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Effectiveness of Aero Cranks
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TOPIC: Effectiveness of Aero Cranks

Effectiveness of Aero Cranks 7 years, 7 months ago #26218

  • dlinenbe
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Ever looking for an advantage in racing TTs, I was wondering if anybody here had any thoughts on the effectiveness of aerodynamic cranksets. For example, Zipp claims a 9 - 25 second advantage in a 40 Km TT for their Vuma Chrono cranks over other cranks.

I was under the impression that in this particular region (BB/Crank/foot) - there was a lot of dirty air - and that it would be very hard to model such behavior. (I wonder if zipp did tunnel tests of the crank/bike alone - or actually put a real person (or shoe/leg/model) in that area, to get a handle on this..)

Thoughts?

Thanks,
Dave Lineberg

Re: Effectiveness of Aero Cranks 7 years, 7 months ago #26221

  • djconnel
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Is the air dirtier on the crank than on the rear wheel? Yet rear disks are clearly effective. Consider the rear wheel: a Zipp disk is worth around 2% relative to a Stinger 9 control. The ratio of the radii of a crank to a wheel is 27 inches:2.1 meters = 32%, but then counting that the crank might only help on one side, that yields a 1/3% advantage to the crank. Super-back-of-the-envelope, okay... but useful. Consider a 1% reduction in wind resistance = a f / (1 + 2f) % increase in speed, where f is the fraction of wind resistance power (90%?). That means 9 seconds / hour = 1/4% increase in speed = 0.8% reduction in wind resistance.



So that's more than my back-of-the-envelope 1/3%. But then the Stinger 9 is already an "aero" wheel wheel and I don't know what Zipp is using as its control.

Re: Effectiveness of Aero Cranks 7 years, 7 months ago #26225

  • kraig
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Thoughts?


I think it's possible that "aero" cranks might not be so "aero". What are the specifics of the Zipp claim?
-kraig

Re: Effectiveness of Aero Cranks 7 years, 7 months ago #26232

  • dlinenbe
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The claim, from Zipp's website is:

"With so much on the line at Kona in 2008, what could convince top pro triathletes to use any equipment that they hadn’t raced on before? For Belinda Granger, Jo Lawn, and T.J. Tollakson, the answer was 9 seconds over 40 kilometers. Zipp's VumaChrono is simply that much faster than other aero cranksets. Compared to traditional cranksets, the savings can exceed 25 seconds."

Re: Effectiveness of Aero Cranks 7 years, 7 months ago #26235

  • kraig
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dlinenbe:

Yeah, I guess I was hoping that there was more meat to the claim - y'know maybe something that addressed your initial questions. Sounds like their _actual_ claim doesn't help us get closer to the "truth" does it?

I'd still say it's possible that "aero" cranks might not be so "aero".
-kraig

Re: Effectiveness of Aero Cranks 5 years, 6 months ago #26940

  • mfrassica
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OK, I know this post is old. However I have been thinking of this topic as of late. So, the crank at the top half of rotation is moving with omega (rad/sec) x crank arm length for a point near the pedal axel.
So the relative wind velocity is higher than just the bike frame velocity. Plus the riders leg is near the crank arms at several positions of rotation. So we may have a effective cross section larger than just the addition of the riders lower leg + the crank arm thickness. This is similar to
a swimmer holding their fingers slightly separated to increase effective area of their hand.

So does an aero crank arm help or hurt? Not sure still. The flow of air around the leg will be separating at about the point that it comes close to the crank arm. Not sure if this would help create turbulent flow around the crank and therefore act as a trip-wire and help the flow no mater what the crank cross section is? Any other thoughts or data out there?

Re: Effectiveness of Aero Cranks 5 years, 6 months ago #26941

  • kraig
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mfrassica wrote:
Any other thoughts or data out there?


I think it would be good to do a first-order type of exploratory test similar to what shoe cover researchers have done in the past when it comes to aero cranks - though, I'd suggest adding an additional component of beta/crosswind angle:

www.intechopen.com/source/pdfs/16686/InT...ntal_approaches_.pdf

..then, I think I might start listening to mfr's making claims about how aero their cranks are. I'm a fan of multiple, independent, lines of inquiry converging...e.g. I'm pretty convinced that aero wheels work due to the multiple lines of inquiry that have converged. when it comes to behind the seat aero bottle placement, pedals, and crank arm geometry, well, I reckon I'd like to see some more convergence by independent research.

Fat crank arms seems to be a step in the wrong direction, if you want my gut reaction, though.
-kraig

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