Rally XC 100 understimate

Hi, I don't see a "Mythos First Climb" segment on the second ride linked (and didn't see any with exactly 197W avg).

Anyhow it's very difficult to compare power measurements from 2 very different types of bikes and courses. The MTB and road bike can require different power for the same VAM because the total bike/rider/gear weight is likely different, and also the rolling resistance (tire and surface condition). That can also affect the average calculation (steady power vs fluctuating power).

We also have to consider the difference in power meters. Assuming the Quarq is a hub-based PM, then it's measuring total power. However the Rally XC100 is only single-sided, which it doubles to blindly estimate total power. If you have a natural left-right power difference then it will over/underestimate the total power. Since the XC100 measures from the left pedal, a weaker left leg would result in a total power estimate lower than actual.

If you're concerned one of the meters is inaccurate (well, for sure the XC100 is guaranteed inaccurate if L/R balance isn't 50/50), then you'd have to try some testing with a dual-side pedal or crankarm PM to see your actual L/R balance. Can also try various recalibrations, static weight tests, etc.

Also from my experience, XC100 (single-side measurement) is much more noisy than XC200 (two-side measurement). I guess it shouldn't be surprising given that it just doubles half of the measurement. With more noise I would expect NP to be inflated.

Not sure if your Quarq is single-sided (spindle) or total (spider), but it could make a difference. Why not install the pedals on your Quarq crankset and compare during one ride?

my Quarq is spider.. it mensuret the total power... 

Hi, if both power meters are assumed to be accurate, then yes putting both on the same bike would let you determine your actual L/R balance: total power (Quarq) minus left power (XC100) = right power.

The problem is if you doubt the accuracy of at least one of them, then it's inconclusive because you have only 2 devices and you don't know your true L/R balance. (My understanding is that Quarq spider-style power meters do not measure L/R balance, they only estimate it, similar to how direct-drive trainers do it.) So at best you might see that it appears to be close enough to 50/50 to satisfy that it's probably ok after all (even though it's technically still inconclusive).

So I think it depends on what your question is. If you want to know why 2 very different rides on very different bikes with very different power meters give different measurements: it's because physics (and also physiology, if you have L/R imbalance).

If instead you believe at least one of the power meters is inaccurate, then you'll have to start doing a lottttt of testing...

it is the typical problem of "what is the exact time if i have 2 wristwatches"... Which Quarq spider do You have, the older one or the newer AXS version? Because the older ones are famous about overreporting the power with 5-15% for sure (bad factory slope calibration, the user can make it more/absolute accurate with a heavy known weight, i had many /bad/ experiences in my life with powermeters and dual/triple measurements and comparisons), and if your left leg is a bit weaker than the right one, the XC100 a bit underreporting, and if you compare these the difference easily can be 15-20%.  IF you can put the XC100 onto the road bike, and can record with 2 head units , pls pedal only with one leg and compare the results (ofc, the XC100 will show double value as the Quarq in this case but with an easy math, you can compare the values on the fly , set the 10s and 30s power avgs as datafield on both headunits and check). Or get a heavy known weight (20kg or more) and validate both powermeters (static weight test, search for it on google) 

But the main rule is if you have more bikes : DO NOT USE DIFFERENT POWERMETERS ON YOUR BIKES, ONLY 1 BRAND and MODEL for all (I have 3 Power2Max NG spiders for my all bikes) otherwise you will have more problems and anomalies and will lose your hair (if you still have some :D)

Luko79 said:

IF you can put the XC100 onto the road bike, and can record with 2 head units

An alternative to using 2 recording units is the Connect IQ datafield ANT+ Power. It can record power (and cadence) data from a second power meter, which can then be compared with the primary power meter data using Garmin Connect or a 3rd-party app.

no more hair sigh!!!

read this: https://www.dcrainmaker.com/2021/03/garmin-rally-power-meter-review-spd-spd-sl-look-keo.html, at the END where it talks about MTB test... this is the answer i think

marconazzi said:

read this: https://www.dcrainmaker.com/2021/03/garmin-rally-power-meter-review-spd-spd-sl-look-keo.html, at the END where it talks about MTB test... this is the answer i think

No, it's not really the answer. The answer was already provided above.

Although Ray's article discusses some of the variability inherent in testing power meters, the key thing is that all of his data comparisons are with power meters on the *same bike* on the *same ride*.

Again: You cannot do a valid comparison from 2 *different bikes* on 2 *different rides* on *different terrain*. A similar VAM does not guarantee similar power, because there are also other variables in the equation (weight, rolling resistance/slip). And finally, a single-sided power meter cannot match a total-power meter unless the rider's left-right balance is 50/50.

And all of those factors are before we even consider the power meter's response in dynamic conditions, like those in Ray's MTB tests.

If you decide to test both power meters on the same bike/ride, it would be great if you could share the results here. I'm really curious to see how it turns out.