I am looking to replace a 15 year old 20" Trek 700 with a Trek 7200 or 7300. I'm in my 50's and 5'10-1/2" or 5'11" with a 32" inseam. The salesman said a 17.5" would be a good fit (perhaps because that's the only size they had in stock). I almost touch the top tube on the 20" when standing over it and have a couple of inches of space from the top tube on the 17.5". I don't know if the 17.5 or 20" is more comfortable, as the shop doesn't have a 20" in stock. Is there a set formula for determining the proper size?
Kind of, there are fitting calculators on the web where you put in your measurements, inseam, arm length, overall height, etc. and out comes a recommendation. But not everyone wants what that program says is best. For me, it is the top tube length that matters the most. I have short legs and a long torso for my size. None of my bikes have any standover clearance. The older 2 inches of standover is the size to get method would make none of my bikes fit me.
If I were you, I would do one of a couple of things.
1. If the bike you ride now is comfortable, measure the top tube length and get the bike that is the same.
2. Get a fitting done at a bike shop.
3. Ride each size at go decide from that.
If you are unsure of what size is best for you, I would not buy a bike that I haven't test ridden.
There isn't a set formula, really: due to some bicycles having differently shaped top tubes (different angles, dropping, so on), the height of the tube shouldn't be the guiding factor unless its uncomfortable to stand over.
Have you taken a ride on the 17.5" and felt if you thought it was too cramped?