Check how your powermeters agree with Dual analyses

For all the newcomers on Zwift, this one might sound weird. But for experienced riders, this new advanced feature of dual power analyses on Zwiftpower.com gives them deeper insight into the quality of their power meter (PM) data as well as some kind of extra confirmation that they are using well-calibrated in-game equipment equalizing them with their outdoor real-life performances.

One of the biggest technical issues on Zwift, at least during social “head to head” events is the quality of PM data. The wattage your equipment is reporting during cycling moves your in-game avatar. And we are just as fast as it is our wattage coming up from our PM. Nowadays, most serious riders already use some sort of PM on their outdoor bike. The more serious, the more into sports physiology oriented they are, the more they usually spend for their PM. While there is a hugely technical, quality and prize span between PM on the market, most manufacturers will claim their PM to be within some respectable margin of accuracy. Most commonly, the typical PM will be labeled within +/- 2% accuracy. But, in reality, comparing PM’s with the same stated accuracy from a different manufacturer, shows they rarely agree with each other. In fact, there might be huge differences even among high-end products on the market as we will see in the following example from my own case.

First, take a quick look at the above graph. There are two sets of power data recorded simultaneously during the same sub 4 minute long hard climbing interval. Purple colored line represents my primary Power2Max NG power meter, with claimed accuracy of +/-1%, and the one I always use outside on the road and inside for indoor rides. It is also the one I do trust as much as I did with my previous two SRM power meters. I have compared countless times my numbers on local hills and they where always spot on. Blue line on the graph represents power reported by Wahoo Kickr trainer, with claimed accuracy within +/-2%. Now, it is more than obvious that these two measure the same power very differently. I was more than 30w stronger when using P2M than Kickr during that climb. That’s almost a 6% difference! And the lower graph shows, what the difference was through the entire duration spectrum.

The question is, which power meter should I trust? Could it be the reason for today’s “virtual” high-performance lousy calibration?

The part of the answer can be explained by the fact, that very rarely I do a “spin-down” calibration on Kickr. I don’t use it for powering my avatar. It’s just a smart trainer who needs to transfer virtual road slope & slipstream under my legs. Thus, after several weeks, once the belt slowly tears down, bearings become looser, temperature drops during autumn, etc., Kickr’s power meter becomes more and more miscalibrated.

But, this is just one of the possible reasons. Even when I do carefully calibrate both units before a ride, I can still observe, a significant difference in power reading. An example can be seen in the graph below.

As it is obvious, in this case, Wahoo’s Kickr was reading much higher than P2M NG. Especially during shorter durations with higher forces on the pedal. If I would use Kickr as a primary power meter instead of P2M in the race and sprint, in the end, I would be much faster “virtually” since Kickr has reported more than 50w higher power output during 15sec sprint than P2M. That’s around a 5% difference. And if I would compare two lower-range level power meters the differences would be even higher. Sometimes much higher.

What’s the take-home message of an article? It’s definitely not a suggestion to spend more money and buy a better, more accurate, more reliable power meter. It is about understanding the technical nature of indoor riding and the reason why we are sometimes faster or slower indoor then our friends or peers outdoor on a real road during summer. And why sometimes, when people upgrade their indoor equipment from beginners setup, say lower cost wheel-on trainer, to advanced ones, which usually incorporates much more precise power meter unit might be disappointed or happy over their sudden typical power output change.

Ride On!