I asked this elsewhere on the forums, and it was suggested I come here:
Why are top tubes longer than the seat tube on small bikes, but smaller than the seat tube on small bikes?
This is what I've consistenly seen, but here is a real world example. Measuring center to center, my Apollo has a 67 cm seat tube, and 62cm top tube. My wife's Miyata has a 49cm seat tube and a 53 cm top tube.
I can barely stand over my Apollo, but I kind of wish that the top tube was longer! Maybe something 65x65 would be more my cup of tea.
Follow up question: Do any large vintage bikes have these dimensions (top tube as long or longer than seat tube)?
- at a time there was a limit to the length of the top tubes so they simply didn't get longer in standard materials, not sure I buy that one. But certainly many of us remember a less generous time when off the rack was not availabe to people of non-normal dimensions, or left handed, etc... To save money tube sets are made to a length that will accomodate most people whether there are any real limits here I don't know. It certainly is true one sometimes has to customize tubes to get various performance goals, even if they aren't specifically driven by rider size.
- There is sometimes a wheelbase expectation out there for a paticular use so the manufacturer's specs may claim all bikes of a certain type have the same wheel base, stay length, and head tube angle just to satisfy expectations. Obviously if you freeze the WB it get proportionately shorter in larger sizes.
- Wheel size is also an expextation, so as bikes get smaller there may still be presure to deliver them in 700c for racing or 26" for MTB which basically does the same thing as above.
- The wheelbase is sometimes optimized for shorter lengths for handling in various kinds of road racing, or for single track, so again as long as you can scrunch aboard there may be a minimum that is desired, this is sorta the underlying reason behind the expectation given in 2 above.
- People say what TM said above, though as a tall guy with short legs proportionatly, I tend to think it is only true in movies.
I don't think it's unusual on a custom bike, though custom bikes are pretty unusual. The ideal for me would be about 60 cm frame toptube and 56 stand over. This kind of geometry doesn't even look all that bad when you consider all the bikes with long exposed seat posts. I can ride a 58 without difficulty, that's what I curently have. It's just one stem size longer than normal, and if I stop in a stand over position I don't have as much clearance as I would like, though I can stand flat footed, as long as there isn't a dip in the road between my wheels... . Dimensionally I have a 33"+ inseam, but my wingspan is 75" which on a normally proportioned person would be a height of around 75 inches rather than 73.5.