garagegirl

I have a bike with a 53 cm top tube, a 72 degree hta, and a 74 degree sta. I have to cram my saddle pretty far back on this bike as I have very long legs for my height, but I am very happy with my fit and reach on the bike. I would not want the reach to be any longer than it currently is.
I'm considering getting a bike with a 54.1 cm tt, 73.5 sta, and 73 hta. The reach will wind up being roughly the same, right? Are there any other factors I need to consider?

The other option is a 52.5 top tube, same angles. I'm afraid if I go with the smaller size, I'll have trouble getting my bars high enough.

Retro Grouch 

Everything on a bicycle works together so you really have to consider everything at once.

I'll muddy the waters by talking about the relationship of the saddle to the bottom bracket. That's the factor that I work on first. If you are delighted with the fit on the bike that you have now, drop a plumb bob from the nose of your saddle and see how far it is behind the bottom bracket. Try to make that match on your new bike when the seat height's right.

When that's done consider the reach and drop from your saddle to the handlebar. Do whatever you have to do to get the drop right and then just buy whatever length stem will make the reach right.

If you have the saddle pushed all the way back now I doubt you're going to like a bike that has a tt that's almost an inch shorter.

garagegirl

The problem is that it's almost impossible to view the bike i'm considering in person. It's an On One Pompetamine, geometry here-
https://www.on-one.co.uk/i/q/FROOPOMP...petamine-frame

The bike I'm basing my fit info off of is an older model Salsa Casseroll, size 49
https://salsacycles.com/bikes/casseroll_double/

I want avoid anything shorter than an 80mm stem. I currently have a 90mm stem on the salsa, but sometimes I use an 80mm. I was able to ride the salsa before a bought it, but I doubt I'll be able to find an on-one to try out.

I know these "what mail order bike should i buy" threads are a pita, sorry.

fietsbob

Rough guess: virtual top tube length + stem extension would remain the same .
as top tube portion is shorter, stem longer , by similar amount.

You may have long femurs in your legs, then the setback, is needing an increase,
To keep the Knee over pedal thing right, so may not be best served by a 73, 74 STA.
need a 72, or less then..

thankfully, Threadless stems are EZ to swap out , buy several, or maybe an adjustable one..

MichaelW

You may be able to solve the long leg/short reach issue by using longer cranks. This would place your feet further forward/down on the pedal stroke. You may need to adjust the saddle forward/down to compensate.
Just make sure that the cranks are not so long as to prevent a nice spinning action and avoid toe-clip overlap.

Rowan

Handlebar height is as much a function of headtube length. It appears to me that the Salsa Casseroll, for the type of bike it is, has a long headtube. The On-One is shorter by quite a long way -- 104mm compared with 125mm for the Salsa.

That to me is significant. You may need to go with a higher cut on your steerer tube to get the same handlebar height. Or if you want the same stack height, you may need to look at a stem that has a steeper rise than the one you use at the moment.

Or opt for another frame

bradtx

garagegirl, Difficult to give a firm answer, but I doubt the frame with the 52.5 cm TT would be better. With a short frame the ST angle won't be too much of an issue WRT the BB as long as you have a couple of cm worth of fore/aft saddle adjustment available with your current bike. Steerer tube length should be at least as long as you have now.

Brad