I've been using a Quarq DZero power meter on my turbo trainer all winter and have noticed a weird phenomenon that arises when I do mechanical work on the bike. Each time, for the first couple of weeks (perhaps around 10 workouts or ~20 hours of riding) after refitting the Quarq to the bike it seems to give erratic cadence and power readings. For example, say I'm trying to hold a constant cadence of 90 rpm, my Garmin GSC-10 will say I'm holding it very steady in the 89-91 rpm range but the Quarq AxCad values will be jumping around all over the place in the 86-94 rpm range. What's more confusing is that after a couple of weeks, the Quarq AxCad measurements sort themselves out and become nice and steady like the GSC-10.
This process has happened to me twice now, once when I first installed the Quarq and a second time when I removed it to change chainrings and reinstalled; same process both times, with a couple of weeks of readings all over the place and then settles down to something acceptable. I trawled back through my TrainingPeaks library to find a nice visual representation for you guys; here's two identical steady-state workouts, two weeks apart before and after this apparent settling-in:
Note: the scales for both power and cadence are identical here and obviously the smoothing slider is set to zero for both.
Furthermore, in between these two workouts, I found another one where the erratic cadence values seemingly fix themselves mid-workout; here the cadence was all over the place for the first 1/3rd of the workout as it had been for the last couple of weeks, I start doing some seated-to-standing transitions (the series of dips in cadence below) and after this the erratic cadence has fixed itself and has been great ever since:
The cadence itself doesn't annoy me so much, more the fact that the Quarq is using this rubbish cadence data to calculate the power and as you can see in the two workouts above, it has a big effect on the variability of that too.
It looks to me like there's some kind of mechanical settling-in going on here and the eccentric stress of the out-of-the-saddle efforts have fixed whatever was wrong and helped the Quarq to read more consistent values. Last time I checked all of the torques on the chainrings, bottom-bracket etc and nothing was amiss so I'm at a loss as to what could be happening, does anyone have any idea? Possibly the chainrings settling in and being forced up tightly against the chainring bolts or something? I've noticed a few complaints about Quarq's AxCad online and initially thought it just sucked compared to a magnet-based system, but then once it settled in it was almost identical to the GSC-10, so I'm thinking it might also be an issue for these people too. Has anyone seen this before with a Quarq or other crank-based power meter?
I'm planning on a full strip-down service of my bike at the weekend to get it ready for the outdoors again and fully expect to see the issue resurface for another few weeks. Just wondering if there's anything that I can do to avoid it returning?
Quarq DZero AL 110 BCD
SRAM Team GXP BSA bottom bracket (no frame adapters etc)
Shimano 105 5750 rings (50-34)
KMC X10-EL chain
N.B. regarding firmware, I was running v2.0 for the first workout here and upgraded to v3.0 that supposedly addresses an issue with erratic cadence values shortly thereafter and it didn't help at all; it was a couple of workouts later (Mont Gosford above) that the issue fixed itself mid-workout as shown.