This is an honest question about adaptive tech. I've got a freind who knows I've ridden and built for years. He has asked about using 2 differnt size crank arms to cover one of his legs now being an 1" or so shorter than other due to injury.
I've never done anything like that. Is it feasible to go 170 on one side and 175 other or would the result be to unsuable?
ideas? Anyone done this? Thanx.
I can speak to this question with loads of first hand experience. I have one leg 50mm shorter than the other. The answer is a little complicated, and it depends on where the leg length discrepancy comes from: Below the knee or above the knee.
If the discrepancy is below the knee, then using a shim (or a lifted shoe) is the answer. It will make your knees even, and your two legs will have a very similar bend at the top and bottom of the stroke.
Introducing a different crank length is not going to help, and in some ways is counterproductive. The problem is that while the shorter crank might even out the discrepancy at the bottom of the stroke (the pedal will be higher), the longer crank pedal will be higher at the top of the stoke, which is the
opposite of what you would actually want. In addition, since the lower part of the leg is longer on the longer leg, the resulting bend at the top of the stroke is deeper. Thus, it makes it hard to set up a saddle height that is optimal for both legs.
If the discrepancy is above the knee: This is more complicated. Adding a lift of shim or lift can help at the bottom of the stroke, but now you have a much longer effective leg length below the knee on the shorter leg than the longer leg, and at the top of the stroke the shorter leg is now bent more deeply. Different crank lengths can help this. The problem is that as the two cranks get to be very different lengths, you are essentially in two different gearings.
In my case, my left leg is 50mm shorter than my right. 15mm is below the knee, 35mm is above. I normally wear a lift that is 50mm at the heel of most shoes. Of course, with bike shoes, it mostly matters how much the ball of your foot (where the cleat is) is lifted.
About 12 years ago, I bought a mess of cheap square taper cranks ranging from 160 to 175, and tried loads of combinations for about two years until I settled on a 5mm difference. Running the same lengths felt a little weird because my left leg felt too bent at the top of the stroke. at 10mm difference, the difference in the gearing and the size of the circle I was spinning was just noticeable. 5mm was just right.
Also, from using shoes with different lift heights under the ball of the foot, 40mm seems to be the sweet spot.
SO in the end it is a combo of lift/shim and different crank lengths that worked for me, but it is never going to be truly "optimal".
Here is my most recent shoe:
BTW, does your friend wear a lifted shoe? If he does not, he better start doing so ASAP. An inch is enough to cause him some really awful back and/or hip problems years down the road.
One last thing: If you want current mid-to-high end crank buying different lengths can be expensive, and a real pain in the ass. Long story that is another thread.