Wheel-on trainer vs Garmin Vector 3S power?

I typically train indoors on a Cycleops M2 wheel-on trainer and when riding outdoors I use Garmin Vector 3S (single-sided) to measure power. It seems that when riding outside, my legs were feeling more fatigued than I would have expected based on the watts I was seeing so I did some indoor comparisons. I rode on the indoor trainer with the Vector pedals and was consistently seeing 20-40W less from my pedals than from the trainer. Both systems were calibrated correctly (I also calibrate my pedals before each ride outside). I also know my right leg is substantially stronger than my left leg and have adjusted the Left Scale Factor on the Garmin pedals to try and account for this imbalance.

Is there a right meter to believe? Thanks!

Always hard to say, I suppose, but personally I trust a direct force power meter more than a wheel-on trainer. Even with spindown calibration my limited experience shows high variability in power estimation from wheel-on trainers.

This is unusual. Is there an injury history or something else going on? If so, you might consider upgrading to the dual sided vectors. Otherwise, it’s hard to say either one is accurate. You shouldn’t mess with scaling factors for things like a 52/48 imbalance, and if you have something more substantial there’s probably a history behind it. Any change you made to the scaling factor could render the numbers you get less meaningful. That’s a red flag to me in terms of power meter accuracy.

There are a lot of variables that go into wheel-on power measurement, and I agree w/ @tnordberg that the direct force power meter (Vectors) is usually going to be accurate. But if you’ve messed with scale factors, then the real answer is practically impossible to know.

My advice would be to use the Vectors as you sole source of power measurement (using TR’s powermatch) since there are so many sources of error involved here. Whether that measurement is accurate or not matters less than its consistency/precision.

Previously had imbalance from years of distance running (XC & TF in high school and college). It becomes apparent when I feel muscles tightening and fatiguing on the right side more than the left. I’ve noticed specifically my right glute and quad will fatigur quicker.

Have you ever had a measured imbalance or used a dual-sided power meter and observed a measurable difference?

In the end, what matters most is consistency of the measurement, hence my recommendation to use the Vectors indoors and outdoors. The scaling factor doesn’t matter that much if you measure with one consistently.

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Never done an actual measurement with a dual-sided (need to). I know that my hip alignment issue has previously caused my right leg to be slightly longer than my left which I think may have some impact here.

Understood. I’d advise against making any modification to a scaling factor if you don’t know for certain what the measurable difference in strength is. You’re introducing error into the measurement with that guess. Again, that only affects accuracy of the power meter, which is secondary to consistency of measurement. My advice would be to set the scaling factor back to default and use the Vectors inside and outside, but ultimately the latter part is all that really matters.

If it’s truly something you live with daily and affects your training, I believe Garmin has a less-expensive upgrade option to go dual-sided. It might be worth your investment. I’ve mentioned in other threads that in the future I will not spring from a dual-sided power meter again, but that’s largely because I’m 50/50 or 51/49 consistently.

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Awesome, thanks for all the insight.

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My suggestion is that which one is more accurate is kind of irrelevant in a way. It’s more important to have consistency. I would suggest using power match in TR with your Vector 3s so that when you ride outside you are using the same power numbers.