Crank length and maximizing efficiency
 

Aside from meeting certain technical requirements that I and other riders may have, and probably for different reasons, it nevertheless is interesting to conjecture about how little we really know when it comes to the interplay between the various factors involved in crank length and power generation. Instead of 165s, traditional thinking would have me pushing 175s at least and probably 180s, simply based on overall height. But why... simply because the my thigh bone length is longer because on average the femur is 26-3/4 % of a person's height?

It is tempting to think in terms of analogies and imagine more easily moving a rock with a longer lever or what happens when we change the stroke on an internal combustion engine and apply what we think about that to human-powered cycles. But, we’re alive in a real not imaginary world.

Obviously, us humans are dealing with a relatively small amount of horsepower compared to an automobile. And, cycling involves more than simply rolling over a big rock. Ultimately, we're interested in maximizing the efficiency of our hips, thighs and etc. to generate the highest sustained amount of power, which modern cyclists measure in watts.

To carry out this power generation we know we need to satisfy oxygen and energy requirements and that we only have so much strength to work with; and, we understand that RPMs over time add up to total watts. Even so, without some sophisticated equipment and relevant controlled scientific studies what can we do with this information other than simply do what 'feels' right?

When it comes to RPMs or cadence, it reminds me of Fredrick Taylor’s science of shoveling whereby he showed that a big man with a big shovel could shovel more coal that a smaller man with the same shovel but more than that, the smaller man with a smaller shovel could shovel even more coal.

Is a longer crank arm the equivalent of a smaller shovel or a bigger man with a bigger shovel? I don't know the answer to that or if ‘big’ in this context even translates to the length of a person’s femur when it comes to cycling. I suspect, however, that the answer is related to cycling cadence at a given resistance and determining the best method to maximize efficiency.

Oh dear.

Crank Length - Cervélo

Increasing crank length will decrease the distance between your chest and legs. This can make breathing difficult when trying to stretch into an aero position. I can't find the study but I heard that someone increased their ftp by 60w, or something similar by shortening their cranks.

Increasing crank length does increase torque, but it doesn't always increase efficiency and power. Try loosening a screw that isn't very tight, if you use a long ratchet the screw will be incredible easy to loosen but it will take a long time to do so. If you use a screw driver, the possible torque drops dramatically, but you are able to unscrew it much faster.

I may have seen what you referred to --i.e., this quote from John Cobb about a rider gaining 65w of power:

In the same article Cobb also is quoted about the results of wind tunnel testing: "one triathlete in which the change to shorter cranks and a correspondingly more compact position yielded a 30% reduction in drag. That means a theoretical reduction in time of 25 minutes over the Ironman bike distance!”

See more at: Crank Length: Coming Full Circle

Greg Lemond would ride 180mm cranks for time trials. My friend, who is a velodrome sprinter and was a Pan Am Games alternate, is 6'5" and rides 165mm cranks on the track and 170mm on the road. Crank length is a difficult aspect to understand.

Over time pedal speed may play a role and perhaps a bigger factor than cadence --e.g., if you're comfortable at a certain pedal speed, then moving to a shorter crank won't feel right until you switch to a higher gear to bring the pedal speed back down. And, if that feels comfortable -- even after going to a higher gear -- you end up pedaling in a higher gear but at the same pedal speed and that translates into more power compared to sticking with longer cranks.

A pedal attached to a shorter crank arm moves through a smaller circle. Thus, at any given cadence, shorter crank = lower pedal speed, not higher pedal speed.

Right. OP either had a mistype or is really mixed up.

there's gotta be at least a million existing crank length threads.

I went from a 172.5 to a 165 and have felt absolutely no efficiency change but it did help my knee problem.

Nope... Cobb was saying it like it is.

What difference it might make is anybody's guess. Even so, Cobb is right in that at any given cadence (rpm) -- e.g., looking at a single revolution -- the pedal that is attached to the longer crank will cover more distance compared to the pedal that is attached to the shorter crank. Accordingly, the pedal speed is higher, the longer the crank.

It's similar to increasing the stroke on an internal combustion engine. For each revolution of the crankshaft, the piston in the engine with the longer stroke will be moving much faster.

Crank length is one thing that I believe could be optimized for every person, but I am not sure anyone has come up with a systematic way to go about it.

Ride different cranks. Ride each one long enough to get to know it. Observe. This is so individual that going to a cookbook for the answer is a crap shoot.

I had always ridden 170s save on my very low BB fix gear which I put 168s on. My second season of racing I purchased a real race bike with 175s. With the same wheels, I matched my best ever training loop time going deliberately slow! And the times kept on falling. Obviously a lot was right about that bike, fit-wise, including crank length.

Ben

A few weeks ago, my girlfriend and I rented road bikes and went off on a forest service road. (We were on vacation.) I got a bike with the frame and stem size I'm used to, but instead of went from 175 mm crank arms I got 170 (?).

The bike felt awful. Seemed to fit at first glance but it felt ... wrong. Sluggish? It was hard to put in words at the time and it's been a month. I think I just hate shorter crank arms.

I would be really shocked if the 5mm in crank arms difference was the only thing that made your rental bike feel sluggish.

Any guess what else might have contributed? I know that's not really a fair question without being able to see the rental, the bike I'm used to, or me. But it's a lot more convenient to rent a bike when you arrive than to bring one with you and we're planning to go back before the summer is out...

Some bike rental companies fill the tubes with snails and slug like creatures. This is known to make rental bikes slug-ish.

Not that kind of sluggish. Wasn't that it was slow or heavy, it was uncomfortable and awkward to pedal.

The mountain bike I got from them after I gave up the road ride was awesome.

I'm sure there's a lot of physics behind it, and perhaps it makes a difference to the very elite athlete, which I am not. I have a road bike with 170mm, 172.5, and a 175. I can't tell the difference when I'm pedaling. My feet are revolving circles with a difference of 1 cm in diameter. The difference in total distance traveled is 3.14cm (106 cm vs 109 cm). For me there are other things about the bike that make a much bigger difference in the riding experience (e.g., compact vs standard crank, choice of cassette, frame geometry to name a few)

Try putting a 170mm crank on one side of the bike with the 175mm on the other. You'll be able to tell the difference.

I don't overthink it. I've got 170 and 175 cranks on various bikes and it really doesn't make any difference to me. I'm figuring my cleats could be off by at least half the 5 mm difference in length, so why worry? Just ride

Thanks, Captain Obvious.

Exactly.

According to the literature, it's about a 5% difference or like going from a 50 to 53 ring or going to a 1-tooth smaller cog on the high end. Mostly, it's not a big difference so if other reasons exist for having to go with shorter cranks, there's certainly nothing to be concerned about, especially in these days of compact cranksets like mine that have a 34 ring, paired with wide-ranging 11/32 freewheels. It could be that there's really no more involved than knowing there's no downside to going with a modestly shorter crank and for some there may be something to gain by it. There are some practical limits on experimentation because it's going to be pretty difficult going with 160mm cranks on a XL bike out of the box but swapping ubiquitous 175s for a 165 is definitely doable. It's interesting too that the traditional thinking on the issue -- which has become what's expected and accepted throughout the industry -- may not amount to much more than superstition and ignorance.

If you are short and ride 170's and they work for you - good for you. Aren't you lucky.
If you are taller and ride 175's and they work for you - glad your happy.
I'm 6'5" and ride 195's for road, 185's for crit and TT.
I do that because there is a significant performance and result improvement.
And I also use longer pedal spindles to widen the q factor because I have a wider stance than shorter narrower stance bike industry standard people.
So if you are taller, and haven't tried nor considered readily available possible performance improvement equipment changes - you are selling yourself short.
Don't buy the kool-aid.
And if you do buy retail and the OEM marketing department or purchasing personnel made the crank length choice for you - Don't drink the kool-aid!!