Jim’s Tech Talk
By Jim Langley
This is the third Tech Talk about cassettes, that cluster of cogs (gears or sprockets) on the rear wheel of any road bike. I’m keeping the topic going because of the comments to last week’s installment.
First, I want to explain something to readers “Henri” and “Walt,” who both shared the idea that there’s no need to tighten cassettes with a torque wrench.
Henri wrote “I’ve done most of my bike maintenance over the last 40+ years, including changing and replacing cassettes. When it comes to tightening the cassette, I have never torqued it after using a large crescent wrench because it is my view that the cassette is going to get tightened as much as it needs just by riding. Am I missing something?”
Walt agreed, “Henri: I do much the same as you. Just tighten with a wrench until tight/no torque wrench even though I have one…the action of pedaling keeps the cassette tight as long as it starts out tight. Never had an issue.”
Freewheels Versus Cassettes
Thanks for the comments, Henri and Walt. Please see the photo here, which will help explain why pedaling cannot tighten cassettes. And, also why tightening did tighten freewheels back before cassettes replaced freewheels on most bikes.
The silver hub in front is the old standard. See how it’s threaded? That hub is a freewheel type hub. Before cassettes became the norm, we used freewheels. These threaded onto hubs. And Henri and Walt, you are absolutely correct that pedaling a freewheel equipped bike would tighten the freewheel. In fact, sometimes they’d get so tight you’d break tools trying to remove them.
However, if you look at the black hub in the back, that is a cassette hub. Notice how the hub has splines or slots on it. The cassette cogs slide onto these splines. The only thing holding the cassette tight on the hub is the lockring that tightens down on top of the cassette cogs pressing them tight against each other.
On a cassette, pedaling the bike doesn’t tighten the cassette cogs. But what it can do is loosen the cogs. That’s because the pedaling force can slightly move cogs and if the lockring isn’t tight enough in the first place it can come loose on its own. That’s where torquing it comes into play to ensure that that doesn’t happen.
I hope this helps explain the difference and why it’s important to get cassette lockrings tight enough. An advantage to cassettes is that they rarely get stuck as badly as some freewheels did back in the day because cassettes are rarely overtightened.
Checking for Cassette Wear
The other comments I want to get back to everyone on is in regards to checking cassettes for wear.
A reader named Gilbert asked, “Will you be able to provide details on how to measure the wear on a cassette and any tools in the market that can gauge when the cassette needs to be replaced. This will be very useful to know.”
Bruce “0le” Ohlson agreed, commenting “I too am interested in measuring cassette wear. My desire is to spend the least amount of money replacing chains, cassettes, and chain rings. Wear on each one influences wear on the other two, so everything is a trade-off.”
Replying to Gilbert and Ole, Greg Przybyl said, “Rohloff makes a very useful cassette checking tool. Used it for years.”
Thanks for asking, Gilbert and Ole, and for the tool tip, Greg. Like Greg, I like and use Rohloff’s cassette checker – but not that much anymore (keep reading). Here it is on Amazon https://amzn.to/35jzSbt While searching I discovered that Unior has one, too: https://amzn.to/3pWiJ14.
Rohloff HG-Check tool
Here’s how the Rohloff tool works. It simulates pedal pressure and shows if a 12- to 21-tooth cog is worn enough to possibly allow the chain to slip off the cog under hard pedaling.
Other Ways for Checking Cassette Wear
If you have a drivetrain with a smallest cog of 12 teeth, you might be perfectly happy checking your cassettes with the Rohloff HG-Check tool.
Modern drivetrains today, however, can have 10-tooth cogs and much larger ones than the 21-tooth cog, which is the limit of the Rohloff tool’s capacity. Maybe Rohloff or Unior will come out with a new updated tool to fit these modern cassettes. Because you can’t look at a cassette and tell with certainty how worn it is. And other than the tools, there’s no surefire way to measure them.
The Ride Test
But there is a check that will work for all bikes, which is to test ride bikes for “skipping.” That’s the sign that a cog is worn out. The teeth can no longer hold the chain and under extreme pressure the teeth allow the links to release and the chain jumps forward and then slams back down – an awful skip that can surprise you and even cause a crash.
You do this test during drivetrain maintenance, most typically after replacing a worn-out chain. Because a new chain will skip on worn-out cogs.
This test should be done on a straight smooth road, relatively traffic- and obstacle-free and ideally there will be a hill to climb at the end. Wear a helmet and keep your weight back toward the rear wheel, never forward over the front wheel. That way you won’t crash if the skip surprises you.
To check for cassette wear you test every cog on the cassette the same way, as follows (keep the chain on the small chainring in front):
1. You may want to spin around a bit and warm up because you’re going to push forcefully on the pedals for all your cogs – starting with the smallest (one of the hardest gears to pedal).
2. If needed, shift and make sure the chain is fully on the smallest cog and running smoothly. Now rise up out of the saddle and pedal hard as if you’re starting to sprint. Or, as if you’re going hard up a steep hill. Do this a few times to make sure there’s no skipping on that cog.
3. Next, repeat the process on each cog in order until you’ve tested every one.
4. At the end you should know if any of the cogs were worn out and which ones they are.
When you’ve determined which cog(s) is worn out, if you’re lucky you might be able to purchase that one cog. If so, you can unscrew the cassette lockring, replace the old cog with the new one and hit the road again.
You might wonder why all the cogs don’t wear at the same time. It’s because we ride in some cogs more than others. So, by replacing the worn cog(s) you can get a lot more miles out of a cassette.
If you can’t find the cog(s) you need to replace, you’ll need to replace the entire cassette. One place you can sometimes find individual cogs is on eBay.com, but be sure to read the description or ask the buyer to ensure you’re not buying another potentially worn-out cog.
Ride total: 10,029
Jim Langley is RBR’s Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his “cycling aficionado” website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim’s streak of consecutive cycling days has reached more than 10,000. Click to read Jim’s full bio.