Bike sizing by a person’s height is bullshit. Some more enlightened types will use pubic bone height (PBH), which is a step in the right direction. But I think it’s still bullshit. I haven’t cracked this yet, but I’ve got one more arrow in my quiver.
It’s not only leg length that determines fit, but also torso length, arm length, posture, flexibility… the list goes on. So why is it that we try to measure bikes by listing one arbitrary measurement of of the frame?! It drives me crazy.
For a while now I’ve been playing with stack and reach and I think they are the most important measurements on a frame, but it gets difficult when you try to compare the relative fit of a bike across multiple sizes. So what I needed was a way of looking at the average ratio of stack to reach across all sizes of a particular frame. Just doing a simple stack÷reach calculation was a step in the right direction but then a fellow bike nerd with a bigger brain and a more than elementary understanding of math reached out to me and suggested a really cool way to do this:
“Obviously, there are many different styles of geometry. Modern endurance road bikes advertise their upright riding position which can make a long ride more comfortable, and modern mountain bikes are all about getting “long and low”. To be able to tell where a bike falls along this spectrum, I measure the angle between a flat line projecting forward from the bottom bracket and the Pythag BB -> HT line and call this, rather brilliantly, “Angle of BB -> HT”.
Angle of BB -> HT = ARCTAN( stack / reach ) in degrees
The larger this number the more comfortable a bike’s fit should be, all other things being equal. For example, the comfort oriented Fargo has an angle of 59 degrees, while XC racer SC Tallboy has an angle of 51 degrees.” – Alexander Sollie from The Earth Remains
So we’re dealing with angles, and that’s cool because we can simply average those up for all sizes of a frame or even all bikes from a particular manufacturer. Finally we can rank bike manufactures by who has the chillest ride (stay tuned for more on this)! I think it’s a profound way to think about bike geometry.
So without further ado, here’s the ranking of frames by c-factor (most chill at top):