repechage

I have run across a few fitters and builders who reference it. I think it is a way to evaluate the bike as how long a stem one will end up with given that the frame is mass produced and one might ignore where the frame is suggesting on where to place the saddle by the seat tube geometry.

Decades ago I called it effective top tube length and measured to the TT /HT centerline intersections from a plumb line through the BB axle. I was going on the assumption that I wanted to place my saddle with respect to the BB the same, no matter the ST angle. The distance forward of the BB to the steering axis gave me a good indication on the length of stem I was going to require. Remember this is from a time of level top tubes and a stand-over height where the top tube did not change one's voice, nor did it feel as you were riding a girls bike.

The frame that drove this investigation was a track bike I had, it had a long top tube, like 22.75" but a slack seat tube angle of 71.5°... (! what were they thinking?) Anyway, I was set up on the bike when I was just learning frame dimensions... A photo by a friend of me on the bike in a race, showed I was cramped up over the handlebars. I went from a 95mm stem to a 135mm, and looked much better on the bike, but too much weight forward. It was time for a bike that fit. My only guess was the frame was really intended for a big gear pursuiter who wanted to be way back and push into the gear, I was on a gear limit, 79", it was hummingbird legs time, the bike was all wrong for that.

Some builders still use "set back" to place the seat tube angle, it is another way of thinking and just as dictatorial as providing a ST angle as the leg, femur and foot length is known and will drive the numbers along with the rider's pedal style and typical cadence. One either takes the what the builder is suggesting or works around it. I think it is better to know what works first, and find a bike that meets those needs in this S/M/L molded stock frame world. Or, go custom and get what you need.