a different offset seat post would have done the same.
So, how do you know if a model truly gets smaller in its smaller sizes? We need to make sure that the head tube continues to move rearward as the frame sizes become smaller. In other words, look for frame reach, not top tube length. The seat tube angle may or may not change, that doesn't matter as much.
You can see Cervélo’s approach below — we make sure the headtube moves closer to the rider as sizes get smaller.
Finding data on body dimensions
We know to use stack and reach to accurately describe a frame's size. To determine what sizes are needed to cover the population, we have compiled anthropometric data, US Department of Defense & UK Ministry of Defense standards and dimensions taken from UK (1987) and US (1988) military personnel surveys.
So, at Cervélo we use definitions of stack and reach, combined with our understanding of riding positions based on biomechanical data, to design a range of sizes needed to fit 98% of the population.
This means that Cervélo has rational sizing: Go up a size, get more reach. Go down, get less.