Yes I am a mechanical engineer, but the math needed to understand bike frames only requires high school level geometry and trigonometry.
Anyone who would suggest fitting a bike based only on a rider's height isn't too sharp. As an example, I have the same 83cm cycling inseam that Cyccocommute has posted, but he's 5-6 inches taller, IIRC. I'm only 5'-6.5" or 169cm tall. My bikes would fit his legs, but not his much longer torso.
The three key dimensions of TTL, STA and HTL (with the headset) or better yet - the frame reach and stack - define the frame's possible range of bar height and saddle to bar reach and permit the comparison of the fit among any number of other frames. They do NOT magically fit the frame to a particular rider. If a rider has a fit that he can define, then I can match it with another brand and model.
When someone posts that they ride a particular size and model of bike and wants to buy a different brand that fits the same is when those dimensions are applied. If a person has never ridden a bike and knows nothing about what type of fit they want, I always recommend a fitting by a professional - not some guy whose main job in a bike shop is selling bikes and may have only a rudimentary understanding of bike fitting.
If a person posts only his height and cycling inseam, I always ask if he has some cycling experience and some idea of his preferred saddle height. Two riders can have the same inseam but if one pedals with a horizontal foot, his saddle height may be 2-3cm lower than a rider who naturally raises his heel. Neither is right or wrong, but it needs to be considered when selecting the best fitting frame. All you can do for a person with no expereince is steer them away from a frame choice that is likely to be much too large or small.
Last edited by DaveSSS; 12-20-09 at 09:57 AM.