jml2

I am a newbie, and this is probably a simple question, but I have a new Trek Domane 2. It is the 2020 model. I from what I can tell from the Trek web site it has a Shimano Claris R2000, 8 speed shifter, and based on the back tire there are 8 gears there so I think that is correct. In the winter 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 a 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?

FTB

Good question. If you want shifting to work across both the rear wheel on your bike and the kickr, the you need the same number of cogs on each cassette. SO since you have claris 8 speed, you need a claris 8 speed cassette.
If you told the shop which you bought the 10 speed cassette from that you needed 8 gears, then they shouldn't have sold you the wrong cassette.
The reason the top and bottom gears are working is because the limit screws (the range left and right which the rear derailleur can move) is the same across both cassettes, but the jumps between the cogs are different sizes hence the imperfect shifting.

You need an 8 speed cassette

jml2

They didn't have. Will any 8 speed cassette work or do I need a claris one?

dsbrantjr

Any Shimano, SRAM or compatible (not Campagnolo) 8-speed cassette should work.

jimc101

Either you asked the wrong questions at the bike shop, they failed to ask the right questions, or their just plain incompetent.

You have a 8 speed bike, the 10 speed cassette won't work, ever with the shifters you have.

As per #dsbrantjr, get an 8 speed cassette

jml2

Thanks guys!! The shop was shady to say the least... I would say incompetent, they just wanted me to know that I couldn't return the cassette if I bought. I'd like to see wahoo tell people to make the cassette on their bike so idiots like me don't make mistakes like this, now I know. Thank you!!

jml2

I think the would be the one: bike.shimano.com/en-EU/product/component/claris-r2000/CS-HG50-8.html

I looked on competitivecyclist.com and didn't find one and amazon.com lists: SHIMANO HG50 8 Speed Mountain Bike Cassette (Nickel - 11-34) which doesn't sound like the right one.

HillRider

People do make cassettes that work on Wahoo trainers but you have to know what number of cogs your bike has. Wahoo has no idea. Yes, your bike shop was either incompetent or dishonest but it's not Wahoo's fault. Consider this a learning experience.

dabac

the MTB part is not an issue. The size of the largest sprocket (34 teeth) can be.
Your chain may be too short to use a 34T largest sprocket and your derailer may not be able to soak up the chain length difference.
easier allround to get a cassette with the same Max-Min tooth count.

jml2

The trek website list this for the drive train which I assume is stock. I'd like to match it:
Shifter Shimano Claris R2000, 8 speed
Front derailleur Shimano Claris R2000, 31.8mm clamp
Rear derailleur Shimano Claris R2000, long cage, 34T max cog

I'm working on military base overseas and since I think the shop here is shady any recommendation where I could purchase one online?

HillRider

Do a Google search for 8-speed cassettes and you will get numerous online sellers. Try to get no more than a 30T largest cog or one no larger than your present cassette. Any Shimano, SRAM or Sunrace 8-speed cassette will work.