Chain repeatedly slips on rear cogs to next higher gear...and sometimes back and forth. What to adjust?

Question

EDIT: Just to be clear, everything has worked fine with the bike and the Kickr up until a few weeks ago (that is, for about six months the combo has worked fine). So something's gone a bit out of whack. I'll try the instructions helpfully provided below.

I have a Merida Scultura 100 connected to a Wahoo Kickr. For a few weeks now I've been having problems where, particularly in a certain combination of gears, the chain won't stay put on the rear cogs, but keeps slipping "up".

Specifically, when I am on the larger (higher?) of the two gears in the chainring in the front, and I am switching from the larger to the smaller cogs on the rear (I believe this is 'lower to higher', but I never get it straight -- basically from easier to harder), when I reach the middle set of cogs on the back the chain won't stay there longer than perhaps 15-20 seconds, particularly if I pedal harder. It jumps up to the next smaller cog, then back down, then perhaps even further down, then back up again.

If I keep slight pressure on the gear lever, the one that would make the rear derailleur move the chain down (that is, to a larger cog), but not pressing hard enough to actually make it change gears, then it doesn't jump but instead works just fine. That is, the slight pressure seems to keep it from jumping up.

I'm wondering if there's something on one of the derailleurs that I can adjust, that is, one of the screws? I know there is one that deals with keeping the actual shifting distance correct, so the chain doesn't jump too far, but I don't know if that's what's going on here. (I am wondering whether the chain is loose, but I had it changed a couple of months ago -- I have an appointment at the bike shop in a week or so to have everything checked.)

The cassette on the Wahoo (the ones where the chain is slipping) are only about six months old, so I don't think there's anything wrong with them.

If there's no screw to adjust to solve my problem, then is my method of keeping slight pressure on the shift-lever OK until I get to the bike shop?

Answer 1

First and most importantly: Are you running a compatible cassette on the trainer? the Kickr comes with an 11-speed and that won't work with an 8-speed bike because the cog spacing is different, plus 8-speed chains are wider than modern 11/12-speed ones).

(Thanks Chris for pointing that obvious one out in the comments)

If the cassette is compatible, I think this is related to rear derailleur indexing, this is done via the barrel adjuster on the cable's end on RD, the screws are for limits, i.e. how far in and out the chain is allowed to move out at the position of the smallest and largest cog but that doesn't matter in the middle of the cassette.

There are plenty of threads related to this topic, for example Adjust a Shimano 105 9x rear derailleur

Or simply check YouTube's most prominent video on this topic:

Coming from a bike that probably worked just fine with its original wheel in, I would focus on indexing only, a slight quarter turn at a time, reassess and repeat as needed. I use the Kickr app, put some moderate resistance (80-100W) on and then try to pedal and shift through the gears.

I don't know the specs of the bike for sure but if your Scultura 100 features 8-speed Claris, you have to fit spacers (the 1,85 mm spacer usually provided with the Kicker) before adding the cassette.

Even if that is the case and properly installed, there is still a slight chance that the indexing on your road bike doesn't match the trainer exactly and it needs slight adjustments (keep that in mind when switching back and forth between indoor and outdoor riding).