I’ve just started using a Wahoo Kickr with Zwift. I find that the back gears quite often change up and down a sprocket by themselves which seems strange. I can also be pushing out around 280 watts (@ 85 rpm) on a flat part and the gears change and when I look around I’m on the smallest sprocket (which I assume is a 12). I also would of thought that the resistance on the smallest sprocket (and big front ring) would be extremely hard to spin at 85 rpm on the flat (i.e. would require much more than 280 watts). It just seems weird that my gears are jumping up and down - I’m not sure whether my gears may need tweaking a bit. As anyone else come across this problem. Any help would be appreciated
Cheers.
Rear gears jumping when they weren’t before is often a cable stretch issue. Follow the procedure outlined here
https://www.youtube.com/watch?v=Bbk5RcH0bbQ
The first thing you’ll want to do is check to make sure that the cassette on your bike and on the Kickr have the same number of gears, for example if you’re running a 10 speed cassette on the bike, you’ll experience skipping if you’re running an 11 speed cassette on the Kickr, same for the other way around.
If the number of gears match up, it could be that the gear ratios are different. For example if you’re running a 12-28 on your bike and an 11-23 or 25 on the Kickr your derailleur limits won’t be correct and may cause skipping.
You can fix either of these first two issues by buying a new cassette for the kickr that matches the other. Just make sure you have the proper spacers on your kickr to fit a smaller cassette if needed.
The third issue could be due to the age of your bike, if you haven’t been diligent in replacing your chain every few thousand miles, the chain and cassette on your bike could be very worn out, and when you put the old stretched out chain on the new cassette on the Kickr, it may cause it to skip. This is a more complicated issue. If your chain is so worn it has caused the cassette and chainrings to wear with it, replacing the chain alone won’t work, you’ll need to replace the cassette and possibly your chainrings as well. A bike shop can check this easily for you and let you know if this is the case or not and what it will take to get you going.
The fourth is cable stretch, but if this is the issue, you will experience your gears skipping both on and off the trainer. A quick basic tuneup at your local bike shop will fix this.
Hope this helps 
Thanks very much for the advice.
I’m running an 11 speed cassette on both my bike and the trainer, however the wheel has a 12-25 and the trainer which is new looks to be an 11-26. Therefore, as Crystal suggests I think this may be causing the skipping. I’ve index the gears as shown in the video and this seems to have solved it.
Both cassette and chain are only a month old and although the cables are older, the jumping is not occurring on the bike. Anyway I’m going to try also putting a 12-25 cassette on the trainer (which matches my wheel) and hopefully this will match up and stop me having to re-index the gears when I swap between wheel and trainer.
It’s just a bit weird because there is quite a bit of difference between the gears on the wheel compared to the trainer. Anyway I’ll see how it goes with matching cassettes. It maybe that I do need to use a small spacer.
Cheers for the help.