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Question about frame geometry

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Question about frame geometry

Old 04-22-08, 11:43 PM
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Asian Sensation
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Question about frame geometry

*puts flame suit on*

just a couple questions.

On my tarmac, what are the benefits of having the kind of "bowing" top tube? advantages/disadvantages? seems like it would enhance the comfort

What are the advantages/disadvantages of a sloping top tube, I've heard what lance says about it but how about people on the boards?

Any advantages/disadvantages to different tube shapes? ex: "squaval" flatter top tube on my tarmac when the rest are all normal circular.
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Old 04-23-08, 01:16 PM
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no one?
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Old 04-23-08, 01:56 PM
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Aesthetics- is about the only advantage I can find. The way it looks. And specialised bikes do look great.
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Old 04-23-08, 03:22 PM
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The bottom tube being extremely thick and round on the Specialized bikes adds a lot of stiffness - the Tarmac is designed to be very stiff, meaning that no matter how much pressure you put on the pedals (due to weight or leg strength) there will be very little flex in the frame.

As for top tube shapes, I think it is largely aesthetic. Many Trek bikes, for instance, have an angular, very elliptical top tube. This is more aerodynamic, but a sonuva***** if you want to mount a battery for a light, an extra water bottle holder, etc...

Primarily it is about looks. There are a lot of different ways you can build a bike frame that still put a person in the correct geometric position to ride - specialized has an edgier look than many frames, and the sloping top tube allows the *potential* for a more upright position.. of course you can simply raise the seat/lower the bars and have the same amount of stretch as a bike with a top tube that doesn't slope.

It's mainly about style. My local bike mechanic also points out that the sloping top tube on specialized bikes allows for a more comfortable standover height. If you have shorter legs and a longer torso (like me) it allows you to fit into a frame size more comfortably - at least as far as clearance issues over top tube are concerned.

The bottom tube is where you will see whether or not the bike is built for stiffness or flex/comfort. The bigger and tougher looking the bottom tube and seat tube are, the stiffer the frame.

Last edited by KidTruth; 04-23-08 at 03:33 PM.
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Old 04-23-08, 04:31 PM
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It makes it vertically compliant yet laterally stiff.
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Old 04-23-08, 04:38 PM
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Originally Posted by Bontrager View Post
It makes it vertically compliant yet laterally stiff.
+1
A tube bent in the plane of bending is easier to bend. A tube bent 90 deg. to the plane of bending is harder to bend.
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