Originally Posted by
deraltekluge
Don't forget that that'd be quite different for bikes with horizontal top tubes than for bikes with sloping top tubes. Which are you considering? Bikes that are really the same size, and fit the same size rider, can have size numbers that are several inches different.
If you stay within the roadbike style you will find that the sizes are about the same depending on where the manufacturer takes the measurement......see post 2 above (the sloping top tube bike will still have the seat tube taken to the same height). MTB's may appear to have shorter seat tubes because their bottom bracket is higher off the ground. (same for most cyclocross frames)
Another significant tube length is the head tube. Shorter head tubes (the TT/TRI frame is the extreme limit of this) places the bars lower in relation to the seat because forks do not vary very much from axle to crown. (road bike forks or basic non-suspension corrected MTB forks)
Very short head tube frames may not be comfortable for most 50+ riders. TT riders ignore that statement please.