Bicycle Mechanics - Frame shapes

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johnwilson09
06-08-09, 11:20 AM
This isn't a specific issue, more a pondering I hope someone might be able to answer! What are the reasons for differently-shaped top and down tubes on road bicycle frames? They can be straight, curved or tapered, with either a circular, triangular or square cross-section. These can be in different combinations on different bikes (as shown in the following images). Is this purely stylistic, or does it have some functional importance?

http://www.comparestoreprices.co.uk/images/bi/bianchi-c2c-via-nirone-7-alu-carbon-ultegra-105-10-sp-double-2007-road-bike.jpg

http://www.giant-bicycles.com/_upload_se/bikes/models/zooms/2008/SCR%202%20Gts%2008.jpg

http://www.daviswheelworks.com/bikes/specialized/images/road/06roubaix_lg.jpg

greyghost_6
06-08-09, 12:21 PM
The mfgr. would claim aerodynamics and strength to yield a stiff ride. These bikes you have pictures of different frame styles not just tube dimensions. The Bianchi is a more traditional style frame geometry. The Giant is a compact frame. The specialized is probably a mix of the two. Every Mfgr probably claims theirs is superior. In reality they all offer something the other doesn't which probably evens out.
Wordbiker
06-08-09, 01:23 PM
What are the reasons for differently-shaped top and down tubes on road bicycle frames? They can be straight, curved or tapered, with either a circular, triangular or square cross-section. These can be in different combinations on different bikes (as shown in the following images). Is this purely stylistic, or does it have some functional importance?

The differences can be just styling and marketing flair, but shaped tubing can and does effect ride characteristics.

By shaping the tubes manufacturers can tune the stiffness of the bottom bracket, tweak aerodynamics, tune out unwanted lateral flex, give more road compliance, allow for more standover clearance while maintaining the same geometry as a bike with a longer or shorter seat tube...or just give the bike a unique look.