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crank/chainring aerodynamics

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crank/chainring aerodynamics

Postby Fourthbook » Fri Sep 30, 2011 2:52 pm

I was struck by the cobweb chainrings (very cool) on the FWB fully custom Crumpton but wonder about their aerodynamics. Does the lace-work create turbulance in the same way spokes do on wheels? In comparison, do the relatively sleek-surfaced DA-7900 cranks/chainrings tend to reduce it in any measureable/meaningful way in the same manner a TT wheel reduces turbulance: AKA, does the design of the crank/chainring have an aero effect of any significance?
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Re: crank/chainring aerodynamics

Postby rlpaul » Fri Sep 30, 2011 11:10 pm

The crank and chainring do have some affect on aerodynamics. However, its probably one of the last places you want to look at changing, as other parts will have a larger difference.

Not sure how accurate this is, but according to an article on Sheldon Brown's website, removing the inner chainring is a .3% difference, or 3 seconds in a 40k TT. http://www.sheldonbrown.com/rinard/aero ... namics.htm

What I don't know, is how this relates to aero vs non aero chainrings. I'm sure there's data on it somewhere in the slowtwitch forums :)
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Re: crank/chainring aerodynamics

Postby Ypsylon » Sat Oct 01, 2011 7:18 am

That's 3 seconds if you TT at 37 km/h, less if you go faster. Also I take it they are talking about changes to a round tubed bike.

I'm rather certain that will be different on other bikes.

I am of the opinion that the crank area is one of high turbulence either way and therefore does not work in the same manner as a TT wheel.

Then, I thought the "myth" of more spokes more drag had been debunked or at least questioned as of lately. The spiderweb is much tighter, so the "spokes" would draft of each other, too.

I'm sure you can optimize crankarms and rings to save some time, but honestly, I would just get the chainrings I like, maybe because they look cool, maybe because they shift well and not let get aero concerns in my way.
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Re: crank/chainring aerodynamics

Postby Fibre-Lyte » Sat Oct 01, 2011 1:15 pm

Firstly, I'm not sure I'd ever look at the 'cobweb' chainrings and wonder about their aerodynamics. I think they are about the look so as long as they shift well, they'd be great on a bike, but I wouldn't consider putting them on a tt bike :D Similarly, I wouldn't really consider the 7900 crankset to be sleek surfaced. Sleeker may be, but not sleek (in my opinion)

As for general chainring aerodynamics, I think that an aero chainring/crankset can make a difference but how much is relative. I think if you're at the top of your game and every micro second counts you will notice a difference. As Ypsylon said there is a lot of turbulence around the crank area, but the area between the crank arm and bike looked at independently is quite clean air. Of course then you have to consider the air coming through the frame, the air coming through the centre of the chainring/crank (if it's not completely aero) and any dirty air 'thrown across' by the crank arm movement.
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Re: crank/chainring aerodynamics

Postby elviento » Mon Oct 17, 2011 8:00 am

Great point, Yps. In fact, if the cobweb is tight enough, it could act like Zipp's dimples...

Ypsylon wrote:That's 3 seconds if you TT at 37 km/h, less if you go faster. Also I take it they are talking about changes to a round tubed bike.

I'm rather certain that will be different on other bikes.

I am of the opinion that the crank area is one of high turbulence either way and therefore does not work in the same manner as a TT wheel.

Then, I thought the "myth" of more spokes more drag had been debunked or at least questioned as of lately. The spiderweb is much tighter, so the "spokes" would draft of each other, too.

I'm sure you can optimize crankarms and rings to save some time, but honestly, I would just get the chainrings I like, maybe because they look cool, maybe because they shift well and not let get aero concerns in my way.
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Re: crank/chainring aerodynamics

Postby Fibre-Lyte » Mon Oct 17, 2011 11:32 pm

You mean like a price multiplier :grin:
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