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  1. #1
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    crank arm length

    Firstly, when I do a metric conversion 170mm shows like 5.5 inches.
    I cant't see how this could be related to crank arm length, they are
    like 12 inches. Secondly, if I have a leg length discrepancy of approx
    4 mm and am currently shimming approx 2mm, would using a 172.5
    on the long leg and a 170mm on the short leg give me just a 2.5mm
    difference, or would it be 5mm. I can see the disadvantage vs shimming
    would be my long leg at 3 and 9 o'clock would be further forward\back,
    but at 12 and 6 I would think it would actually work out better and my
    stack height on the short leg would be reduced to the same as the
    other.
    All I said was that our son, the apple of our three eyes, Martha being a cyclops, our son is a beanbag, and you get testy!

  2. #2
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    The length is from center of the BB axle to center of the pedal axle.

    In previous answers to questions like yours about leg length discrepancies, the consensus was for keeping the cranks the same length and compensating by shimming your cleats, although to me 4mm doesn't seem like a lot.
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  3. #3
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    what Julio said, and also, you may want to check your math, since 170 mm equals a shade under 6.7 inches.

  4. #4
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    Thinking further about the question, I'm not sure that 4mm which is less than 3/16" (and how do you even measure that?), is worth bothering about. It's not the same as walking. Think about it, at the 12 and 6 positions your legs aren't producing any power, so why worry about it? If the discrepancy is evenly split between your upper and lower leg, then a 2mm shim would equalize femur angles at 3 and 9, but I'm not sure how such a tiny difference would matter.
    There's sometimes a buggy.
    How many drivers does a buggy have?

    One.

    So let's just say I'm drivin' this buggy...
    and if you fix your attitude you can ride along with me.

    http://www.youtube.com/watch?v=GekiIMh4ZkM

  5. #5
    Cycling induced anoesis
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    Quote Originally Posted by bikerjulio View Post
    Thinking further about the question, I'm not sure that 4mm which is less than 3/16" (and how do you even measure that?), is worth bothering about. It's not the same as walking. Think about it, at the 12 and 6 positions your legs aren't producing any power, so why worry about it? If the discrepancy is evenly split between your upper and lower leg, then a 2mm shim would equalize femur angles at 3 and 9, but I'm not sure how such a tiny difference would matter.
    CT scanograms are (to my limited knowledge) the most accurate method of determining LLD.

    JMO, but what would dictate whether or not a 4mm discrepancy is worth bothering about would be the degree that it affects activities. Mainly, walking and cycling. Assuming something (like a fit issue/ pro fit) led to their employment, if the shims are proving effective, I'd leave well enough alone and not experiment further.

    Re: the comment on not producing power at the 12/ 6 positions, that's pretty much true, but to avoid hip/ knee issues, maintaining the correct saddle height is important.

  6. #6
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    Measurements

    Quote Originally Posted by phoehn9111 View Post
    Firstly, when I do a metric conversion 170mm shows like 5.5 inches.
    I cant't see how this could be related to crank arm length, they are like 12 inches. Secondly, if I have a leg length discrepancy of approx 4 mm and am currently shimming approx 2mm, would using a 172.5
    on the long leg and a 170mm on the short leg give me just a 2.5mm difference, or would it be 5mm. I can see the disadvantage vs shimming would be my long leg at 3 and 9 o'clock would be further forward\back, but at 12 and 6 I would think it would actually work out better and my stack height on the short leg would be reduced to the same as the other.
    As has been pointed out, there are 25.4 mm to the inch so a 170mm crank is 6.69 inches (center to center). Your "they are like 12 inches" comment doesn't make sense. What are you saying is "like 12 inches"?

    General rule of thumb for correcting for leg-length differences is to shim half the difference. If your difference really is 4mm then you would shim 2 mm under the cleat of the shorter leg.

  7. #7
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    Thanks for the replies, I am going to have to get a CT scanogram, not sure about the 4mm
    I have short leg knee problems as well, As to the other question, I think what I am reading
    is that the effective differential would be 5mm if the cranks are 2.5mm difference?
    All I said was that our son, the apple of our three eyes, Martha being a cyclops, our son is a beanbag, and you get testy!

  8. #8
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    The effective difference for leg extension is only 2.5 in the example you are citing. The upstroke position does nothing for you with respect to developing power and evening out the pedal stroke.

    You need to evaluate whether any true LLD is in the upper or lower leg, evenly distributed, or functional due to hip or muscle issues.

    If you run longer cranks on one side it will affect your KOP on the longer side. If the upper leg is longer, that would be fine. If it is all in the lower leg, you will be reaching out more and could develop knee issues. This is one reason to stick with shims for small variations.
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  9. #9
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    I have a leg length discrepancy also, and the sit bone on my shorter leg protrudes out a bit further, so when I sit on the saddle the problem is amplified. But your body can adjust. Increasing my flexibility has certainly helped me. Stretch your hamstrings, the inside portion of your leg, your glutes, and your lower back. Be gentle with your stretches, but do them daily.

    I currently run 175 mm cranks on both sides, and 6 mm of spacers (3) on my shorter leg. My knee angle at lowest point of the pedal stroke is different for both legs, but this is where flexibility comes into play. You can have a shallower knee angle on your shorter leg vs your longer leg and still be within the 25-35 degree angle for both knees, which most fitters recommend. But to be able to achieve shallow knee angles you need to be flexible.

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