Crank Arm Length setting?

I recently started using Favero Assioma power pedals and I’m also a tall rider with 180mm crank arms. I set the crank arm length in the Favero Assioma setting app on the my iPhone per the instructions and I think everything it working correctly.

I’m curious…why doesn’t TrainerRoad have a crank arm length setting in their PC/Mac app? Seems like it would make sense?

The power meter is what needs to know what the crank length is so it can generate a correct power number. The apps only care…and need to know…what that power number is.

I’m using an indoor trainer with my Favero Assioma power pedals sending power data directly to my laptop via Bluetooth. So there isn’t a power meter…just power data from the pedals going to Apple/Mac TrainerRoad app via Bluetooth.

  • Your Assioma pedals ARE the power meter, since (by definition) they are what measure and report the power data.

But to your original point, I do think it makes sense for TR to offer the option to alter the crank length setting associated with pedal or crank arm power meters.

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Checking in with the team to get some info about why we don’t have this as an available setting/if we plan to work it in. :+1:

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From having been down this rabbit hole due to getting some P1 pedals, and not having an iphone for the app, I do think that TR should control the crank length for power meter pedals it uses. Garmin and wahoo head units do this, and the configuration sent by the head units overrides what was set by the configuration app.
As I understand it, the pedals will use whatever value was last set, so if you move them between bikes with different crank lengths this is something you need to be careful with.

Also, if TR adds this, you should be able to specify an arbitrary crank length (ie it should be a numeric entry field, not a drop down list.) This allows for crank length to be (ab)used to adjust the readings of pedal based power meters. In this regard the Garmin head units are superior to the Wahoo ones.

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Ok @mcneese.chad…why? If the crank length is set in the power meter profile what would, or even could, potentially change in TR with a user adjustable setting? I mean, in my head, that would be like telling TR you’re on a heavy Tri bike rather than a light climbing bike. What difference would that make to watts?

I don’t have the specific issue, but could foresee a need to swap pedal power meters between a road and TT bike for TR use, that happen to have different crank lengths. In those cases, you’d need to have the correct crank length set whenever you swap bikes on the trainer, if you want accurate power data.

Same could happen for a Rider A bike swapping the same power meter to Rider B bike (like between spouse or significant other) that has different crank length.

We already have access to info like battery levels, and Zero Offset options, so having crank length setting seems a handy feature on par with the existing info/control.

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So you’re talking about TR sending a signal BACK to the power meter to set the crank length like how a Garmin does it. I had two Garmins, one for the road, one for the Tri bike. The bikes had different crank lengths so each Garmin was set for the bike it was used on. It was never a big deal to me to swap the pedals since the Garmin already overroad the app setting. What I thought you were talking about would be taking the wrong crank length data and adjusting it “in app”.

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  • Yes.
  • Not what I meant or what I took as meaning from the OP.

We just want the same access to alter the PM crank setting like the Favero app or head units like you mention.

I suspect this may have happened today on my first ride using Powermatch. Used my Garmin pedals on my road bike (175 mm cranks) then transferred to TT bike (165 mm cranks) on my Neo 2. The resistance was never quite right, all of my power readings were a few % higher than target at a consistent cadence and the resistance wasn’t adjusting.

Interesting… I thought the crank arm length was used by the app to calculate power, but it sounds like the pedals (or whatever is the power meter) needs to be told what the crank arm length is via a configuration app or a cycle computer in order for the power meter to accurately report power data to the app/computer. Is this correct?

If so, yes, it would be very helpful for TR to be able to query the power meter (pedals) and display the configuration settings (like crank arm length) and also be able to modify any configuration settings (like crank arm length) that might need to be changed depending on the riders bike setup…

Not sure that explains your issue. The power reporting is effectively the same, but just “wrong” by the proportional difference in the crank length. Presumably you have PowerMatch in use, and experience with that can vary, not to mention that we have at least 2 versions floating around (original and the “new” one in the beta apps). So any variation above or below the power target is from some other source, IMO.

Might be best to contact TR for review of that issue, because I don’t think crank length is the source.

  • Yes, pedal or crank arm power meters MUST know the crank arm length. That distance is used to calculate the torque value, which is one main component in the power equation (crank speed / RPM is the other).
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Hi Ivy,
Any update from the development team? Thanks!

Hey! So I believe we haven’t added the ability to adjust this within TR because we’ve known athletes are able to do this from their head unit or from the native app for the power meter (e.g. PowerTap for P1 pedals, Garmin app for Vector pedals, Favero Assioma app for the Assioma pedals).

Setting it in the native app will make things accurate everywhere except a garmin/wahoo (if the garmin/wahoo settings are different from what’s set in the native app).
Changing it on a garmin/wahoo alone won’t affect the device for other 3rd party apps from what I’m seeing, basically, the garmin/wahoo will override whatever crank length they’ve been set to in the native app (172.5 is default). :+1: