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Overthinking Crank Length


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So after a lengthy time riding only my HT I've noticed how rarely I have pedal strikes on it vs my FS. Lots of differences between the two from Rear shock vs No Shock, 26" vs 27.5", BB height, 175mm Crank vs 170mm, etc. 

It got me thinking that one of the only things I can change is the crank Length. Will I see much of a difference only loosing 5mm? Should I consider shorter?

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Yes - you are overthinking it. But I have had such difference in experience I'm not sure what I've learned.

I put 180 cranks on my single speed when I built it. I liked them. So I replaced the 175 cranks with 180 cranks on my geared bike. My climbing actually got worse. A few months later I went back to 175 cranks on the geared bike. I found my climbing improved with the 175 cranks. Actually I think my climbing improved over what it was when I started on 175 cranks. So what I think I've learned is that my fitness / skill / practice made more of a difference than the crank length.

If you google crank length I found articles that said about the same thing - unless your crank length is way way off, it does not make much difference.

Of course - YMMV.

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Ive found that on my FS I get strikes but only when I dont lock out the suspension,, the more squishy I adjust it, the more I strike

When I lock her out ,, No strikes , the BB and ride height is higher. If I adjust the suspension to allow squish I strike 

On my HT,, I never strike

It isnt my bike geo  necessarily but the fact my frame fluctuates up and down 

 

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There are old articles out on the internet with tables that show the correct crank length for your size (inseam measurement).  But the best thing I ever read was an article that explained that a person’s optimum spin RPM was directly related to their % fast twitch and slow twitch muscle composition.  It also said that geared bicycle racing was the great equalizer.  A normal person could effectively compete with an Olympic sprinter in a bicycle race by picking the correct crank length, gear ratio, and spin cadence.  In other words a naturally quick/fast person might be spinning a 160 rpms in a small chain ring & big cog while being passed by a bigger stronger slower person spinning at 80 rpms in a big chain ring & small cog.

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When I built my last FS bike I used 170 cranks. after only ever using 175.  5mm made a very noticeable difference.  It felt like I lost a low gear, but I definitely had less pedal strikes.  I think over the last year I've adapted and learned how to peddle in even more precarious situations and get the pedal strike number back up.

Edit: I'm 6'2"

Edited by Kyle
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Ultimately I believe that the human body will adapt to any bike over time. This is both a good and bad thing. However, I do believe that if you can get the crank length/gear ratio optimized before your body figures it out you'll be in great shape. Pedal strikes tend to happen on new bikes and new cranks, generally they work themselves out in my opinion. 

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I'm not even going to try to make sense of all the mechanics, but until last year, I had ALWAYS ran with 175 cranks. To me, it was just the size I should use (probably read and magazine article in the mid 80's). Fast forward 30 years and by chance when I ordered a new crankset for my upgrade to 1x12 they sent me a 170 set by mistake. Figured WTH, and gave them a try. Wow, from perceived pedal efficiency to reduced pedals strikes it was pretty significant. I'm just shy of 6' in height. Not sure what the moral of this story is....

maybe don't trust all the numbers you read in magazine articles from the 80's?  

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Thanks for the input everyone. You've convinced me its worth the experiment to swap my cranks.  Fortunately for me both my cranks are the same brand so I just swapped the 170mm cranks on my HT to my FS and the 175 cranks from my FS to my HT. 

Interestingly enough, I measured the static BB height at the (27.5) HT is ~12" and the (26") FS is ~13.7" So I guess suspension sag must play a big role too. 

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22 hours ago, RedRider3141 said:

Thanks for the input everyone. You've convinced me its worth the experiment to swap my cranks.  Fortunately for me both my cranks are the same brand so I just swapped the 170mm cranks on my HT to my FS and the 175 cranks from my FS to my HT. 

Interestingly enough, I measured the static BB height at the (27.5) HT is ~12" and the (26") FS is ~13.7" So I guess suspension sag must play a big role too. 

25-30% sag will affect your crank length, or so she says.

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At the risk of more goat jokes, here are my initial results:

I rode Mulligan and 1/4 notch last night and was amazed at the reduced # of pedal strikes. Sure I had a few, but I was peddling much more aggressively. Might have been 90% mental but I was able to clear a lot more of the uphill, notchy climb last night than I ever had before (riding E to W or Clockwise). I also felt that I was more able to get "back on the power" no matter where I was in my stroke.

I'm surprised at how different 5mm felt. There I said it, are you happy?

The only downside Is my longer, nearly new, XT cranks are now on my HT with an almost 2" lower BB. More testing is needed.

image.thumb.png.889dfbcaa0a3e6f2dd3852164b627c3f.png

 

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