Stories of Zane riding his Mountain Bike in New Zealand

Update on the short cranks

Ok, so after reading some stuff, I tried out some 170mm cranks on my mountain bike after riding on 175mm cranks for many years. See my previous post for more info https://mountainbikingzane.wordpress.com/2009/09/21/shorter-cranks/

What I noted.

Shorter cranks are easier to spin when you get up to speed. Things like hammering across a piece of fast singletrack it was easier to spin at a higher RPM.

There was no appreciable difference in my ability to climb up technical climbs. I had thought that the reduction in leverage would make a difference, but I could find no difference in my ability to grunt or power over stuff, even from a standing start on a steep hill in a hard gear.

I still bashed my pedal just as much as with 175’s. The 5mm of extra ground clearance did not make me less of a Muppet when it comes to avoiding rocks with my pedals.

With a reduction in crank length, you are supposed to drop your seat a little to maintain the same femur/tibia angle at the beginning of the stroke. I did not do this, and I my pedaling really did not feel much different to normal.

My conclusion for my situation. Shorter cranks are probably more efficient than longer cranks if you are able to get up to speed and spin at high RPM for a large amount of your riding. For example time trialling or triathlon (like what they were talking about in the original article). For mountain biking where I tend to spend a lot of time at a lower RPM (even my climbing is quite low RPM) I do not really see much advantage to using the shorter cranks.

Doing measurements of my femur , the formulas for crank length actually suggest I should be on a 180mm crank, so maybe I am already on a short crank riding my 175s!! Some discussion of this crank length being proportional to femur length

here http://www.billbostoncycles.com/crank_length.htm

and here http://www.cyclefitcentre.com/pdf%20final%20docs/CRANK%20LENGTH%20ARTICLE%206_final.pdf

and here http://www.nettally.com/palmk/crankset.html

However, having a read of Sheldon Browns website it kind of all puts it into perspective. http://www.sheldonbrown.com/cranks.html

So that was interesting, and my girlfriend now has a shiny pair of 170mm cranks on her bike, as she really should be riding something shorter than the 175’s her bike came with. I will be sticking with the current Shimano M760 XT 175mm crankset  for a while yet.

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