Technical FAQ: Crank stiffness and bottom brackets

Do cranks flex, or is it the bottom bracket and frame?

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Technical Q&A with Lennard Zinn - Crank issues galore

Technical Q&A with Lennard Zinn – Crank issues galore


Dear Lennard,
Your recent response on regarding Shimano’s new HollowgramII bottom bracket mentioned VeloNews’ (print edition) crankarm stiffnesstest. I must have missed that one, but it did bring to mind a persistentquestion that I have: Can the average mortal – or pro for that matter –actually flex a quality crankarm, especially considering the planes offorce being applied?

It seems to me that what riders’ interpret as crankarm flex is muchmore likely to be frame and BB flex. I’m not an engineer, but it seemsimpossible for a human to apply the power through a pedaling action thatwould be required to flex a crankarm in a plane along its main axis (i.e.,BB to pedal).

From the test engineer who performed the tests

We ran several tests at Schwinn and GT where we had riders of all sizes sit on exercise bikes and pedal against loads. We also rantests where the chain was clamped to the frame of the machine. The firstperson we tested was a National-caliber track rider (he actually has a national championship in pursuit) we were all floored when he could easily hit 800 lbs. When he actually tried he could hit 1000 lbs. Most of ou rtesting was based on an old Schwinn standard and none of us were presentfor the original testing so we wanted to be sure that the test standardwas valid. It’s valid.The loads I used (in the VeloNews Vol. 32/No.19 November10 2003 “Black Gold”) were in the maximum 250-pound range to test crankarm stiffness, and I feel that most people given the right conditions could feel the difference between the stiffest crank and the least rigid crank.Many of the cranks were so close in rigidity that it would be tough to tell any difference. Larger and stronger riders would be more likely to notice a difference since they will apply higher forces. As far as whetheryou could tell the difference between a flat tire and a flexible crank, well let’s just say I’d be keeping my tires inflated!

Mark Rhomberg
Bike Testing, Inc.

Dear Lennard,
I just want to put in my $.02 worth regarding comments you made inthe magazine about the S-Works FACT carbon crankset. These cranksare closer in comparison to cranks than to Kent Carlson’s Sweet Parts cro-moly cranks! The Sweets are like Roger Durham’s Bullseye cranks, asymetric and steel! They don’t fasten in the center, or use the same type splines. If only the Isis type spline was around when Kent was making his Sweet Wings, the cranks might still be manufactured today!

Dear T.G.,
Actually, that is incorrect. The Sweet Wings, while they are completelymade of cro-moly steel as you said, do indeed fasten in the middle. Eacharm has a partial-length hollow spindle attached to it with a male splinedend on one and a female splined end on the other. There is a short bolt that you reach with a long 8mm hex key that holds the two together. I actuallyhave three sets here still, and I just snapped this photo of them. I have no more non-drive-side cups for them, since people have begged them off of me over the years, so I can no longer install them. They were way ahead of their time with oversized bearings and integrated, oversized spindles.

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