Different Crank Lengths

unichall

Just wondering what the effect of different crank lengths has on cycling?

Do greater levers create more torque? Do smaller levers make it easier to spin more?

On that note, without getting a professional bike fit how does an amateur work out the right length for them or is it really that important?

FISMA

unichall wrote: »

Do greater levers create more torque? Do smaller levers make it easier to spin more?

Technically, yes, since there's a greater moment arm.

Longer cranks would also, technically, make the lean angle while pedaling less.

I use a 172.5 - right in the middle.

sherlok

I'm 6 ft 2 and use 172.5 cranks. A bike fit told me that I was on the borderline between 172.5 and 175 cranks due to my shortish legs.

i have bikes with cranks ranging from 170 to 175, and if i'm honest, i really can't tell the difference.

165 cranks often get used on the track for those fast spinning legs.

camaghead

Unichall

Here is a pretty good site on the matter - there are a variety of formula that can be used - each giving differing results.

http://bikedynamics.co.uk/FitGuidecranks.htm

I myself previously used a 177.5 but sore knees have me at a 175 or 172.5.

Lumen

Look ZED 2 cranks have adjustable length.

http://www.lookcycle.com/en/us/route/accessoires/pedalier-zed-2.html

doozerie

Lumen wrote:

Look ZED 2 cranks have adjustable length.

http://www.lookcycle.com/en/us/route...ier-zed-2.html

That's clearly witchcraft, but I like it! And it can take compact or regular chainrings too. In fact, the bike that they seem to have first shipped the ZED 2 cranks on also had an adjustable stem (more here), and it manages not to look like most adjustable stems which could pass for something that you built yourself out of Mecano when you were 12. The entire bike got a very positive review by Cycling Plus at the time, for what that's worth.

It's a shame that the crankset seems to only be compatible with Look frames/BB's though, it's also a shame that the rest of the industry can't be bothered producing something similar themselves.

macnab

I am 5'5" and recently moved from 175mm cranks to 165mm cranks. The obvious difference is less leverage when you put the power down. I have raised my saddle by 10mm which means there is less flex in my knee. I have noticed that I can stay out of the saddle much longer than before presumably because I am not moving my body mass through the same distance. Overall my average speed has not gone up or down that I can prove. I find it easier to spin faster with the shorter cranks. My knees dont ache as much as they used to. I can get into a better aero position.

doozerie

For what it's worth, I've had 175mm cranks on my mountain bike and perceived a difference to the 170mm cranks on my road bikes. They are several other variables involved there though, such as different bike geometry and wider bottom bracket on the mountain bike, which muddy things a bit. The single biggest unknown though, is whether I simply imagined the difference due to convincing myself that it should be detectable.

From what I've read, there is no a simple answer to the question of what is the "right" crank length. There are certainly very strong and opposing views on whether length is even significant at all but there doesn't seem to be a consensus for any of the arguments so you are left with basing your choice either on one of the various formulae for optimum crank length (based on height and/or leg length and/or and/or femur length and/or foot size, etc.) or going with what "feels" right (which, of course, is subjective). Those Look ZED 2 cranks above look like they'd be great for giving the opportunity to try out some different options easily and reliably, it's a pity that they are not compatible with a range of bikes.

Here are some sources that might inform, or confuse, further:
* www.customcranks.de discussion of crank length
* Peter White Cycles discussion

Lennard Zinn seems to have some strong views on this topic. In this article he supports the formula of (inseam x 0.216) to derive the appropriate length. At 6'6" he says he himself raced on 180mm cranks but prefers 202.3mm cranks since then. Nowadays his company makes custom cranks of up to 220mm in length. With those kinds of length though you also have to consider whether the bottom bracket shell of the frame needs to be higher in order to avoid the crank/pedal clipping the road when taking a bend, so significantly longer cranks potentially introduce other headaches (which might help explain why the industry generally doesn't serve the needs of those needing/wanting longer cranks very well).