Quote Originally Posted by jobob View Post
Frames vary a lot, in geometry, bottom bracket placement, you name it.

So don't automatically assume that if you fit on a X size frame from one manufacturer, the size X frame from another manufacturer will fit.
Well said, Jo!

If you're a complete newbie to buying bikes, get a professional to help. There are so many measurements plus musculoskeletal issues (such as how flexible and where you flex) that must be accounted for. Don't depend on numbers alone.

The labeled sizes I ride for both mtb and road are all over the ballpark -- it's just an arbitrary label used by the builder.