An 11 speed cassette won't work with your bike.

Bummer, unfortunately I was correct. I'm glad I bought a 9-speed cassette.

If the chain is pretty old, only run it on the old cassette. In that case, you'd want to get another chain and swap it on when you ride on the road with the new cassette.

If the chain is still pretty new, put the new cassette wherever you ride and shift most...with preference going to the road. (a slipped chain on the trainer is no big deal. It may crash you on the road.)

Since you are in the money saving mode, why not return the new cassette and simply swap the cassette when your switch from road to trainer?

That's a very good question. I didn't even think about it. I've never replaced a cassette in my life before (but I bought the required tools: chain whip + lockring tool). I can see how much time I will spend doing that.. but the cassette was pretty cheap $28 from REI (https://www.rei.com/...g50-9-speed-cassette).

You've got the tools. Give it a shot! There are countless youtube videos but GCN or park tools are consistently good and reasonably streamlined. (It is crazy how there are 10 minute videos for a <5 minute procedure.)

It is as simple as:

1. insert the lock ring tool in the slots on the hub
2. wrapping the chain whip on the correct way - something I consistently fail at, but becomes immediately obvious when you start the next step
3. loosen the lock ring tool (lefty-loosey)
4. pull the cassette off the hub
5. put it on the other hub
6. put the lock ring tool in the slots on the new hub
7. tighten pretty tight...less than lug nuts on a car, more than a jar of pickles (you do not need the chainwhip since the freehub won't spin...just hold onto the tire. You may need it for the trainer if you can't hold the flywheel still)

If you are slick, you can pull the cassette off and place it back on (steps 4 and 5) without the cluster falling apart! Even if it does, no worries. You can't put anything important on upside down and just make sure all the spacers go in. If you miss a spacer, you'll look at the installed cassette from behind, and see some cogs will be visibly way too close together. Take it off, put the spacer where it needs to be.

A common problem is that the lockring is on there way too tight or has seized in place. Otherwise, if you get the lock ring off and the cassette won't budge, maybe try one at a time and they may need a little back and forth encouragement.

Give it a shot and post back if you have troubles.

9 speed stuff is stupid cheap nowadays. I'd just buy a new 9 speed cassette on ebay for 20 bucks, a new chain for 12 bucks, put new cassettes on the trainer and the bike. Depends on how much you value your time, but you can switch the cassette as others have said.

https://www.ebay.com/...8:g:6-kAAOSwsupckYy5
https://www.ebay.com/...1&frcectupt=true

Thank you for your detailed explanation! I really appreciate it. :)

Yeah, it's really cheap. I bought this cassette from REI: Shimano Tiagra CS-HG50 9-Speed Cassette (https://www.rei.com/...g50-9-speed-cassette) for $28. I could spend another $30-ish to get new chain and new cassete. The question now is: should I get new chainrings?? I might as well huh?

I wonder if this is what I'm looking for: https://www.ebay.com/...ainring/223438636891

Is everything compatible?

You do not need to change the rings as often as the chain/cassette...which should almost always be changed together.

Do the chain ring teeth look like a shark fin at all - straight on the trailing edge and rounded on the leading edge? Another test is to put it on the big ring and pull on the chain from the 3:00 position. If anything is worn, the chain will lift away from the ring. If you replace the chain and it does not lift anymore, the rings are probably OK. If the chain still lifts, then the rings are worn and need replacing.

Wow, those rings are cheap though. To ensure that they are compatible, look at the numbers on your installed/old rings and buy the same!

Depending on the crankset, you will almost certainly need to remove the crank to change the rings. This is also quite easy with hex head bolts on the non-drive side. You might need a rubber mallet to pop out the drive side (or a hammer with something on the head to make it slightly squishy). Also, you'd want to have some assembly grease on hand to clean everything and re-lube it before putting back together.

I wouldn't get new chainrings. The only reason I've ever bought new chainrings is because they were bent, not because they were worn. Unless you've ridden like 20,000 miles in bad conditions on those chainrings I highly doubt you need to change your chainring(s).

I took a photo this morning.. the teeth look very different from each other. Is this normal? Or should I get a new chainring too??

Perfectly normal.

Hey guys, last question!! And this one is probably going to be stupider than the others... I'm still learning. :)

The original cassette I bought is 12-25T and the cheaper one suggested above is 11-25T. Will I be able to use the same chain length for both cassettes (the one on the trainer 11-25T and the one on the bike 12-25T)??

Thank you so much. Everyone has been extreme helpful. I really appreciate that.

Yes.

Thank you! Just bought new chain + new cassette (second).