2 high end Power Meters display different power data...?

Hello team,

I have a MTB and a Road Bike.
Both have PMs, the MTB has a Quark, while the RB has a Stages one.

But I feel that it is much harder to produce power on the RB. In other words, 300w on the MTB feel much more pleasant than 300w on the RB.

I always calibrate them before I start my rides… And the baterries are not the issue, either…

But is this possible? I thought they were both well-renowned PMs but this difference seems evident every time… Or could it be that because the body position is a little different and I use the MTB more often than the RB, the latter just feels harder for that reason?

So… It’s hard to train (literally…) because I would love to mix them up, using the MTB some days and the RB other days… But I tend to gravitate more towards the MTB as the zones/feelings seem more accurate…

And what should I do, guys?
I could do two FTP tests but that would make things complicated… I would have to change my settings every time I switch bikes so I record the correct zones…

Please enlighten me.

To be sure of their relative readings, you’ll need to find an alternate power measurement means common to both bikes - can you borrow some power pedals or a smart turbo that you could put both bikes on?

I won’t claim to be an expert but it sounds pretty feasible to me as they’re different bikes. I put out slightly more power tucked down on a TT bike than I do on a road bike.

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First, Stages is hardly a “high end” power meter. That is, unless you are willing to accept problems with power spikes and water ingress and still call it high end.

Second, you are comparing twice the power generated by your left leg with the sum of the power created by both legs. The apparent discrepancy you feel - which isn’t great to go by - could be entirely due to a left-right imbalance.

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So it’s entirely possible that they differ by a bit. They are being calibrated around a different ‘true north’.

Is the Stages a left only? A strength imbalance could account for part of the difference. If your left leg only puts out 49% of the total power then the stages will read 98% of the power you are truly putting out.

Your position is most likely different between the two bikes. Saddle setback, stack, reach, etc can all affect your power and RPE leading to your perceived difference. Does the gap close if you sit up on the tops of the road bike to open up your hips a bit?

The only way to be sure there is a difference is to compare them both to another power source. So power pedals or a trainer. If your quarq says 300w and the pedals say 300 and then you put the pedals on the stages and it reads 300 while the pedals say 310 then you can reasonably say that they read differently.

What does the bike fit look like? Crank length difference between the two?

I also find it easier to produce power on my MTB despite the P2M that it has reading slightly lower than the Quarq.

Can you put both bikes on the same trainer and do some comparisons?

You might check out this topic I started. My Stages read low and after doing a factory reset (remove battery, short terminals, see topic) the power came up in line with my Powertap wheel.

It could very well be that you just put out more power on the mountain bike because of its upright position. Usually you lose power as you get more aero. This can be mitigated by training in the aero position, but that is a contributing factor. Moreover, if you are close to FTP, even a 2-3 % difference is substantial (which would be the largest margin of error you may expect between the two power meters).

A few questions:

(1) Is your stages left-only?

(2) Do you happen to have a recent Shimano crank on your road bike and you have left-right measurements?

If the answer to either question is yes, then your Stages power meter may be off by a few % points.

Yet another contributing factor is cadence: on my mountain bike, my average, self-selected cadence is lower than on my road bike. This means you will fatigue differently and be able to produce power differently. A higher cadence taxes your cardio-vascular system more, a lower cadence puts more emphasis on your muscles’s ability to produce force.

Wow. Didn’t know that about Stages…

Interesting… Thank you.

The answer is yes to both. Great insights. Thank you.

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This is a trace showing the difference between my Assioma’s and a Stages ( Both left sided only). 8% difference.

Screenshot 2020-07-27 at 09.39.07