Originally Posted by
Unca_Sam
Some advice that keeps popping up on the forums here is that the top tube length (virtual TT is better) is more of a final 'bike size' measure as opposed to the seat tube. You have more adjustment in your seatpost than you do with stem length. While seat to pedal is absolutely critical for performance and efficiency, the top tube length is acceptable even if you place too much weight on your hands.
I paid for a bike fitting at a local bike shop, and I believe that the knowledge of recommended frame proportions has paid dividends in my [self destructive addiction] adventure into cycling. You can make some precise measurements at home with some help and use online calculators to approximate what you should look for, and then pay attention to geometry tables when shopping.
'Vintage' road frames were frequently described as 'square' meaning the top tube is parallel with the ground. The top tube length was frequently proportioned (within a size bracket) to the seat tube length. Modern bikes utilizing sloping top tubes still measure center to center for the seat tube, rather than to the imaginary point where a horizontal top tube will join an extended seat tube. The result is that a '52cm' frame might have a 55.5cm virtual top tube, and ride like a 'square' 56cm. I use the geometry charts and the virtual top tube length to figure out a bike's 'true' size, but I only know what sizes to look for because of the bike fitting. To help you evaluate bikes for yourself, compare the geometries of the bikes, with the emphasis on where the saddle is in relation to the bottom bracket and handlebars. A long level is useful when you're top tube is sloping.
That is more or less how I consider sizing. One might also look at the headtube length (stack height).
But, in general, the length of the top tube seems to be the most representative.
However, one can look at riding the bicycle as 3 contact points.
Pedals (bottom bracket), Saddle, and handlebars.
You can make a bunch of adjustments. Stem length, stem height, saddle height, saddle fore & aft, crank length, etc.
The OP didn't mention his height. I'm not convinced a high top tube is a major problem, at least for riding on the road. Unless, ou are trying to do a top-tube descent.
At stop lights, I take one foot off the pedals, and sit one cheek on the top tube, and it is nice to have it a bit high. But, riding, you're not on the top tube.
Stack Height will affect bar drop if that is a concern.