Bicycle geometry question

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annirak

Veteran
Location
Cambridge, UK
What is the advantage of seat stays that join the seat tube below the top tube?
E.g.
csm_Bike_Zoom_Headerimage_3800_1441_MY16_TMR01_Ultegra_side-1_abcfc3ef59.png
 

Slioch

Guru
Location
York
I'm not an engineer, so can't comment from a performance perspective, but from a purely aesthetic viewpoint, it just looks "wrong". I'm sure there are those who would disagree. :smile:
 
Location
Loch side.
There is no advantage and any such "advantage" stated by the manufacturer is post-rationalized in the marketing department, not created in the engineering department. You have to realize, with wheels, frames etc, where the shape has been refined over decades to perfection, every bicycle looks the same. The only way manufacturers can really distinguish their products is to break away from the norm and the proven. We see this all the time with every single bicycle component you can name..
 
Location
Loch side.
A shorter seat stay will flex less than a longer one. Simple materials science.
It is actually structural science, since the material has nothing to do with geometry. Nevertheless, why not just move the anchor point of the stays to just above the BB - that will give you the shortest seatstay possible and also expose the flaw in your reasoning.
 

midlife

Guru
Take it further and imagine the seat stays are as short as possible and joined at the bottom bracket, I have a feeling simple materials science will take a back seat :smile:

Shaun
 

deptfordmarmoset

Full time tea drinker
Location
Armonmy Way
In the picture, it's very very close to making an equilateral triangle. I know nothing about engineering but I'd guess that it's an economical, light and strong structure, with the proviso that it might need a strengthened seat post to compensate for the unequal stresses.
 

midlife

Guru
Might work if the frame was a geodesic structure?

Shaun
 

Citius

Guest
A shorter seat stay will flex less than a longer one. Simple materials science.

Not so. The length of the stay just means the stress points are re-located, that's all. Bugger-all flex in that frame anyway - especially from the stays.

If there's any reasoning behind those stays being where they are, it is probably to do with shorter stays presenting less profile to the airflow - given that it's meant to be an 'aero' frame n all...
 
Yup, it's aero. Though, by the time it's got to that area the air is pretty dirty so any advantage will be infinitesimally small. They've used an 'aerodynamic' seat tube, with greater fore and aft stiffness (than a round section tube of the same weight) which means that the seat stays can lean more heavily upon it.

There are some extremely unaerodynamic aerodynamic tubes being churned out at the moment.
 
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