Originally Posted by Finn04
Well I have learned quite a bit the last few days. I really appreciate the input. It is a difficult thing to determine and there seems to be many theories as to the right fit. I have a better idea at what I should be looking for now, and I do realize that my entry was rather vague. Apologies to those who had to endure yet another newbie's inquiries and thanks to those with the patience and time to share a little insight. I'll hit you all with more questions as they come up. Cheers!
There's nothing wrong with a newbie question. Sometimes the answers are, however... unfortunate.
There are three common sizing theories that I find highly suspect:
1) If your seat tube is some percentage of your leg length, the frame is the right size
2) The right frame size is based on a bike that gives you some amount of standover height
3) Frame size should be determined based on top tube length.
There are a number of secondary guides to fit that are equally suspect including handlebar hub obscuring tests, forearm length comparisons to cockpit length and plumb lines from knees to pedal spindles.
They are all oversimplifications. "Fit" is a matter of finding the right riding position for the intended use and the physical abilities and disabilities of the rider. Since two riders with the same measurements may have vastly different objectives, and be different in age, fitness and general health, the exact same frame may be perfect for one and awful for the other.
As different as riders are, so are frames as well. No single frame measurement is sufficient because there are no standards and a "56" from one manufacturer may be radically different from the same size from another manufacturer. A frame size is an exceedingly rough guideline.