I’m 5’9" with a 31 inseam - last year I rode a 54cm Jamis road bike and a 52cm Aegis as my tri bike (road frame built to be a tri bike). This year I picked up a 50cm Raleigh Competition Carbon road frame and built it up as a tri bike and am now converting the Aegis to a road bike.
Before I bought the Raleigh frame, I compared it to the Cannondale Slice in 51cm config and found that they were almost identical.
No matter what you do, plan to be on a smaller tri than road bike - you’re going to be more compact.
Go get fit and tell us what happened!
Like a lot of people have mentioned, get a fit and see what they suggest.
I’m 5’9’‘-5’10’’ with about a 33’‘-34’’ inseam. I have relatively long legs, short torso, and definitely short arms (always have to get my shirts hemmed and my sister calls me a “T-Rex”). I had a 54 P2C for awhile, but it was definitely too “long and low” and I had to quite a bit on the front end to get a higher stack, plus I was in the back seat position.
I’m riding a 56 Tarmac SL and had a Retul to fit aerobars and my position was actually a lot more comfortable than on my P2. The Retul suggested a size Medium Shiv or Trek SC for me, mostly b/c I need a pretty high stack. I’ve actually opted for a 56 Cervelo S5 and converting it into a Tri bike. I can get a little lower than my Tarmac and stay AERO the entire 112 miles, but still stay comfortable to run.
Good luck!
Buy my old frame and find out. Kidding. Sort of.
http://forum.slowtwitch.com/gforum.cgi?post=4590039;search_string=insidious;#4590039
I’m quite close to you in height but probably proportioned differently. 5’10.5" with a 34" inseam and a 6’5" armspan. I rode a 57 litespeed tuscany off and on for a decade and liked it. I was fit at a high quality roadie-centric shop when I bought the bike. It was what I’d now call a good club century/credit card touring position for me, comfortable but really pretty upright. I got back into riding a few years ago and started lurking here. I learned the basics of the F.I.S.T. system and finally reassessed my own fit on the bike. Basically it fit well in the traditional sense, ie the seat tube was “appropriate” height for me, the “standover” was good etc but the position did not work well in the more functionally accurate X-Y axis perspective. I couldn’t get into a comfortable, low position on it owing to my extremely long arms propping me up, even when bending them excessively. I tried stretching out the cockpit at one point and I’d get really sore shoulders from the shoulder angle it would force. Finally I just went out and bought another Tuscany, 55 cm, of the same era. I swapped parts and now I’m feeling dialed, low and comfortable all on one bike.
Anyway, sorry this is a non-answer. I’d probably go get fitted then start trying out bikes that fit within that size range. If you don’t value your time and energy or just like tinkering, buy a used frame, build it up, try it out, tweak it and repeat until you have a winner.