seejohnbike, I think the guy responding to my post is confusing straight vs flat top tube line. Of course the top tube itself is straight, but whether the tube is level in relation to the horizon is what I was referring to. And the top tube is part of the frame's geometry as it happens to be like you indicated effected by all other dimensions of the frame.

Wearyourtruth: As for chain tensioner inefficiency, I subscribe to it, whether you do is your belief:

http://en.wikipedia.org/wiki/Single-...ain_tensioning

"If neither of these are an option, when working with an existing frame not originally designed for single-speed use for example, some kind of idler pulley, such as the Surly Singleator[3] , can be used to take up chain slack."

Just me, but running a chain thru the tensioner, there is additional friction, friction = resistance, you can't tell me that with that resistance, that no matter how minute the inefficiency is, it is occurring. And not a myth when compared with a system that has no tensioner in the drive line. And if we go by a direct drive definition that you indicated, I'll disagree with that definition as there wouldn't even be a drive shaft, it would be 1 or even 2 gears interfacing at the tooth level with nothing in between, shaft, belt or chain. I say 1 gear, because a unicycle is a direct drive. 2 gears interfacing with each other to drive the wheels is a true gear driven drive train. Otherwise there is a shaft drive with universal joints in between, or a belt or a chain that can have derailers and tensioners involved.