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Technical FAQ with Lennard Zinn: Feedback on crank length, ChainGate II, and BB30

2011 Tour de Suisse, prologue, Andy Schleck
Andy Schleck in the Tour de Suisse prologue starthouse. Photo: Gregg Bleakney |

Q. Dear Lennard,
I’ve been working on my own bikes for over a decade. Still use your book frequently for all those tips tricks compatibility guides and exploded diagrams. Love it.

Your crank length article was aptly timed for me. Just yesterday I happened across Frank Day’s blog and a lengthy (pun intended) writeup on crank sizes.

Frank starts off with Courtney Ogden, a 6-foot-2 pro triathlete who won an Ironman in Australia on 145mm Powercranks and is now riding 115mm cranks, which Frank assumes will shave another 10 minutes off Courtney’s time. I’ve seen the video, which explains that you can crouch into a more aero position (dropping the front end 10cm!) when you combine the shorter cranks and raising the seat for fit. It seems you would have a substantial loss in leverage, but Frank also claims an increase in power. How can this be?
— Mike

A. Dear Mike,
Well, since power is equal to work done per unit time (or energy expended per unit time), which you can otherwise understand as the rate of doing work on an object, in this case the bike and rider, then the fact that he has less leverage can be made up for by pedaling (with the same force from his feet) at a higher cadence, by pushing harder on each stroke (at the same cadence), or by a combination of both.

I can’t see why he would be able to push any harder due to the shorter crank, so in order to generate higher power with the shorter crank, I would guess that he is pedaling at a cadence that is enough higher that it makes up for the fact that the torque he is applying on the crank is less. (The torque he is applying is equal to the component of his pedaling force perpendicular to crankarm at any instant times the length of the crankarm, so if the force is the same and the crankarm is shorter, the torque is less.)

And, of course, if he can generate the same or more power and has less aerodynamic drag, then he will go faster. And unquestionably, the comfort on the crotch when the shoulders are level with the hips would be higher with a shorter crank than with a longer one. This is because the body would not have to be rotated so far forward around the bottom bracket (putting the pressure farther out on the nose of the saddle) to achieve that position without constraining hip movement. If your taint doesn’t hurt as much, you can probably pedal harder. Seems a stretch to me that it would work at 115mm or even 145mm, but if it works for him, more power to him (pun intended).
— Lennard

Q. Dear Lennard,
It appears not many have commented on the chain skip or whatever it was during the prologue of the Tour de Suisse by the Leopard-Trek team members Cancellara and Andy Schleck, who both use Trek bikes and Dura-Ace electronic shifting. The question is, what caused the problem?

Andy could not get his chain back in or on and switched bikes. Cancellara skipped right away out of the start, though he recovered.

Do you happen to have any details on this, as I could not locate any?
— Stefan

From Shimano’s Wayne Stetina:
I had also inquired and just got confirmation that Andy was running an unmatched combination of chainrings — the wrong 54 for the 42 inner. So his problem apparently was big/big crossover shifting to inner ring. Mick Rogers had same problem in Monaco TT opening stage of 2009 Tour while riding a 46-tooth inner selected for that course. When the chainrings are not a perfectly synchronized set, it can cause even more shifting problems with Di2 because the riders are accustomed to shifting under very high power, including while standing.

Regarding Cancellara, I still don’t know. I replayed the TV footage several times. Complete speculation on my part — I wonder if he might have shifted the wrong button to a lower gear instead of a higher gear??? Because he recovered instantly.

Q. Dear Lennard,
I have a question that is half technical and half asking for your opinion. I have plenty of experience building custom bikes and servicing bikes in the industry. Now I am trying to build a spectacular road bike for a friend.

The problem arose when my friend ordered a 2009 Ridley Noah frameset. What a beautiful frame by the way! He then proceeded to order another complete bike for the SRAM Red group, with plans to sell the frame off that.

The problem he was not aware of before coming to me was the Ridley has a English threaded (1.370″ x 24tpi) shell with a 68mm width, and the SRAM crank that came with his group is a BB30 design.

Now as far as SRAM Red cranks go, I have not determined if there is a different one that will work with the traditional threaded bottom bracket that he could purchase for a simpler solution and try to sell the BB30 crank.

My question is, if he wants to be able to use the cranks he has or even another SRAM Red design, what is the best long-term solution regardless of price. Are there any? And what is out there for conversion kits? Does anything convert traditional English threaded bb shells to work with the newer BB30 standard cranks?
— David

A. Dear David,
He should simply buy a SRAM Red GXP crank and bottom bracket. There is no such thing as a conversion BB for a BB30 crank to fit a threaded bottom bracket shell. If you think about the width constraints, it will be obvious why it won’t work; perhaps an article I wrote earlier this year will clarify it.
— Lennard