Stages PM - compensation for leg difference?

neeb

I’m considering a stages one sided PM as an affordable option for a second bike. I have powertap g3 and P2m on other bikes. Obviously the one-legged measurement is an issue, especially as I have a shim under one shoe and suspect that I am consistently slightly different between legs.

Of course I could measure the discrepancy by comparing the stages data with the powertap wheel on the same bike and then (assuming the difference is reasonably consistent) apply an offset / calibration to the stages. However I can’t find any mention of a facility to do this with the stages, only with the 4iiii system which isn’t available for campagnolo.

I just can’t believe that someone would design a one sided PM without providing a facility to calibrate for power difference between legs.. surely the stages must be able to do this?? It’s simply a case of allowing a multiplication factor to be applied to the data. I could always edit the ride data in excel before uploading I suppose, but that wouldn’t help while viewing power on the road..

Please tell me that the stages isn’t that useless!!??

joey54321

Just a thought; but what makes you think you are consistent? And are you consistent at all power zones? And at all times of the years (i.e. differing fitness levels & training blocks)?

neeb

joey54321 wrote:

Just a thought; but what makes you think you are consistent? And are you consistent at all power zones? And at all times of the years (i.e. differing fitness levels & training blocks)?

The PM2 provides an estimate of leg balance (it doesn’t really measure each leg independently but works out a figure from power distribution over the pedal stroke), I’m always 49/51 or 48/52.

But in any case, having the ability to calibrate the stages to a known accurate two sided system can only increase the accuracy of the figures, assuming any inaccuracy isn’t completely random.

neeb

Just to elaborate - I’d probably use the bike mostly for time trials, so I would think it should be possible to calibrate it reasonably accurately for that particular type of effort at least.

imposter2.0

Unless you actually know your power imbalance/difference, then the discussion is academic. Most people have a few % asymmetry anyway. I would have thought that knowing your 'exact' power output is not as important as being able to 'improve' your power output. The stages meter will still help you do that, either way.

neeb

Sort of agree, but it monumentally confuses things if I’m using different power meters that give different figures.

Arguably I don’t need to know my exact leg imbalance to compensate for it - as long as I can calibrate the stages with a known-accurate two sided system and get comparable numbers. Presumably the bulk of any discrepancy being calibrated for is leg difference, but if some of it is something else that doesn’t matter much.

slowbike

TBH - in your case - I'd stick with a P2M or powertap as you're not going to be happy with a Stages and will always question the results.

For me - a Stages is absolutely fine as the numbers it churns out are comparable to my turbo and seem consistent enough to be trusted as accurate - I've tried a wattbike and seen the "imbalance" I have - which wasn't much at the time ...

joey54321

What about swapping the P2M between the two bikes? I used to do it with my Quarq before I bought a second (and that was only so I had a compact and a double). Takes maybe 30-45 seconds.

superkenners

Does it matter, if you are X watts on your powertap and y watts with stages after cycling bit you'll know what the difference is between them, so it'll make no difference. It's not as if you'd be thinking ah I'm 10 watts down on my stages, need to pedal harder as powertap on different bike told me to.

If you don't know what you L/R imbalance is now, that's not going to matter either.

neeb

Superkenners wrote:

Does it matter, if you are X watts on your powertap and y watts with stages after cycling bit you'll know what the difference is between them, so it'll make no difference. It's not as if you'd be thinking ah I'm 10 watts down on my stages, need to pedal harder as powertap on different bike told me to.

If you don't know what you L/R imbalance is now, that's not going to matter either.

But if the data all feeds into the same apps it screws up all the stats and metrics.

Actually it would make sense if all power meters allowed callibration to compensate for consistent, measureable inaccuracies and/or differences between meters. This would be a second, user-controllable callibration on top of the automatic callibration for temperature etc. My guess is that it’s not encouraged because it could be used to cheat on Zwift etc.

joey54321

neeb wrote:

But if the data all feeds into the same apps it screws up all the stats and metrics.

Actually it would make sense if all power meters allowed callibration to compensate for consistent, measureable inaccuracies and/or differences between meters. This would be a second, user-controllable callibration on top of the automatic callibration for temperature etc. My guess is that it’s not encouraged because it could be used to cheat on Zwift etc.

Most do; I know Quarq and SRM 100% do. Oh, and there are far easier ways to cheat on Zwift, if that's what anyone is into.

I think it would screw up your data less than you'd think. If the stages is in the right ballpark then your 49/51% split will likely not be meaningful. Probably not more or less consistent than between your P2M and your powertap.

Edit: just looked and P2M do too.

neeb

joey54321 wrote:

neeb wrote:

Edit: just looked and P2M do too.

Didn't know that - useful, thanks.

sungod

as joey54321 said

the head unit can take calibration from the pm (not zero setting which is sometimes misnamed calibration)

but srm head units certainly allow this to be overridden (it's needed if you calibrate the pm but can't reset the internal slope the pm sends, you instead set the correct slope in the head unit)

worst case you can export the data files into a format excel/spreadsheet of your choice can open, modify the data then import back

neeb

I've never noticed such a setting for my P2M NG (used with a Wahoo Bolt). Will need to delve further..

joey54321

neeb wrote:

I've never noticed such a setting for my P2M NG (used with a Wahoo Bolt). Will need to delve further..

I believe it's done through their app.