# Define "virtual" vs. "effective" top tube



## 03Vortex (Jun 17, 2003)

I have a simple question relative to a sloping frame geometry What exactly do these two terms mean and when deciding on a frame size, which of these two should you base your decision on?

Thanks


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## av8torjim (May 11, 2009)

Actually, I think the distinction is "virtual or effective" versus "actual". "Virtual" and "effective" mean the same thing.

With traditional geometry, the top tube is parallel with the ground. The distance between the centers of the head tube and seat tube running through the top tube is both the "actual" and "effective" top tube length. With a sloping geometry, the tube generally is lower at the seat tube. With a sloping top tube, the "actual" top tube length is shorter than the "effective" or "virtual" top tube length, which is the length the top tube would be if it were parallel to the ground. So, you need to focus on the "effective" or "virtual" top tube length when you are looking at a sloping geometry frame. I hope that made sense.

Jim


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## krisdrum (Oct 29, 2007)

Yep. Virtual and effective are the same thing. A horizontal line drawn between the seat tube and head tube. Since reach is one of the most important aspects of geometry as it relates to fit, that horizontal measurement is key in helping to determine how well a particular frame will fit you. Very few makers provide actual on a sloping top tube design as it is relatively meaningless unless you know to what degree the tube is sloped and then you'd need to do some math to get to effective/virtual anyway, which is a stretch for the average consumer.

av8torjim - actually I believe you have the longer/shorter relationship reversed. Actual TTs on a sloping tube frame are shorter than virtual/effective. The seat tube angle radiating away from the front triangle of the bike. The sloping TT hits the seat tube lower down its length and hence closer to the front end of the bike and doesn't have to travel as far as a horizontal tube would to make the same intersection. Or at least that is my understanding of it.


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## 03Vortex (Jun 17, 2003)

Thanks for the clarification and that all helps


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## Mel Erickson (Feb 3, 2004)

You've got it backwards. The actual TT length is shorter than the effective TT length.

Edit: I can't even spell right! Effectibe? :blush2:


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## av8torjim (May 11, 2009)

Oops. Sorry. That's what happens when I don't drink enough coffee before I post. I edited my post. The picture really helps. Thanks.


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