Bike ≠ Car ≠ Ped.
Join Date: Jul 2007
Location: Washington, DC
Bikes: Some bikes. Hell, they're all the same, ain't they?
If you're like me, and the frame's too small for you, your lower back will start hurting within half an hour or less.
That being said, after figuring out my own fit on my road bikes (the two I have with drop bars), along with the recent hybrid and newest MTB, the saddle-to-handlebar distance on all of them is nearly identical to my MTB that I got in college (and I picked that one after trying out three sizes of one bike model). Specifically, since I ride the drop bar bikes most often with my hands at the hoods, which adds about 2" beyond the handlebar clamp, that's the distance that I measured -- and I didn't even measure them until I got them comfortable for ME.
Frankly, I was surprised. The riding position of each bike definitely feels different, and the drops give me a more aero option, but I can't think that it's just coincidental that every bike measures 30" on the nose.
I'd say to check that distance to see if you're in the ballpark with the ol' Schwinn. On your flat bar bike(s), measure from the middle of the saddle to the handlebar clamp; on the old roadie, measure from the middle of the saddle to a virtual line that runs between the middle of the bars where they point forward (basically, about where the brakes clamp onto the bars, or maybe an inch rearward from that point).
If they're within a couple inches (at most), you're probably going to be fine. If it's a bigger difference than that, you'll feel it, and it might hurt after a long enough ride.