Mine is similar to yours. Between 200 and 300 watts, it consistently reads 20 watts higher than the powertap. There is also a bit of drift, presumably from the tire warming up. So by the end of a 60 minute session, the difference gets closer to 30 watts. I've just learned to ignore the Wahoo and focus on the powertap, ratcheting up the resistance as necessary.

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The Kickr Snap has a 5% accuracy from what I remember. Was the Powertap the P1 pedals, or the hub?

Could you amend your spreadsheet with the % error in each case?

I have been down the whole rathole of trying to get my Smart-trainer and power meter to match. Can you just use your power meter as your primary power measurement, and use Powermatch to control the trainer?

30w sounds a bit much to me. If your other meters read close to each other and this is the outlier I'd say you are in the "right" in your claim about the trainer.





Sorry to sidebar, I'll put this thought down here.........
Almost 30w difference at 320/290w..........yeah, now you know why I'm not a huge fan of "racing" with Zwift.

It's an equipment issue, not a "people cheating" issue, but still. You get people sitting there racing against people who both may be cheating their weight AND have a bum power readout from their trainer. 30w isn't trivial if you're close to the limit, aka racing.

I get similar power differences with my Tacx Vortex. Always reads 20-30 watts higher than what my power meter does. I just don't pay attention since I am always using my P1 pedals anyway.

I was using the powertap hub, but I've done similar tests with my power2max and found the same result.

I have not yet tried the powermatch function with the wahoo app - will try that next. Will that then also work when riding Zwift?

Outside sending it back I think it is either set the offset to something like 18 (it has been a while but I think this is still a possible setting) or always use your PM to drive it.

My Gen I is like this as well, but I use a P2M to drive it and it is great that way, less an occasional spike.

I fully agree with you and given that the difference gets larger at higher power the effort required at high intensity moments gets less.

Your data looks like what I would expect if you were having an issue with tire slippage/not clamping the tire down hard enough. Put on a new tire and see what happens.

Before the system and tire are warmed up you are off by about 18%. As the system comes to temperature you hit the 5% difference expected for a Kickr. The change is consistent with expansion of the tire as it heats up. Wheel-on trainers generate a lot of heat at the tire/resistance wheel contact which gets transferred into the tube causing air expansion and a better contact with the trainer.

However the same process eventually heats up tires to a point they begin to deform and slip at a higher rate. Your data shows a gradual increase in the offset between the power meter and your trainer from 5% to about 8% after your first 250 watt interval. I would guess your tire is shot and that Wahoo are correct in stating you trainer is working properly. It is possible the firmware update altered the way the trainer reports peak power pulses associated with tire slippage, but it may be a total red hearing.

There's a couple of ways to do this.

Until a week ago I got the kickr to read my p2m (setup using the wahoo fitness app). That worked pretty well, Zwift happily talked to the kickr which aligned itself.

But recently I started getting an offset between the p2m & kickr (10 or so watts), not the end of the world, but it does make a difference over extended intervals.

So I turned off the power match in the kickr & set zwift to read the p2m directly.

Bingo, no more offset.

The tire has been clamped down at 2.5 full turns and with a spindown comes to a stop in around 13.7 seconds. It should be under 15 per Wahoo. Note that after I started recording the data from the table I already rode on the trainer for 10 minutes, again per Wahoo instructions. The first numbers are before doing a first spindown and then after an advanced spindown and then a simple ladder. I doubt your reasoning about tire warming up first not enough, then too much - because my ladder going UP and DOWN give the same average values at a given setting. For example at 220, 240, 260W... the first interval at that (kickr) power resulted in the same powertap power as the second time I rode at that (kickr) power, meaning that the riding in between did not effect that value. If nothing caused a change in powertap measured power there isn't a mechanism happening that is changing power transfer - like tires heating up / expanding / etc.

Plus, my tire isn't 'shot' at the end of a training to then miraculously 'heal' again when I take it out on the road and when it still has substantial life left per the wear indicators. And I get the same thing with 3 different wheels with 3 different tires...

I'm on my 3rd wahoo kicker and I finally gave up. They were all off 30-50W and showing my wattage higher, in comparison to Garmin vector pedals and Power tap pedals. Wahoo solution was to disable my string gauge (and use software calculated power) which is total crap.
Good thing is, I was always kicking ass on Zwift, with 50W more I was really humming along :) After this season I'm switching to Neo, maybe I will have more luck.

I'm considering getting a Cycleops Hammer instead. My experiences with powertap have been very good and since they are essentially using a technology they've refined over 20 years I think it should be working well. I've had a Tacx trainer before, the 'Flow' which wasn't a smart trainer (bluetooth didn't exist yet and ANT+ was only just becoming available in HR monitors) but it had a power measurement and could ride in ERG mode. It was not very accurate and the roll got incredibly worn out by the tire very quickly. When they came out with the Bushido there were many cases of the motor catching fire... all just to say, I'm a bit reluctant towards Tacx products...

My journey went like this. Early adopter of Garmin Vectors. Bought a Wahoo Kickr after that, and could never get it to match the Vectors. Spent many hours looking at the problem etc..

A few years later heard good reports about the Tacx Neo, so bought one of those. Got perfect match between Vectors and Neo. After that I have spent zero hours calibrating, measuring etc..

Folllowing. My Snap has about the same variance, and seems to get worse at the end of a workout. Sometimes it will almost lock up near the end and is almost impossible to turn over the cranks.

Has anyone of you who also had problems ever tried to get support from Wahoo to address it? I found their customer support to be very poor. The only thing I got were instructions how to do a spindown and besides that, absolutely nothing. (One of these cases where the lack of customer service makes the situation much worse and leaves you with such a bad taste you'd consider any other company before them...)

My experience is that they are (were) responsive, but not very effective. Speaks to some likely limitations or variance in the product. I eventually did get it to work satisfactory, but only with P2M power match driving the Wahoo.

It has been a while but I was accessing the beta releases of the software (TR) and utilized the "secret pinch" on the Wahoo utility app and eventually landed on one that closed the gap. I do not use it standalone as the Wahoo power data (actual) is always much lower than setting. I always know if I get a communication drop from the PM before even looking on TR because the watts will drop 20-30.

Heat-induced drift is an issue for all electromagnetically-braked ergometers.

IOW, it isn't just the tire that needs to come up to a stable temperature, but the entire system.

The now-ancient Velodyne had a pre-heating mode (i.e., full current applied for 30 min or longer) to help deal with this, but AFAIK nobody else has ever included such a feature.

They were helpful when I called. Turns out that an advanced spin down can be harmful, especially as I was doing one almost every time.

Improved things quite a bit, but I still have results similar to the OP. Sounds like the issue is inherent to these devices based on what a few others have said.

What is the "control by power meter" a few folks have referenced?

I just want mine to be a "treadmill" and control the amount of power I need to exhert when doing intervals. I use ERG mode, but have to set it above the target number due to the ~20% discrepancy.

Well, heating the part of a trainer (while probably acceptable solution way back then) might not be that bright of idea now. Sticking some kind of a real torque sensor can cure it in a little bit more adequate way.

I've had a Cycleops Hammer since December and pretty happy with it overall. It's built like a tank and the 20 pound flywheel feels great. Previously I was using a PowerTap G3 hub and "dumb" fluid trainer. With the Hammer I use my Quarq power meter for power (in Zwift) because it's more accurate and that way it's consistent with power data from outdoor rides. I don't think the Hammer or the KICKR are all that stellar with the power reporting accuracy, so if you're picky about that kind of thing I suggest a crank or pedal based power meter. I would never rely on power data from a wheel-on trainer like the SNAP for all the reasons other people have mentioned. I would just use my PowerTap hub and call it good. As for Tacx, they do have a bad reputation with their wheel-on trainers but the Neo is different and something special as far as smart trainers go. I know a couple guys out here on the Big Island that have the Neo and love it.

PowerTap tried that with one of their early indoor bikes (perhaps they still do). The problem is that strain gages are also subject to heat-induced drift, so instrumenting the hub didn't solve the problem.

The only way to make this sort of approach work (well) would be to place the strain gages somewhere that they would be well-isolated from the actual braking mechanism that is converting the cyclist's input energy into heat.