It all comes down to geometry. A bike frame is essentially two triangles. Both the angle and length of each side of the triangle is determined by its two other sides. EcoVelo has an interesting look at how geometry affects the fit of a bike, even when the triangle is the same height, the angle can yield very different results. This is true to determine stand over height, which the article discusses. Another consideration in bike fit is the reach. Very often a road bike will have a longer reach for better aerodynamics. When I bought my last road bike, the shop owner pointed out that Felt bikes tend to have a shorter reach, which is a better fit for me and that became the deciding factor. It’s a good idea to consult a good shop that will be able to fit you for a bike, and also know the differences between the different brands, for example the reach on Felt bikes, and be able to help you find the bike that will fit you best rather than just make a sale.
via [EcoVelo]