Mountain Biking - Crank Arm Length???

I was just wondering how you all determined a preferable crank arm length. I currently ride a 175mm and was wondering what the downs and ups were of switching to either 180mm or 170mm arms other then the obvious differance of tourque delivery to the rear. Thanks!

its all based on your legs and the lengths of the various parts, there is a lot of info on the net about it. Longer arms give better torque but you cant spin as fast. There is always a compromise. The key is to find what works best for you. There are formulas to help determine the right size.

The formulas say I need 175s (I'm 5'11"), but I prefer 180s on my mountain bike. I don't spin much, preferring to turn a higher gear slower. If you can find a set of inexpensive 180s, give them a try. Good luck on that though. I got lucky and found a set of square taper Icon 180s at MTBR for $40, but I doubt I'll get that lucky again.

Chris

i've heard that this is actually a bit controversial of a subject. there doesn't appear to be one generally accepted rule. but, one thing for sure is that you don't want a crank that's too long. it can mess up your knees and even though you will get greater leverage out of the longer arm, you will have to bend your leg so far that your effective power actually goes down.

a shorter crank will allow you to pedal at a higher rpm, which can have a similar effect to the increased leverage of longer cranks. it seems like a personal decision to me, depending on your riding style. if you're tall and like to pedal at slower speeds, go with longer cranks. but, if you like to spin at high rpm's, then go with shorter cranks.

some potentially helpful links:

http://www.cptips.com/crnklth.htm
http://www.bicyclesource.com/bike/fitting/crank-length.shtml

for more info than you're probably looking for:
http://www.arniebakercycling.com/pubs/Free/Optimum%20Crankarm%20Length.pdf