
Why not tube length?
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A few folks have contacted me about geometry questions, and I want to help clear up why stack and reach are used to hone in on the right frame size and not top tube length or head tube height or other angle/variable-dependent metrics.
We use stack and reach to make things easier to compare by giving you a straightforward method of checking the fit of different bikes.

Stack: The vertical distance from the center of the bottom bracket to the top of the head tube at the top tube intersection.
Reach: The horizontal distance from the center of the bottom bracket to the top of the head tube at the top tube intersection.
As bicycle design has evolved dimensions like head and seat angle, bottom bracket drop, and fork length are no longer standardized so we want to use fit coordinates that better compare a bike fit.
A frame with a 75 degree seat angle and 72 degree head angle and 55cm top tube is going to have a different reach than a frame with 73 degree seat angle and 74 degree head angle with that same 55cm top tube.
Stack and reach do have some limitations- namely not all track bicycles fixture the stems to the fork with a traditional steerer tube. Hinged forks like the Look, Felt, and Argon among others would need to account for the added stack of the stem assembly. Nonetheless, stack and reach are the best way to compare relative fits between different models and between the various sizes.
If you need any assistance determining the best size, don’t hesitate to connect with us.
-Dave