And I stand by my assertion that your data are crazy close for two power sources, and even more so for a wheel-on trainer. I had a Tacx Vortex which was light years worse than that - I could clearly observe the thermal effects within intervals. I was training in a very cold room though, so it would cool off markedly within each rest interval. I’m guessing you are in a warmer place, or else the Snap has much better temperature compensation.
I am in the process of doing a full analysis of Kickr Snap versus Quarq and intend to do an in depth separate post with all the setup details. That was not my intention when I posted. Rather people have suggested that the “wheel on” trainers were way off and users should expect a big drop in FTP when they get a “real” PM. My point was to show that’s not the case.
OK cool - that will be interesting. I can add my experiences with my Tacx Vortex (but that could be quite brief and involve swearing!). From mine and others’ experiences, I think my overall conclusion is that wheel-on trainers are more likely to suffer from differences from ‘truth’, and are more vulnerable to variability, but (a) direct drive in itself doesn’t guarantee an overall improvement (especially in training usefulness), and (b) careful understanding and use can often improve things, and (c) there’s at least one out there that’s amazing, but mine was not.
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That is probably all true regarding direct drive versus wheel on. I’m finding oddities when I change up gears and vary cadences…
Interesting - were you in erg mode for that test? It’s very hard to guess what’s going on without knowing which trace is which power source, and whether your were in Erg mode or manually holding a given power (e.g. from looking at Quarq data). Did you change gear to change cadence, or just let Erg mode do its thing?
On my Vortex, it would take a while (maybe 20 minutes) to get warm. During that time the effective calibration factor would drift as things get softer / less viscous. After then the effective calibration factor would be affected by both power output and wheel speed. In general, faster wheel speed => more fluid resistance => more heat build up and thermal drift (so Vortex would tend towards reporting higher virtual power). More power => more electrical brake resistance => more heat => higher reported power for a given true power.
Things didn’t always precisely follow that simple model, but I ended up concluding that most of the crapness that I saw with my Vortex was down to heat management / compensation. I don’t actually know if it has any active temperature compensation (i.e. other than just saying “warm it up before doing a spin down”). It was actually not far off my Powertap and pedals if I e.g. held a steady 200W for 20 minutes, did a spin down, and continued at 200W. Trouble is, even then my body heat would warm the shed up… 
Erg mode for all my tests, letting it do it’s thing. Red is Quarq and Blue is Kickr Snap Erg. Didn’t change gears in this test, only cadences. Using a 1x setup at 32 x 18. All tests are proceeded by a Quarq calibration and a separate Free Ride warm-up workout (about 115 watts) then a Kickr spindown calibration inside of TR. I’ve tried to isolate every variable to include temperature. Trainer is in my basement and temperature doesn’t vary more than 5 degrees day to day (about 65 degrees on average).
So the most interesting bit to me is what happens at 30 minutes…? The target power steps up, the Snap (obviously) thinks the delivered power has gone up accordingly, but the Quarq simply says “meh” - it seems to notice the spike (probably cadence change related as much as anything) but settles out at the same level as it was before.
If you ignored everything after that point, then things hang together perfectly - from say 5 mins up to 30 mins, as you decrease the cadence, the Snap reports lower power relative the to Quarq, which would be consistent with it warming up a bit relative to the lower intensity freeride portion, and with cadence effect manifesting directly through wheel speed. However, after 30 minutes things kind of go in the opposite direction! 
It’s all very odd and there are so many internal variables being adjusted by the Kickr Erg and the Quarq. In the end, the NP and Avg Power between every test workout so far has been within 0-2% difference. Which means within the margin of error of any power meter. So no matter what device I am using, Kickr’s Erg, Powermatch or the Quarq, I am essentially getting the same workout benefits in the end, even if the two devices may stray from time to time. I will continue testing, but my early conclusion is that;
- “If” you hold all variables constant, do daily trainer warm-ups, spindowns, PM calibrations, tire pressure checks, firmware updates (should they be needed), be completely OCD regarding setup;
- Then at least in my case the Kickr Snap and Quarq are the same and inserting the Powermatch variable is not needed. Resulting in my ability to use the same FTP for inside or outside rides as well as continue to use the Kickr’s Erg for ramp tests.
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