So I bought a new chain for my marin rift zone. I removed links to make it the same length as my old chain, But it just seems to roll over the cogs on the rear cassette. It’s ok when I pedal slowly but under pressure it just crunches 🤢 my original chain is KMC X12 that says 9c on some of the links and my new chain is KMC X12 but says 9L on some of the links. I’ve tried searching online for info and cant find any. What does 9c and 9L mean??
You may have waited too long to change the chain. What often can happen is that the chain wears out and the distance between rollers gets longer (widely referred to as “stretch”, though that is a misnomer). The wider space between rollers causes the teeth in the cogs to wear down very quickly. That wear let’s a proper spaced chain slip. You may need to replace the cassette as well. To avoid that going forward, check your chain wear frequently and replace your chain before it gets too bad. This will prevent cog and chainring wear.
As Texmex explained, your cassette or chainrings could be worn some. Before you start replacing parts first determine whether it is the cassette, chainring, or both. If the crunching sound occurs in all gears, it is likely your chainring. If the crunching sound only occurs in some gears, it’s probably the cassette. However, and this is an important point, a new chain is nearly always crunchy on even a lightly used cassette and chainring. But, after a few rides that noise goes away. It takes a few rides for the new and old parts to mesh. So take your bike on a few rides before buying a new chainring or cassette. If you still have noise, then only replace one part at a time. Picking the most likely part that is worn.
I ride a lot and I replace my chain every 6 months. A 1x aluminum chainring lasts about 2 years, a 1x steel chainring lasts about 4 years, and a cassette lasts about 3 years. However, if I didn’t replace my chain for 18 months the cassette and chainring would also need replacing. The more often you replace your chain, the longer the cassette and chainring will last.
I never mix and match parts. If I have a Shimano drivetrain, I use Shimano parts. Sram drivetrain, Sram parts only. Maybe, I’m just being superstitious but I think it makes a difference—reduces wear and improves shifting.