Crank Arm lengths

JesseD
JesseD Posts: 1,961
edited July 2008 in Workshop
Does this really matter?

I use 172.5mm crank arms as thats what I was told I should use when I was fitted for my TT bike, but if I used 175mm carnk arms would it make much difference and if so what would the difference/potential problems be?

Thanks

Interested of Bristol
Obsessed is a word used by the lazy to describe the dedicated!

Comments

  • dennisn
    dennisn Posts: 10,601
    I don't think you will gain or lose much by switching lengths one way or another.
    Conventional wisdom seems to be that longer cranks "may" cause knee problems
    because the knee bends more. Conventional wisdom also states that you are able
    to spin easier at a higher cadence on shorter cranks. I know that bike fit people claim
    to have formulas for figuring crank arm length. I have heard of at least six different ones.
    So I don't believe that there is any real consensus on the subject. I recently switched
    from 170 cranks to 172.5. Didn't notice a thing. The only reason I did it was to see for
    myself if it made any difference. I now have a lighter bank account, if that helps. :D
    If you have a serviceable crank and are not having any physical problems with it I
    don't see any reason for switching to longer and / or shorter. 170, 172.5 have been around for years and seem to suit most of the cycling population.

    Dennis Noward
  • JesseD
    JesseD Posts: 1,961
    Cheers Dennis.

    As I thought really, 2.5mm can't really make that mush difference (I'm getting ready to be flamed by the techie guys :wink: )

    The only reason I asked is because I have seen loads fo 2nd hand chainsets advertised cheap with 175mm crank arms and always thought I like the look of that but it's the wrong length etc.
    Obsessed is a word used by the lazy to describe the dedicated!
  • cougie
    cougie Posts: 22,512
    I really cant see that 2.5mm can make that much difference. Having odd socks could almost account for that thickness ?
  • James_London
    James_London Posts: 530
    cougie wrote:
    I really cant see that 2.5mm can make that much difference. Having odd socks could almost account for that thickness ?

    You need socks that are 7.5mm thick at the top of the pedal stroke and 2.5mm thick at the bottom to properly simulate the effect!

    And no, I don't adjust my saddle height for different chamois thicknesses :)
  • bill57
    bill57 Posts: 454
    I've heard stories of riders having 170 on one side, and 172.5 or even 175 on the other, and not noticing the difference. Suggest Dennis should try it and report back?
  • meagain
    meagain Posts: 2,331
    172.5 or even 175 might be of some benefit c.f. 170 if one has relatively (to overall height) long legs?

    Closest I ever came to a custom build was an Airborne some 6years ago. When finalising spec with the Airborne guy in Netherlands (who was the best bike customer service man I've ever come across) he suggested 172.5s ratjher than 170s on the basis of my leg length/height. Certainly seemed a "good idea" in practice!

    PS Also seem to recall that "monster" gear pushers like Indurain used to run 180 cranks!
    d.j.
    "Cancel my subscription to the resurrection."
  • dennisn
    dennisn Posts: 10,601
    bill57 wrote:
    I've heard stories of riders having 170 on one side, and 172.5 or even 175 on the other, and not noticing the difference. Suggest Dennis should try it and report back?

    I'm going to pass on that. Both my legs seem to be pretty much the same length.
    Although there was a guy in our racing club with one leg shorter than other
    by almost 2 inches. When I last saw him he was actually having his short leg
    lengthened. Worked out pretty good for him.

    Dennis Noward
  • meagain
    meagain Posts: 2,331
    "he was actually having his short leg lengthened."

    Surely quicker and easier t'other way round?
    d.j.
    "Cancel my subscription to the resurrection."
  • dennisn
    dennisn Posts: 10,601
    meagain wrote:
    "he was actually having his short leg lengthened."

    Yes, the doctors preformed an operation where they sawed the bones in his lower leg completely in two. They then put a pin in each sawed off end. These pins extended out through the his outer skin about a inch or so. The pins were then connected to a sort
    of hi-tech turnbuckle between them. Each day he turned the turnbuckle one click and
    this moved the sawed off bones apart about 1 mm. After about 60 days of this his leg
    was about 2" longer than it had been and the bones grew back together just fine, filling in the gap. It was weird, but it worked. I think I've heard of this being done on midgets and dwarfs also. Or to be a bit more politically correct "small people" .

    Dennis Noward
  • giant_man
    giant_man Posts: 6,878
    LOL great story Dennis! Love your humour especially the midget and dwarfs bit at the end. Oh did i chuckle, all comes of having a warped soh!
  • dennisn
    dennisn Posts: 10,601
    giant mancp wrote:
    LOL great story Dennis! Love your humour especially the midget and dwarfs bit at the end. Oh did i chuckle, all comes of having a warped soh!

    Not sure if you believe me or not, but it is a true story.

    Dennis Noward
  • dennisn
    dennisn Posts: 10,601
    dennisn wrote:
    giant mancp wrote:
    LOL great story Dennis! Love your humour especially the midget and dwarfs bit at the end. Oh did i chuckle, all comes of having a warped soh!

    Not sure if you believe me or not, but it is a true story.

    Dennis Noward

    As an afterthought I looked up the name of the procedure - distraction osteogenesis.

    Dennis Noward
  • Bugly
    Bugly Posts: 520
    change your shoes and you will change the geometrey by more than 2.5 mills. do the maths the extra torque due to the longer crank is minute.
  • kevin_stephens
    kevin_stephens Posts: 184
    Crank Length can make a very big difference for some people

    I have short legs and changing fro 170 to 165 cranks on my road bike made a great improvement, eliminating knee pain from too acute a knee angle at top of stroke

    5mm difference in crank length actually means 10mm diffeenct to knee at top of stroke (after changing saddle height by 5mm for correct height at bottom of the stroke)

    Having said that I find 170 cranks OK on my TT bike because of the much more forward saddle position
    I want to climb hills so badly;
    and I climb hills so badly
  • Not Another Hill
    Not Another Hill Posts: 382
    I have 175 cranks on my MTB and 172.5 cranks on my road bike and I 'feel' the difference. I certainly spin easier and faster with the shorter cranks. The arc my feet rotated round felt much shorter. It felt weird when i first started riding the road bike. However I don't know how much of that was due to reduced rolling resistance, lighter bike etc. Even now (7 months later) I notice the difference.
  • whyamihere
    whyamihere Posts: 7,715
    18.5% of the length from the floor to the top of the femur is the best measure I've found. However, from personal experience, both 170mm and 175mm cranks seem fine. Then again, I am in between the two, and have yet to try 172.5mm cranks... I could be missing out on a revolution in my riding.

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