Powermeter and asymmetric crank arm length

[jmfreeman535]

I have a fairly significant leg length discrepancy, with my right leg being ~2.5cm longer than my left. I've had a couple bike fittings and have gotten my fit to the point that it is decent. But not perfect...it's either hip pain on one side, or inner knee/quad pain on the other,

So, my next step is to try going with a larger crank lenght on the right side (corrective surgery after that)...my question is, how would this effect my power readings? I have PowerTap P1's, and it does require you to imput crank arm length...would asymmetric cranks really mess things up?

I'm currently using 170's and would first go up to 172.5 on the drive side, so not a huge jump.

[TobinHatesYou]

Maybe time to get in contact with the various pedal PM brands and computer brands and tell them to implement separate saved settings for each pedal.

[Karvalo]

I do wonder about assymetric crank lengths. Unfortunately with such a large discrepancy you're probably damned if you do and damned if you don't - but with different crank arm lengths you may be able to use it to equalise knee angles at BDC, but then you'll be increasing the hip angle difference at TDC. Is that going to help?

[refthimos]

There are other more widely accepted ways to address leg length discrepancy, including shims (Speedplay is especially suited for this) and adding additional thickness to the sole of the shoe of the longer leg.

[TobinHatesYou]

It's...complicated, especially with 2.5cm to make up.

Shims would effectively be adding length to his tibia, which may or may not be desirable depending on how much shorter each bone in his legs are. It's possible his tibias are similar in length and all of the discrepancy is in the femurs. Shims would make things worse.

[jmfreeman535]

There are other more widely accepted ways to address leg length discrepancy, including shims (Speedplay is especially suited for this) and adding additional thickness to the sole of the shoe of the longer leg.

I already have the maximum amount of shims possible, along with a thinker insole on my shorter leg. Increasing the sole thickness simply is not pratical on a cycling shoe...you'll either completely compromise the stiffness of it, or spend a fortune having someone build-up an extra 25mm worth of carbon fiber to it.

I do wonder about assymetric crank lengths. Unfortunately with such a large discrepancy you're probably damned if you do and damned if you don't - but with different crank arm lengths you may be able to use it to equalise knee angles at BDC, but then you'll be increasing the hip angle difference at TDC. Is that going to help?

The main issue right now is in the 12-3 o'clock posititon, on my longer leg.

Since I have to have a shorter-than-ideal saddle height, the knee/hip angle of my longer leg is forcing my foot to be too far back, which is putting extra strain on my knee/inside quad. But, if I raise my saddle, I'm then streching too far on my other side, which strains my hip flexor (I've sprianed it twice this year. It's not fun). Thus the longer crank arm. It should help to get my foot into a more natural position.

And my cleat is already moved as far back as I'm comfortable with (ideally, I'd like it to be further forward)

[jmfreeman535]

It's...complicated, especially with 2.5cm to make up.

Shims would effectively be adding length to his tibia, which may or may not be desirable depending on how much shorter each bone in his legs are. It's possible his tibias are similar in length and all of the discrepancy is in the femurs. Shims would make things worse.

Correct, all of the discrepancy is in my femur. I fractured it when I was six, and during the lenghtly debacle that was the healing process, the bone lengthened.

I've ridden, and been fitted, with and without the shims, and they definitely do help. They are far from perfect, but are a major improvement.

[TobinHatesYou]

Yeah I suppose 2.5cm is so much that shimming is a quick'n'dirty way to prevent listing to one side or rocking (and too low of a saddle position.) However I think crank length and cleat fore/aft is the more complete technical solution.

[Nefarious86]

Spider based power measurements?

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[TobinHatesYou]

Even dual crankarm PMs would be fine. Pedals are the only PMs that require user defined crank length, and there’s currently no facility to define separate lengths for each side.

[jmfreeman535]

I'd like to advoide buying another powermeter until I know that asymmetric cranks help, and that I won't need to have surgery in the near future...it's too much money to spend on something that may not work out, and/or will not need it in the near future.

Plus, would a manufacturer be willing to build one for no additional charge? Or would I have to send a crankset in to someone like 4iii?

[TobinHatesYou]

You can buy Stages L and Stages R cranks separately and then pair them. You could also probably contact them and tell them your situation and maybe they’ll give you LR pricing. Or you could do what you mentioned and send them your own crankarms.

[mikemelbrooks]

I don't think a 2.5 mm difference will totally fix your problem, Shimano cranks are pretty interchangeable from model to model and year to year, and buying different length cranks of eBay to experiment with seems like a good way to go, Also if you know anyone with a smart bike several come with adjustable cranks, Stages seem to have the biggest range.
RE powermeter I don't think a 2.5mm difference in crank length will throw off your power reading much. I power balance is important for you then your out of luck, but I guess with such a leg length difference it is what it is.
can you average out the crank length measurement for the pedals IE one 170mm one 175mm put in a crank length of 172.5mm. Do you have the same fore-aft position of your cleats I imagine placing the one on the shorter leg further forward and allowing your ankle to take up some of the discrepancy might help.

[Mesorhizobium]

I think if you have a splider based power meter it should be all fine. There is only one place of powere measurement which is not affected by crank length. The way the powers is transferred into it is, the measurement is not. For training with power the numbers do not have to be accurate. ( I mean calibrated accurate, who cares if the watts are the same as your friends watts... ) The numbers only have to be consistent every time you ride! So that you can track progress. I dont think there is going to be an issue with that at all. If there is a measurment error, the error is going to be always there. So it doesnt matter too much for performance tracking.

[Nefarious86]

Then the simplest way forward will be re test once you change crank arms and just accept your numbers wont be perfect for the interim period.

Failing that I'd be moving to a spider based power meter.

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