Geometry flunkie needs help!

[woody_1029]

I have a road bike that I would like to use for triathlons. The seat tube angle is 73 degrees and the top tube is 56cm. If I were to use a Thomson Setback seatpost -- reversed that is -- what would my effective seat tube angle be? The Thomson has 16mm of setback.

Can any math majors help me here?

Thanks.

Woody.

[MrCjolsen]

The angle would be the same. What the seatpost would do is not change the angle, but rather have the effect of moving the whole seat tube forward.

It's hard to explain. let's say you are boldly raising your seat in 1" increments. Your seat will move back each time you raise it vertically. The horizonal distance it moves back each time will increase the further your seat gets from your bottom bracket.

That increase will be less with a greater seat tube angle because your seat tube is more straight up and down, rather than slanted back. It doesn't matter what post you use or how far forward your seat is positioned on it, it's still going to move back in relation to the angle of the seat tube.

The only real way to tell is to measure it where your seat is in relation to your bottom bracket, and then compare that to what it would be if you had a TT bike with a more vertical seat post.

Oh, and I majored in History.