Repair notes
Removing and Fitting a Cassette in Your Garage
I put off learning how to remove a cassette for way too long. It seemed like one of those jobs that required special knowledge or expensive tools, but honestly once you've done it a couple times it's pretty straightforward. I'm writing this from my cold garage where I just swapped out a worn cassette on my commuter bike, and I figured I'd share what actually works for me. You'll need a couple specific tools, but this is absolutely something you can do at home without taking your bike to a shop.
Why You'd Need to Remove a Cassette
The most common reason is that your cassette is worn out. If your chain is skipping under pressure, especially on the cogs you use most, that's usually the sign. I also remove mine when I'm doing a deep clean of the drivetrain, or when I need to replace a broken spoke on the rear wheel. Sometimes you'll pull it off just to inspect things back there or to swap to a different gear range. Whatever your reason, the process is the same.
The Tools You Actually Need
You need two specific things: a cassette lockring tool and a chain whip. The lockring tool fits into the splines on that threaded ring holding everything together. The chain whip is basically a length of chain attached to a handle that lets you hold the cassette still while you loosen the lockring. I also use a big adjustable wrench to turn the lockring tool, though some people prefer a specific socket wrench. Wear gloves if you have them because everything back there is usually covered in grime. I learned that the hard way with black hands that took ages to clean.
Taking the Old Cassette Off
First, pull your rear wheel off the bike. I flip my bike upside down or put it in a workstand, whatever's easier. Wrap the chain whip around one of the larger cogs so it's gripping tight, with the handle pointing counterclockwise. Fit the lockring tool into the splines on the lockring, then put your wrench on that. The lockring loosens counterclockwise, so you push the wrench counterclockwise while holding the chain whip steady. It might be tight at first, especially if it's been on there a while. Once it breaks free, you can unscrew it by hand the rest of the way. Then the cassette just slides off the freehub body. Some cassettes come off as one piece, others have spacers between some cogs, so pay attention to the order if yours does. I take a quick photo with my phone just in case.
Fitting the New One
Before you slide the new cassette on, I wipe down the freehub body with a rag. There's usually a buildup of gunk back there. Look at the freehub splines and you'll notice one is wider than the others, and your cassette has a matching wider gap, so it only goes on one way. Slide it on, making sure it's seated fully. Thread the lockring on by hand clockwise as far as it'll go. Then use the lockring tool and wrench to tighten it down. You need it properly tight, but you don't need to hulk out on it. I usually give it a firm snug and then maybe another quarter turn. Use the chain whip again to hold the cassette while you tighten. Put your wheel back on, and you're done. The first ride after, I always check that everything's shifting smoothly and there's no weird noises.
Questions people ask me
Do I need to replace my chain when I replace the cassette?
Usually yes. If your cassette is worn enough to need replacing, your chain is almost certainly stretched and worn too. A new cassette with an old chain will wear out much faster, and an old chain on a new cassette might skip. I always replace them together. It's frustrating to spend money on both at once, but you'll save yourself hassle in the long run.
How tight should the lockring be?
Tight enough that it won't work loose while riding, but you're not trying to make it permanent. I've seen specs around 40 newton meters thrown around, but I don't have a torque wrench for this. I just tighten it firmly with my adjustable wrench. If you can't budge it with reasonable force when you go to remove it next time, that's about right. Don't use a massive breaker bar or anything.
Can I mix and match cassettes and freehub bodies?
Mostly yes, but you need to match the standard. Shimano and SRAM cassettes generally fit on the same freehub bodies for road and mountain bikes. Campagnolo uses a different spline pattern. Some newer stuff uses different standards too, like XD or microspline for certain wide-range cassettes. Check what you have before buying a replacement. The number of speeds doesn't matter for fitment, it's the spline interface that counts.