OK I’ve looked into this and even have graphs if anyone’s interested! (they’ll be going into a review I’m doing of the unit).
I put the Powertap wheel on and measure power off the Snap, Stages and Powertap simultaneously. Since the Powertap and Snap measure at the hub effectively I would expect them to read a little lower than the Stages. And also the Stages is doubling the left leg measurement, so some possible inaccuracies there.
The Stages reads about 15W higher than the Powertap - perhaps I’m a little left leg dominant. I doubt there’s that much drive train loss.
The issue with calibration seems to be the tyre used. I use Conti GP4000 tyres which are a very supple rubber, and I also use the 25mm variant. I tried some other tyres, both in 23mm: an old Specialized Mondo and a Conti Gatorskin.
With the 25mm GP4000 my spindown even at 120PSI was 26s with 2 turns of the knob from contact. With the 23mm tyres (with a higher contact pressure due to the smaller contact area), at 110PSI 2 turns gave me around 16.5 seconds, and another quarter turn gave me 15.5s.
In this article (and speaking with their support):
Wahoo state that with the 2nd gen Snap (2017), only spindowns under 10s and over 45s are of concern. But as suggested above even with the newer Snap, I’ve also found that if the spindown is significantly over 15s, power readings will be around 15-20W lower than a calibrated power meter, and in my case on average 35W lower than a Stages power meter.
To get my GP4000 to 16s took 3.5 turns of the blue knob. Which feels too much to me, and seems to put unnecessary strain on the unit and wheel. And also makes the ride less smooth. I’d also be worried about stripping the thread on the standard.
So thank you for the suggestions above.
In summary, having spent far too long looking into this, I’d recommend:
- Use a 23mm tyre at 110PSI (or perhaps a harder compound 25mm tyre?)
- Despite Wahoo’s recommendations, it appears you still need to get spindowns to somewhere close to 15s to get accurate results.
I’m fortunate enough to have a second wheel I will use with the Snap. And I’ve ordered a Conti 23mm trainer tyre to use with it now. And with my sons bike (the main reason I got the Snap was so I could use other bikes too!), the cheap 23mm hard tyres get a spindown of 12 seconds :).
Sorry, a bit long winded!