Technical FAQ: Crankin’ through the air

Tech writer Lennard Zinn gets an answer on aero crank effectiveness from FSA

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Editor’s Note: Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.
Dear Lennard,
I keep seeing aero chainrings and “more aero” cranks and now Campy has released the Bullet Ultra and Bora Ultra aero cranks, and it makes me wonder, how much drag is actually caused by chainrings and cranksets?  How much is the drag reduced by putting a bulbous carbon fairing over your crankset?  Do you know of anyone who’s published the numbers on this?  I have a hard time believing it’s more than a second or two per kilometer in a TT, if that.
— Scott

Dear Scott,
Here’s FSA’s response on this from marketing manager Fletch Newland, who says, “This came before we began to realize how important higher yaw angles are so only goes up to 15 degrees.”

FSA’s chief engineer Ron Correa wrote:

“The data is from 2009 and was done at A2 (wind tunnel in Mooresville, North Carolina). Our Vision cranks show a two-to-three watt savings of power compared to our FSA SLK cranks, which, from our calculation, in a 40K TT (rider power 390W) using the Vision Tri Max Crank, would save them six seconds compared to a rider using our SLK-light. ([This was] at highest YAW angle tested (15 degrees) and closest to real world situation.) Considering TT’s can be won or lost in less time than this, six seconds could be the difference between standing on the podium or going home early.

To note: this result can vary depending on the side of the bike exposed and the YAW vector. So the data shown is an average of the DS (drive side) and NDS (non-drive side) of the bike because wind doesn’t always come from the DS. As you well know, very small changes can make the drag rise or fall, and there are so many variables it can make one excited and get a headache all at once. For instance, shoe make/shoe covers, size 12 or size 9, air off the front wheel and tire, bike frame used, cleat forward/back, cadence, chain position, head up/head down, knee flexing outward… It could go on and on and on… And there is that friction drag from a rainy day issue that can throw it all out the window or make it better. (In the Pacific Northwest we just tack it on in the sake of training purposes.) These reasons alone are why most companies do not test with a rider on the bike… Parts are easy to control… humans are not!”


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