Originally Posted by
Andrew R Stewart
Var 171 measures set back from the vertical line extending up from the BB's center to the seat tube's center. This is the short side of a right triangle. The vertical is the long side and the seat tube is the hypotenuse. From these you could derive the angles but this set back, for a certain "frame size", could be directly transferred to a drawing (and therefore establishing the seat tube angle). Here's a shot of my 171 and it's horizontal scale. Andy
I think we all understand how it works, just not what you do with that 14cm result it produces, especially when the tool doesn't give a vertical distance to go with it. What process does that number assist?
In theory, you could build a frame just by knowing all the center to center lengths of the tubes without referencing angles at all.