Powermeter and asymmetric crank arm length

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StephanP
Posts: 7
Joined: Tue Dec 26, 2017 8:00 pm

by StephanP

I deal with roughly the same thing, but have gone about it slightly differently. I first started with same length cranks and them progressed to different crank lengths, and finally back to the same crank lengths. I have one bike with rim brakes so I have a powertap hub built up into my carbon rims. On my other bike I have disc brakes so I went with a Quarq powermeter.

Your situation is different from mine but my right leg is about 2.5cm shorter then my left, and with my disability my right ankle is fused. With that problem as well I have almost 3cm of shims under my right shoe. I think the best way to go about it, is get in contact with your national paracycling team. The coaches have dealt with all kinds of different situations and they can direct you in the best way to go, or to someone that maybe has dealt with the same issue. Just this past year I have been in contact with the fitter for the Canadian paracycling team and he was the first fitter that actually understood my situation, and produced a solution that has benefited me and allowed me to progress as a cyclist.

Stephan

RECAR77
Posts: 102
Joined: Mon Jun 10, 2019 7:12 am

by RECAR77


jmfreeman535 wrote:
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.
The following is disregarding biomechanics and only looking at the mechanical aspect.

Power is proportionate to torque

Power = Torque x Angular velocity

Torque is proportionate to the radius / crankarm length

Torque = Force x Radius x sin(α)

So if we have a deviation in the Radius by X%, the same deviation will directly show in the power reading.

172,5mm / 170mm * 100% = ~101,47%

BUT this deviation only happens on one side and not on both. With a perfect L/R balance you'd have a power reading inaccuracy for your total power of ~0,735%.

I'd say that this falls very much within the margin of error on all powermeters on the market.

Also like I stated in the first paragraph this is only the mechanical side so take it with a grain of salt. It is entirely possible that with the now more optimized crank length you can actually produce more power than you did before, even with a 0,735% penalty...




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jmfreeman535
Posts: 361
Joined: Wed Sep 18, 2019 8:12 pm

by jmfreeman535

RECAR77 wrote:
Mon Dec 30, 2019 10:08 am
jmfreeman535 wrote:
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.
The following is disregarding biomechanics and only looking at the mechanical aspect.

Power is proportionate to torque

Power = Torque x Angular velocity

Torque is proportionate to the radius / crankarm length

Torque = Force x Radius x sin(α)

So if we have a deviation in the Radius by X%, the same deviation will directly show in the power reading.

172,5mm / 170mm * 100% = ~101,47%

BUT this deviation only happens on one side and not on both. With a perfect L/R balance you'd have a power reading inaccuracy for your total power of ~0,735%.

I'd say that this falls very much within the margin of error on all powermeters on the market.

Also like I stated in the first paragraph this is only the mechanical side so take it with a grain of salt. It is entirely possible that with the now more optimized crank length you can actually produce more power than you did before, even with a 0,735% penalty...




Sent from my moto g(6) using Tapatalk
This is an excellent explanation, thank you.

My L/R balance is consistantly at 50/50, so a 0.735% deviation in accuracy is not something that I am worried about. Also, I do think that it is very possible that my power will increase with the longer crank, as well. My toqure effectiveness is usually ~5% lower on my right (longer) side, and I do think that is due to the combiniation of the less-than-ideal saddle height/crank lenght, and position that it places my right leg in.

I did end up ordering a 172.5 crankset, so hopefully it does help.

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