170mm or not 170mm? (pros and cons with shorter cranks)

qpcr8tiv · **Posted:** Sat Oct 25, 2008 7:46 pm

I've been hearing how some riders are going down in length for their cranks (170mm). I believe this is more true for Freeriders; it could be more stable to have your feet closer together going through the air as well as being planted more over your pedals (this is my theory anyways).

I remember racing on 190s on my 20 inch bikes back in the day and if you rode anything shorter that that, it was just lame (since that's what Stu Thompson and Greg Hill were rockin) :bang:

-- the belief was, the extra length, produced more leverage for getting out of the gate faster and helping getting you that holeshot.

That said, for mountain biking, XC or Freeride, you really don't need longer crank arms, gears help you with that. Just curious what the general thought is out there and what some of you might have read or experienced yourself.

OffRoadie · **Posted:** Sat Oct 25, 2008 8:03 pm

If you get shorter cranks, you'll have to spin faster at the same pedal force for the same amount of power.

Red Hot Sloth · **Posted:** Sat Oct 25, 2008 8:09 pm

My belief is that if your not DHing (race stuff) or free riding or bmxing...basically doing mostly trailriding stuff then the crank arm length is dependant on leg length. Riders with longer legs need a longer crank and riders with shorter legs need shorter cranks. There are a few varibles that come into play, but the article I read several years back explainded what sizes for what leg length. It stated that when riders were prone to sore knees that crank length played a part in it. Too short of crank length for a rider with longer legs will put too much pressure on the knees causing pain, and vice versa.
Some riders will opt for 170mm cranks to help decrease chances of pedal smack on rocks and other obstacles but at the cost of comfort and leverage.
I think the article was in Bicycle magazine about 7 years ago..maybe a little research will help.

took · **Posted:** Sun Oct 26, 2008 8:43 am

I think most XC bikes run 175mm . DH bike or bikes with 7" or more travel might want to run 170 or 165mm cranks . I race BMX and run 180mm on my bikes alot of people swithched to 175mm few years back saying your spin is better . I like the leverage out of the gate with 180mm . I have 175mm on all my MTB bikes though and will have 170mm on my Demo 8 .

qpcr8tiv · **Posted:** Mon Oct 27, 2008 10:38 am

took wrote:

I think most XC bikes run 175mm . DH bike or bikes with 7" or more travel might want to run 170 or 165mm cranks . I race BMX and run 180mm on my bikes alot of people swithched to 175mm few years back saying your spin is better . I like the leverage out of the gate with 180mm . I have 175mm on all my MTB bikes though and will have 170mm on my Demo 8 .

Nice. I almost picked up a demo 8 on craigslist but it wasn't exactly my size. I think it'll come down to primarily what kind of riding I'll be doing. If I use my bike for all purpose bike, even though its a freeride, I might tough it out and stick with the 175mm it comes with and see how it goes. but it the long run, I think I might pop for 170s later.

My curiosity for going 170 route is that when I'm speed pedaling over terrain and lean back past my seat, I feel like I have to extend more on the stroke as I'm going over the obstacle. So, with 170, that movement should be easier (less extension and more power).

I heard on this forum that guys are going out to kearny with their kids racing but didn't know what age group. Just curious, do they have a class for 40 yr old and up in BMX these days :lol:

luna · **Posted:** Mon Oct 27, 2008 8:43 pm

Yeah the old dudes are running cruiser class, usually first moto so you can help with your kid ready for there moto.

For BMX Crank Arm length here's a good article. http://www.bmxbasics.org/new/bmx1297.html

qpcr8tiv · **Posted:** Mon Oct 27, 2008 10:17 pm

luna wrote:

For BMX Crank Arm length here's a good article. http://www.bmxbasics.org/new/bmx1297.html

Thanks for the article. More to chew on. I'm guessing I'll make the switch to shorter cranks sooner rather than later.

Ld00d · **Posted:** Tue Oct 28, 2008 8:21 am

i wouldn't mind a longer crank...

DHP · **Posted:** Tue Oct 28, 2008 10:25 am

I'm running 170mm SLX on my new Rogue and they feel great so far. It even has a high bottom bracket (14.75 unburdened I believe) but I prefer the shorter cranks for huckin and jumpin.

170mm or not 170mm? (pros and cons with shorter cranks)

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