Informative article on crank arm length

William Jenks

Nice little article on ST's front page about crank arm lengths and gearing:  http://www.slowtwitch.com/Tech/Crank_Length_and_Gearing_4095.html

Take home lesson:  If you shorten your cranks, you make your gears "harder" because of the shorter lever arm.  (You have to increase the Torque/Force, but over a shorter distance, so it's the same total energy...just more force = "harder"..  I did a calculation on my bike when I went to 165s that suggested that all gears would be about 1 gear "harder".  For me it seems to have been a good trade off, because it allowed me an aggressive low fit without squashing up my torso angle and lowering FTP.  You'll see a discussion of this.

Nonetheless, going "one cassette easier" is something to consider if you make the change.

Al Truscott

It's enough to make my head hurt. There are two many variables to make a reasonable equation, much less guesstimate in some sort of logical fashion.

I switched last year from 172.5 to 167.5 cranks. I kept the 650 wheels and the 53/39 chain rings. I alternated between 11/28 and 11/32 cassettes. And, as a result of the shorter cranks, I was able to lower my front end substantially - either 5 - 10 mm, with a higher saddle to account for the shorter crank.

So my hip angle was more open - "stronger" - and my position was more aero. It's very hard to know if those two effects more than made up for the increased force I needed to apply due to the shorter cranks. Which I calculate @ 1.7% when riding @ 20.3 mph, a common speed on the flats in an IM for me.

Anyway, I came away from the article not convinced that I needed to change my gearing at all when going 5 mm shorter in my crank length. The change in position (lower front end, more open hip angle) I believe enables me to go the same speed with less fatigue (the only metric that matters to me: how fast can I run afterwards?). It's kinda like comparing road to tri power - the aero position changes more than compensates for the decrease in power which the TT position imposes.

Gaudir8

Everyone sees what the pros are doing and think they must follow. The majority of the people that switched to short cranks appear to be going back. There was an interesting thread about this on ST a few months ago. When I first started the macho thing was how big a crank you were using. And I mean bicycle crank.

tim cronk

I have 2 tri-bikes.... My race bike is compact gearing and 165's I train and race on this bike..... But when I sent my bike to KONA I had to ride my old bike with standard gearing and 172.5 cranks for a week, and then ride it for 3 weeks inbetween KONA and IMFL.... Bottomline.... I could hardly tell one from the other!!! I also did some 7 mile TT loop tests and couldnt tell the difference between the bikes then either!!!

John Withrow

I disagree with his findings. I don't have time right now to go into details of why I think so, but basically with a smaller crank arm but same foot speed you can actually promote a higher cadence. with this higher cadence, each rotation actually produces less power. So if you wanted to maintain constant power with the same foot sped, then each single rotation with the shorter cranks will produce less power, thereby changing his crank arm torque equation.

Given how easy it is to change Cassettes or pick a different gear, I don't see how it would really matter for "most" AG'ers anyways.

And As Tim observed, these differences are so minute that most people wouldn't notice them anyways.

But what Al commented on is spot on. Going to a shorter crank nets you 2x the difference in length in lower bars, assuming one of of your limiters is your knees hitting into your belly at the top of your pedal stroke. This also allows you to open up your hip angle as well like he noted.

The Geeky BioEngineer in me loves this topic...

jendat

I have a 165 ultegra crank set that I am selling if anyone is interested in trying it out

William Jenks

I'm a convert, as I said...much like Al's experience.

John...be careful with your units. :-) there has to be the same amount of energy per full rotation. (Imagine the case that you are going on a 100% grade with an otherwise frictionless system.)

John Withrow

I still need togo back and do the math... But Jenks, In a 100% grade with an otherwise frictionless system, it takes the same energy per distance the bike travels. but if your cadence is different, then each full crank rotation will have a different amount of energy used. He was assuming constant RMPs and yes, the math would then mean that the "force" needed to be applied on a shorter crank arm would need to be higher. That is simple physics 101. However, I was considering constant foot speed, not RPMs.

Lets imagine a super extreme example, where the circumference of the shorter cranks was exactly half of the circumference of the long cranks. In this case, at the same foot speed, the shorter crank would need to make 2 full rotations for every rotation of the longer crank. Then if we were going up the same 100% frictionless incline, the same amount of work would certainly need to be done, but the shorter crank would need 1/2 as much work per rotation but do 2x as many rotations to get there. Now I haven't actually muscled through the equations to determine if that's better or worse, but it's not as obvious as he makes it sound in the article. I would think that If I were doing bench Press and needed to lift 2,000lbs, I'd be less tired after doing 20 reps at 100 lbs than I would be after 10 reps at 200lbs. And I know I couldn't even do 8 reps at 250lbs.

My nuance is I'm assuming the same effective foot speed. This will simple help to promote a "slightly" higher cadence at caonstant power. Obviously there would be limitations to this as you get to the extremes, but the difference we're talking about in circumference is not that great when we're only talking about a 5-10% change in crank arm length. But I'm guessing that the it makes the differences mostly negligible.