Correct Crank Length- Mtbr.com
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  1. #1
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    Correct Crank Length

    I ride a Rig, size small and have a 28" inseam. The stock Bonty crank is 170mm which is the same lenght I have used for years on all my geared bikes. I decided to try a 175mm crank on the Rig to see the effect of more leverage. I definitely like it with one exception, more pedal strikes. I ride very rocky terrain in New England the the pedal strikes are starting to bother me. With my EBB setup, I run the crank at the 10 oclock position so the crank is near its top point.

    My question is, as a rule of thumb, do SS'ers generally run 5mm longer cranks than their geared bikes? While I like the extra leverage, I'm not sure about the pedal strike issue. Maybe a 172.5mm crank is the answer. Any thoughts would be appreciated.

  2. #2
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    Proper crank length is one aspect of proper bike fit. If your cranks are the proper length for your legs but your pedals are hitting the ground, then the problem is not crank length, it is your frame's bottom bracket height (too low). Replacing the frame is a more costly solution to be sure, but nonetheless the real solution if that is the case.

    Here's a link to one crank length formula:
    http://www.nettally.com/palmk/Crankset.html

    Personally I use the same length cranks on my singlespeed and off-road gearies (195mm cranks @ 36" inseam). I do run shorter cranks on my road bikes.

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  3. #3
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    You'll learn the proper timing to avoid pedal strike with the new crank length eventually. Main thing is comfort. Take the bike out for a long spin on smooth trails or the road and see what the joints in your lower body tell you. Some would say that the advantage in leverage you gained can be had by running 1 or 2 teeth smaller cog or c-ring and spinning, which will be easier with the shorter cranks.

  4. #4
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    Thanks for the feedback. I'm beginning to think that years of riding 170mm cranks has me programmed for that length. I noticed a bit of knee pain yesterday when riding as well so the fit may be a stretch for me.

  5. #5
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    Quote Originally Posted by LIV2RYD
    Thanks for the feedback. I'm beginning to think that years of riding 170mm cranks has me programmed for that length. I noticed a bit of knee pain yesterday when riding as well so the fit may be a stretch for me.
    I have a ~27 inch inseam, and have been using 175mm cranks on my Soma B-side single speed for about 6 months, thinking that leverage of the longer crank arm was the key. Was bored last week and a friend (experienced rider) recommended trying some 170mm cranks to help with the spin and to alleviate the slight knee pain i was beginning to have recently. Two rides and I think the 170's are it, I'm running a bigger cog (winter riding) so it is easier to turn over, but my spin is better with the 170's. Another bonus is that with the shorter cranks, my legs don't come up as far at the top of the pedal stroke which feels better on my knees and throughout the entire pedal stroke.

    It is definitely a matter of body size and personal preference. I'm just glad i found mine!

  6. #6
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    My inseam measures 32 inches, and I've been using 175s for quite some time. The formula above agrees -- 175 is what it suggests. I was thinking of trying 180mm awhile back (the 5mm thing I heard too), but I worry that I'll be putting pressure on my knees (while seated) when they are too far flexed, which does cause knee issues. I'm curious about other folks impressions concerning running longer cranks on the SS...

  7. #7
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    I have a 30" inseam and run 170 mm cranks on road bike and all MTBs except the SS. The SS has 175 mm cranks. However, I am getting knee pain when riding the SS, so converting to 170 mm cranks on it as well. I'll see if this clears up the knee pain, or if the greater "mashing" up hills on the SS is what's causing the sore knees. Really don't want to go with easier gearing.

  8. #8
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    32 inch inseam, 180mm on SS, 175 on geared bike and road SS, 175 on geared road bike. No problems so far 9 years. Just have the seat and bars in the correct position you will be fine.

  9. #9
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    28" inseam and have been on a 170 for some time now, SS. I am a believer! LEverage is WAY over rating in regards to moving a single speed bike.

  10. #10
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    I ride some of the same trails as the OP. Have been for years and I've always ran 175s geared or SS. Granted I'm 6' with a 33" inseam. I can spin the 175s fine and like the extra leverage when mashing. Pedal strikes have just always been part of the deal for me.

    On my other bikes I have 172.5s on my CX and 170s on my roadie. It's just what works for me. If your frame fits well and you are set up correctly (saddle height etc.) you should be OK.

  11. #11
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    i have a 34.5" inseam and roll 180 ENO's on my rigid bike....they work great...couldn't be happier...

    BUT the formula says I should be pushing 190mm cranks!....do they even make those?...it's something i'd like to give a serious shot as I'm having a custom frame being built and would like to let the builder know what length cranks I plan on using
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  12. #12
    AZ
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    Quote Originally Posted by CHUM
    i have a 34.5" inseam and roll 180 ENO's on my rigid bike....they work great...couldn't be happier...

    BUT the formula says I should be pushing 190mm cranks!....do they even make those?...it's something i'd like to give a serious shot as I'm having a custom frame being built and would like to let the builder know what length cranks I plan on using

    Zinn's for the crankset , spendy though .

  13. #13
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    Quote Originally Posted by AZ.MTNS
    Zinn's for the crankset , spendy though .
    Buy the Zinn label if you insist, but you can save ~$120/crank by going directly to the people who make the Zinn cranks:
    http://www.hscycle.com/Pages/customcrankset.html

    I've got 3 or 4 sets of HSCC cranks in 195mm length -- they're awesome.

    --Sparty
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  14. #14
    AZ
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    Quote Originally Posted by Sparticus
    Buy the Zinn label if you insist, but you can save ~$120/crank by going directly to the people who make the Zinn cranks:
    http://www.hscycle.com/Pages/customcrankset.html

    I've got 3 or 4 sets of HSCC cranks in 195mm length -- they're awesome.

    --Sparty


    Cool , thanks for the link .

  15. #15
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    I run 177.5 on my road bike. I ran 180 on my 26" and I even run a 175 on a fixie. I just like long cranks

    6'4" 36 inseam

    I have been on a 177.5 for about 12 years on the road. Cadence was not compromised. Cadence was not a concern on the 26 and I like to lever and climbed at a slower cadence. It was recommended by a fit guru after watching me ride and measuring my femur length, seat angle and reach.

    I put the 180 on the 29er I bought. for whatever reason the 180 on the 29 felt odd. I just couldn't get over the top quickly as I liked. didnt spin well and give the power I thought tried to fit the bike better and still had the issue. I went SS for a bit like the 26 and still didnt feel it with the 180.

    for whatever reason I like a 175 on the 29 both geared and ss. It just feels like I can spin better and come over the top

    I am a bonehead when it comes to the techie stuff so I ask is there a relation to wheel size and gearing with longer cranks on a 29? I spin high on the road and on the 26 geared and ss I liked to lever and push at a slower cadence. On a 29 I really feel like I need to spin more. 32 20 SS and 12 34 geared....
    Why are there so many threads about cheap ass bikes?

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