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  1. #1
    Senior Member toneb's Avatar
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    Crank arm length 170 172.5 175 whats the difference?

    Crank arm length 170 172.5 175 whats the difference? I might be getting a new bike and the sales rep asked which size I wanted. I told him standard size but is there a reason for different sizes?

    I'm 5'9

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    I am 5'6 and a 1/2 and have 170's.

  3. #3
    Senior Member 04jtb's Avatar
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    I'm 5'11 and i ride 170s on all my bikes too
    Quote Originally Posted by cc700 View Post
    i jam my thumbs up and back into the tubes. this way i can point my fingers straight out in front to split the wind and attain an even more aero profile, and the usual fixed gear - zen - connectedness feeling through the drivetrain is multiplied ten fold because my thumbs become one with the tubing.
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  4. #4
    You blink and it's gone. rbart4506's Avatar
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    I'm 5'9 and ride 172.5's. I had 170's on the old bike, but got 172.5 with the new bike. The 2.5mm extra in length added a bit of leverage on my pedal stroke and I think my body prefers it. I know that whenever I switched bikes I could feel the difference at first, but then as I rode it would normalize.

    I also noticed that with the 172.5's I have less knee issues, which seems odd...Since the longer lever should be putting more stress on the knees...Only think I can think is it alters the angle at which my knee bends through the pedal stroke and it seems to prefer that.

    With all that said, I don't think there is a gospel rule as to what crankarm length you should use. It tends to be that shorter rides use shorter crank arms and that taller rides use longer ones, but that doesn't mean that two people of the same proportions will use the same crank arm length...

    Are you confused?? I am...

  5. #5
    NW Georgia Mountains On Your Right's Avatar
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    Longer Crank = effectively more bottom end gearing due to increased leverage.

    Shorter Crank = more torque when beginning sprinting. After that the longer crank helps you maintain speed better.

    With that said......... The everage rider will feel little or no difference.

    So just as with the other posts so far....... There's still not a definate answer.
    He who makes a beast of himself gets rid of the pain of being a man.

  6. #6
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    2.5

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    I'm 6'7" with a 34" pants inseam and ride both 172.5s and 175s.

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    i'm 5'9" and ride 172.5 because it's what i have.

  9. #9
    has a Large Member Campag4life's Avatar
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    Quote Originally Posted by On Your Right View Post
    Longer Crank = effectively more bottom end gearing due to increased leverage.

    Shorter Crank = more torque when beginning sprinting. After that the longer crank helps you maintain speed better.

    With that said......... The everage rider will feel little or no difference.

    So just as with the other posts so far....... There's still not a definate answer.
    The part about the average rider not feeling a difference just isn't so.
    OP, you are average sized and likely would do best on average length cranks.
    I am long legged and much prefer 175's.

  10. #10
    Flatland hack Flak's Avatar
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    I think the whole thing is miniscule. 2.5mm is literally less than an 8th on an inch.

  11. #11
    Senior Member 04jtb's Avatar
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    Quote Originally Posted by Flak View Post
    I think the whole thing is miniscule. 2.5mm is literally less than an 8th on an inch.
    Exactly
    Quote Originally Posted by cc700 View Post
    i jam my thumbs up and back into the tubes. this way i can point my fingers straight out in front to split the wind and attain an even more aero profile, and the usual fixed gear - zen - connectedness feeling through the drivetrain is multiplied ten fold because my thumbs become one with the tubing.
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  12. #12
    Senior Member mikeE46's Avatar
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    I have been told,
    Riders inseam of less than 29 inches = 165 mm cranks
    Inseam of 29 to 32 inches = 170 mm cranks
    Inseam of 32 to 34 inches = 172.5 mm cranks
    Inseam of 34 and higher = 175 mm cranks
    Used to ride BMC SLC01, CAAD, FELT, Cervelo P3 but now I ride FUJI SST and TCR advanced and CAAD9.
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  13. #13
    Senior Member Retro Grouch's Avatar
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    When I ordered the compact crank for my Klein the 172.5 size wasn't available but I could immediately get the 175 size. I worried myself to death but decided to take a chance on the 175. Honestly, I can't tell a difference. How much difference can 2.5mm make?

    I'm 5'10", by the way, so that's right on the average for American men.

  14. #14
    Senior Member Ramjm_2000's Avatar
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    As previously mentioned it is related to leg and femur length.

  15. #15
    In beaurocratic limbo urbanknight's Avatar
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    Quote Originally Posted by Ramjm_2000 View Post
    As previously mentioned it is related to leg and femur length.
    I've read that here multiple times, but does anyone have information for correlating femur length with crank length? In place of the inseam generalization, I'd like to see a general chart where XXcm femur goes with XXX.Xmm crank length.

  16. #16
    Senior Member nathbdp's Avatar
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    eddy merckx ran 175s

    no more questions

  17. #17
    ride lots be safe Creakyknees's Avatar
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    I've gone from 180's (mtb) to 165 (track bike) and every size in between. Sometimes on the same day (yes I have a garage full of bikes). And I can never tell a difference.

    The science, what little there is, seems to say 'meh' about the whole issue.

    So I just go with whatever's stock, unless it's a track bike where 165's are a potential safety requirement.

  18. #18
    Senior Member Old School's Avatar
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    While that extra 2.5-5.0mm length gives you additional leverage according to the laws of physics, I found no detectable difference going from 172.5 to 175. Now the lighter carbon frame and compact crank was a different story!
    Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW! WHAT A RIDE!"

  19. #19
    In beaurocratic limbo urbanknight's Avatar
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    Quote Originally Posted by nathbdp View Post
    eddy merckx ran 175s

    no more questions
    But what size frame did he ride? Stem length? Handlebar width? Seatpost height? Please tell me, as I want to win the Tour de Franc this year!

  20. #20
    has a Large Member Campag4life's Avatar
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    Quote Originally Posted by Retro Grouch View Post
    When I ordered the compact crank for my Klein the 172.5 size wasn't available but I could immediately get the 175 size. I worried myself to death but decided to take a chance on the 175. Honestly, I can't tell a difference. How much difference can 2.5mm make?

    I'm 5'10", by the way, so that's right on the average for American men.
    By virtue of miniscule size differences being available...2.5mm increments is telling about what a difference it makes. It is quite noticable to me and I have ridden most of them.
    If you have an average inseam 170-172.5mm is fine.
    If you are Shack, likely 180-190 would be better. I have a 35.25 inch inseam and 175's feel perfect to me.

  21. #21
    has a Large Member Campag4life's Avatar
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    Quote Originally Posted by urbanknight View Post
    But what size frame did he ride? Stem length? Handlebar width? Seatpost height? Please tell me, as I want to win the Tour de Franc this year!
    You miss the point. In Eddie's day 175 were considered very long cranks. Eddie is a tall man with long inseam and it makes sense he would prefer longer cranks.

  22. #22
    Mad Town Biker Murrays's Avatar
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    Quote Originally Posted by On Your Right View Post
    Longer Crank = effectively more bottom end gearing due to increased leverage.

    Shorter Crank = more torque when beginning sprinting. After that the longer crank helps you maintain speed better.

    With that said......... The everage rider will feel little or no difference.

    So just as with the other posts so far....... There's still not a definate answer.
    I'm not sure what you mean here, but shorter cranks can't provide more torque at any point since torque is equal to pedal force X crank length. You might be able to generate more power (torque X RPMs) with shorter cranks because you can spin them faster, but not more torque unless you're pushing on the pedals harder.

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  23. #23
    Senior Member enjoi07's Avatar
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    Quote Originally Posted by mikeE46 View Post
    I have been told,
    Riders inseam of less than 29 inches = 165 mm cranks
    Inseam of 29 to 32 inches = 170 mm cranks
    Inseam of 32 to 34 inches = 172.5 mm cranks
    Inseam of 34 and higher = 175 mm cranks
    best answer thus far, all it needs are photos of cranks.

  24. #24
    Flatland hack Flak's Avatar
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    Quote Originally Posted by mikeE46 View Post
    I have been told,
    Riders inseam of less than 29 inches = 165 mm cranks
    Inseam of 29 to 32 inches = 170 mm cranks
    Inseam of 32 to 34 inches = 172.5 mm cranks
    Inseam of 34 and higher = 175 mm cranks
    Not an attack on you Mike, but just on the conventional wisdom you heard.

    So we're supposed to believe that a 5mm (less than 1/4") is going to compensate for a 4" difference in inseam? Absurd.

    Ride your bike.

  25. #25
    Time for a change. stapfam's Avatar
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    Two similar bikes and both with compacts. One has 172.5 and the other 165's. I find I spin faster with the 165's but even that is not great. Swopped cranks from bike to bike- both do hills and they both climb hills as easy as each other- No matter what crank fitted.

    Only difference to me is the mental bit. Smaller cranks spin.
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