A new FS frame with the same TT, but 1″ shorter ST

or

a new FS frame with the same ST, but 1″ shorter TT

?

Top tube length. Sort of. You also need to factor in seat tube angle and all the other variables but assuming similar then top tube length matters while seat tube length doesn’t really.

Seat tube length is dependant on how high you have your saddle surely ie the theoretical seat tube length is different for every one.

As above geometry of seat tube is much more important.

Ok, well I have an 18.5 ST 24″ TT.

Note sure whether to go 17.5 & 24″ TT or 18.5 & 24.7 TT.

Intense Tracer current vs Trek EX 9 new. The trek sizings are the actual not the advertised sizes (which are 18.5 & 19.5 respectively).

I’m 5’11, 31 inside leg if it helps. And yes I know I should test one first.

I’s say the 17.5 for sure: you’re keeping the same reach and gaining valuable clearance for your danglies.

I’s say the 17.5 for sure

+1.

you can always stick your seat up.

yep I’d definitely go on TT length. ST is mostly incidental imho and so long as it’s within a reasonable range isn’t going to cause a problem.

I’ve found that top tube length determines how a frame feels to ride and seat tube length determines how likely it is to break. 🙁

Assuming you’re happy with the toptube length you already have, then stick with that. I size my bikes with tt length rather than seattube

Famous Vegan – Fat Mike from NOFX.

Reports of him snapping bikes with his bulk are, as yet, unsubstantiated.

Are we talking actual TT length or Effective TT length?

ETT obviously. Does anyone ever quote ATT when referring to fit?

24″ effective top tube is pretty long anyway for someone your height. I’m the same height and my bikes have all got between 23 and 23.5″ effective TT length, any longer feels too big. 24.7″ TT will be enormous! That said I have got longer legs than you, and obviously a shorter torso.

Seat tube length, within reason, is largely incidental as they make decent seatposts in lengths up to 410mm these days.

P.S. Is it just me that thinks Trek has gone excessively long in the Top Tube lengths these days? 24″ TT on a medium sized bike? 😕