Using external power meter with Kickr Snap & Zwift


(Francisco Martinez) #1

Hi all,

I have 2 general questions about using external power meters with Zwift. I’m totally new to external power sources, so apologies if these are silly questions:

Power discrepancies between Kickr and power meter

I have recorded several several activities, using the Wahoo Kickr Snap as both the controllable and power source connected to Zwift, but I also recorded that same activity with my garmin that’s reading the power from the power meter… to be able to compare. In all cases, the power readings are very different, sometimes even 30w difference… with the power meter readings always being higher than the Kickr one.

Note: I did calibrate the pedals, but I didn’t perform a spin down on the trainer.

Is this normal to happen, like such big differences? Is spin down THAT necessary to get more aligned power readings, or is there something funky? I’m thinking on using the pedals as the single source of truth, but not sure if that’s the right approach.

Power meter oscillations

Again, I’m totally new to external power meters, so not sure if this is “normal”. I did interval work, over/unders to be specific, and this is what I got:

As you can see the Kickr Snap is what I would expect, clearly defined power sections. While the power pedals are full of oscillations/spikes. If you squint your eyes, you kind of see the underlying intervals, but still… full of oscillations. Torque effectiveness and pedal smoothness all within normal values. So not sure… if this normal?

Current setup

  • Kickr Snap
  • Favero Assioma Duo
  • Zwift
  • Activity recording: ERG mode, Kickr as both power source and controllable, but recording same activity with my Garmin (which reads the power meter pedals)

Thanks,
FM


(Vincent W.) #2

Hey Francisco,
Are you on the latest firmware for the Favero? Also, you are going to get pretty different results in power from the Kickr and the Favero because of the technical differences in the devices. As long as they’re consistent among themselves then they should be working correctly.


(Francisco Martinez) #3

Yeah, latest firmware on the Faveros. Thanks Vincent.


(Nigel Doyle) #4

The Snap is artificially smoothing out the power. The reality is what the Assioma pedals are reporting. I’ve got Assioma Duo pedals and these agree almost watt for watt with my 4iii power meter.

Only 30w difference, you’re lucky. I went through 2 Kickr snaps and power was way off what my power meter was reporting and it got worse as the power target went up i.e. 250 watts on Snap = 350 watts on power meter.


(Michael Hentges) #5

I have a Kickr Snap, and PowerTap P1 pedals. I got the pedals because I wanted to be able to take power measurement out on the road - not just at the trainer. I quickly found that the power on the Kickr didn’t match the pedals. I thought it was because my FTP was so low (out of shape…), but I just couldn’t get the Snap to be consistent.

If you are religious about topping off tire pressure, and doing a spin-down every session, I think you can keep the Snap fairly consistent - but I gave up and just have the pedals measure power now. At a minimum it ensures that my FTP is the same between the road and the trainer.

The Snap would regulate power more accurately when I didn’t have the power pedals - having the power measurement on the same device that is controlling resistance evened out things nicely. With the P1’s there is a lag between the power reading and the Snap, and it oscillates a bit - but it’s pretty close. I don’t know what causes it, but there are times where there might be a 30s delay getting power right when an interval changes, but usually it’s 2-3 seconds.

I ran into these same problems with Trainer Road - this is not a Zwift issue.

I’m thinking of upgrading to the Kickr Core - getting rid of the need for a tire-on trainer. But, I have a different rear axle on my road bike - 135 x 12 - that I’m not sure would work on the Kickr Core. I’ve rigged it to work with the Snap, but needing the derailleur to line up on the Core is a different thing all together.

Good luck out there!


(Russell Gowers [CRANKS] KISS (C)) #6

Yep, treat the power meter as the canonical point of reference. The only downside is that it makes ERG mode almost unusable because of the lag between the external power meter and the trainer resistance