How easy to get diff. length crank arms on each side?

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04-17-06 | 12:13 PM

#26

Dave Moulton

ex frame builder

Joined: Aug 2004

Posts: 523

Likes: 5

Quote:

Originally Posted by donrhummy

There's over an inch in difference between the two legs. I'm still not 100% certain whether the longer crank arm or something on the bottom of the shoe is best, but the crank arm diff. does make some sense to me. The problem is that if the crank arms are the same length, when the longer leg is perfectly angled at the bottom of the pedal stroke, the shorter leg is not. There's no way to get them both set up properly (pedal under knee, leg slightly bent at bottom) if both cranks are the same length.

I agree that over an inch difference in leg length is extreme and your saddle height has to be set to suit your shorter leg. Having a longer crank on the opposite side would appear to help at the bottom of the stroke but your knee is going to be higher on that side at the top of the stroke. With your saddle already being too low for the longer leg you may be beyond the limit of your hip movement on that side.

Read this piece on my website on saddle height here. Then test the limit of your hip joint movement as described in the piece.

One suggestion I would make is to move the shoe cleat nearer the toe on your short side. With you toe pointing down at the bottom of the pedal stroke this will have the effect lengthening the foot on that side. Not a perfect set up I agree, but your riding position is always going to be a compromise. The object in your case is not a perfect riding position, but to maximize your riding experience.

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04-17-06 | 06:02 PM

#27

rruff

Senior Member

Joined: Jan 2006

Posts: 1,359

Likes: 2

From: Ruidoso, NM

Quote:

Originally Posted by donrhummy

There's over an inch in difference between the two legs.

I have a similar problem (left femur shorter by ~.75 inches or 20mm). It isn't easy to correct for this problem, compared to having the difference in the lower leg. The trick is to try to compensate to some degree during the power phase of the stroke, without making it worse elsewhere. There is no way to make the entire stroke match your other leg, but you can get close. This is what I tried:

5mm shorter crank on the left.
5mm shim on the left.
5mm forward cleat adjustment on the left.

At 0 degrees (top) I have zero vertical correction because the crank length and spacer cancel out (good!) and 5mm of horizontal correction from the cleat adjustment, which directly helps compensate for my short femur.
At 90 degrees (power stroke) I have 5mm of vertical correction from the cleat shim and 10mm of horizontal correction. Both of these help compensate for my short femur.
At 180 degrees I have 10mm of vertical correction from the shim and short crank, plus 5mm of horizontal correction from the shim position. Since my leg is almost fully extended at this point, the correction is pretty good.
At 270 degrees I have zero horizontal correction, and 5mm of vertical correction... this is not ideal, but it's the best I can do.

Note that I'm only correcting for about half of the actual leg difference... and for you it would be even less than half... but I think this is a good place to start.