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I have been riding Mountain Bikes since 1981 but as far as technical knowlege is concerned I would probably be considered a newby. I am planning on using most of the components I now have to build a Pivot Mach 5. My question is: I will be buying a new crankset, probably xtr m970. I am 53 years old, in pretty good shape but knees are beginning to show signs of age(not alot of cartilage left). Would I be better buying a longer or shorter crank??? My instincts say shorter so there is less knee bending but not sure if leverage is maybe more important. Maybe it doesn't make any difference. Thanks in advance for the advice.
 

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I'd say go with what fits your leg length and riding style best. I run 170mm cranks, but had always used 175's previously. The extra 5mm is for ground clearance, since where I ride is steep/rooty/aggressive. I think the issue with the knees is best resolved with proper seat height as opposed to crank arm length.
 

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Agree. Adjusting to a different riding position to use more of the upper leg muscle and glutes (usually means a little more forward) if that doesnt make your ride more sketchy would relieve the stress off the knee area. Used to have weak knees but it doesn't plague me anymore after I adapt to each bike's setup, regardless of crank length.

My 3 bikes-
roadie: 170mm
AM bike: 175mm
SS: 180mm
 

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ColoradoCoolBreeze
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Are you a spinner or masher?
Shorter allows for a faster/smoother cadience. Much like raodie or XC racing.
Longer gives you more torque and slow your cadience for a more relaxed ride..

I'm a 5'9" masher, 55 years old and prefer 180mm crankarms.
 

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I'm 6'3" masher, 56 y/o, and run 180mm cranks.
I initially went with the 180mm crank in an attempt to give me more leverage to make up for the larger 29" wheel on my RIP9, then experimented with the 180 on my 26" wheel bikes, and ended up putting 180mm cranks on all my bikes. I can tell the difference between 175mm and 180mm on long Granny-1 climbs and even long uphill fireroad mid-1 or 2 climbs, the longer cranks have my legs hurting less and breathing and heartrate is slightly reduced vs. 175mm cranks.

Mathematically it's about a 3% change on the leverage arm, but I can definitely feel the difference. I recently got an 05 Specialized Enduro frame that came with 175mm cranks so I left them on during my build up. Taking the bike up a 1.6 mile technical Granny-1 climb, I noticed more stress and strain getting up that vs. my other bikes. I swapped in a 180mm crank and was able to do the climb with my 'normal' effort.

If you are a spinner that pulls up with one leg while the other is pushing down, you might notice the longer crank arm has more rotational degrees of 'dead power area' (the areas of top and bottom of stroke).
 
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