Problem with cassette and derailleur I am a newbie and this is probably a simple question. I have a new Trek Domane 2. It is the 2020 model. From what I can tell on the Trek website, it has a Shimano Claris R2000, 8 speed shifter. Based on the back tire (where there are 8 gears) I think that is correct. In the winter I want to be able to ride the bike on my wahoo kicker. When I went to the bike store the only cassette they had was the Shimano CS5700 11-25T which is has 10 gears. I've adjusted the tension on the derailleur, but that doesn't seem to do anything. The 4 big gears and the small one seem to work, but not the other ones. What do I need to do to fix this?
You can't use an 8 speed shifter with a 10 speed cassette. The space between individual cogs is smaller than on an 8 speed cassette. The shifter moves the derailleur a fixed distance per shift, so it can't reach the position of the cogs if they are not spaced as intended.
The fact that you can use the 4 biggest and the smallest gear is pure luck, because the derailleur seems to match those positions good enough to make it work.
In conclusion, try to find an 8 speed cassette that fits on your trainer.