Other than attempting to estimate whether a chainring or cog is worn and needs replaced by eyballing it, is there any way to measure the wear on chainrings or cassette cogs so you can determine whether they should be replaced? What tools can be used to accomplish this?
There are tools such as the Rohloff HG-check for measuring cassette wear.
And of course there is visual inspection and also how it rides and indexes, particularly under heavy load (when it will usually slip if worn).
Because there exist so many cassette styles and tooth profiles, no universal tool for measuring cassette wear exists.
One good advice to assist visual inspection is this. We all have our favorite speeds. Compare the most worn 2-3 cogs with the rest. If their profile is very different, the cassete is probably worn. The video documents how to do that.
There are many variables in determining whether you have a worn chainring or cassette. Visual inspection helps and noting the kilometres racked up should also help.
If you have set up your mech's correctly you will know you have wear when your chain starts to skip, especially under load. Visual inspection should then confirm your wear.
Park Tools do a CHAIN wear tool [CC-2] but that will not determine cassette or chainring wear. When replacing a cassette you should always replace the chain. Worn chains will reduce the life of cassettes and chainrings considerable due to the chain being worn to the form of the previous cassette/chainring.
Here is a video of the pro's confirming the above:Hope this helps.
Not an easy solution, but you can use a photocopier to measure wear on chainrings. This doesn't tend to work with cassettes cos they're thicker.
First, remove the worn chainring from the bike, and clean it well to keep oils off the copier's glass and undercover. Then photocopy it at 1:1 size.
Flip the chainring and lay it on top of your copy, so that the bolt holes align. There will be at least one position where the teeth line up.
Then trace around the rear of a tooth. Remove chainring and visually compare the overlaps, and then use a caliper to measure the differences for a numeric result.
ANSI standards for industrial chains are that a sprocket should be replaced when
the depth of the erosion along the tooth (x) has reached a value equal to 10 percent of the tooth width (Y) across the pitch diameter (PCD)
This would roughly translate into 0.5 mm, probably impossible to measure while the cassette is on the wheel.
A better way is to compare your old sprockets to new ones, when you clean your ageing cassette. Keeping in mind that sprocket wear is inversely proportional to the cog count, I'd suggest focusing on the 13T-15T-17T (or so) sprockets. Put the old sprocket on top of a new one (you have to purchase a replacement anyway, so buy one as a reference until the old one is worn) and inspect visually to appreciate the amount of wear at the back of the cog.
A few distinct camps appear here, regarding how and whether it is feasible to adequately measure cassette cog and chainring wear. The Rohloff tool looks to be basically an eight link chain bit attached to a small lever arm with the short end effectively adding another link-lengthed arm that is positioned so as to provide force on the chain segment. I would think this is little different, really, from a regular cassette sprocket tool, which might be finagled into position to check the same concept, as long as the chain portion is not old and worn. As shown with chain wear tools, there are often other methods that indirectly measure things like wear, by indirect but sufficiently accurate means for normal use. Wrapping a section of chain around a suspect cog and applying tension should be fairly easy to do, and the principle that a worn cog will not allow more than five pins to move in and out without binding, seems crude, yet the concept automatically incorporates the wear into something observable and apparent, while measuring tiny surface changes is exceedingly tedious and prone to error. http://uniortools.com sells their version with a regular bit of HG chain used as the checking segment, so there is nothing magical or special about the Rohloff, or the concept itself. In practice, I've seen folks intentionally ride chains and cogsets about twenty years past due, and while they had to fight chain hop and shifting issues, they could still ride the bike. The fear over a calamitous event is likely a waste of energy; if you regularly clean and lube chains, replace before the wear point is passed, every five to ten years you might eventually reach a point where a new chain starts misbehaving on an older cassette, when pedaling hard. Just replace your cassette then, before putting too many miles on the new chain, and you're good for another five to ten years. Do not overthink the problem.
I'll just comment on chainrings, since the other responders have already provided good guidelines regarding rear cogsets. My ways of "measuring" chainring wear, without any tool, would be to first wrap a new chain around the suspect ring, then inspect it carefully, to see if the chain sits tight in the bottom of the gap between the teeth (good, so far), or if it appears as if it "lifts" out of the gap (not so good). A worn ring will usually have an irregular, asymmetrical appearance. You will see that the tips of each tooth look as if they are beginning to look sharp & pointy, and the ends of the teeth will have lost some of the nice, square edges where they were originally machined & finished.
The worst effect of a worn-out chain is that it stretches over time with use, and so the pivot bushings are no longer correctly spaced at 1/2-inch intervals. This is what causes the eventual failure of the teeth to grip the chain. The chain will tend to "ride high" on the teeth.
Perhaps the best way to see if your rings are really all that worn is to compare them visually to a new ring of the same brand. Bring your crank to a bike shop and look around. Some types of rings feature somewhat-odd tooth profiles. Do a careful comparison of your rings with some new ones and pay particular attention to the leading edge of each tooth, where it does the work of pulling the chain along. You'll almost certainly see some wear here.
Another way of checking is by merely giving the ring a good cleaning. Does the black gunk built up on the teeth come off easily, or is it practically ground into the metal? Do the teeth have good, square edges, or do they seem as if they were rounded over? Is there a semblance of a flat end on each tooth, or do they look like shark's teeth? Are the teeth finger-friendly, or are they riddled with sharp burrs ?
Compare the way a new chain will stay close to the rings of a new bike when you attempt to lift a section of the chain up and away from the teeth. It should only lift a tiny amount. If the chain can lift off the ring excessively, then it's no longer fully seating between the teeth during rides, and this will cause wear. This is because a stretched chain will ride near the tops of the (worn) teeth, and will chew into the leading side of each tooth. The chain's contact region is being concentrated on a too-small area of each tooth.
Finally, the last one I can think of is to try moving the chain lengthwise, back-and-forth on the ring. A worn ring will allow some movement, while a new ring & chain will seem stuck together. In the end, it's a matter of cost, convenience, and whatever you deem as acceptable ride quality, and you'll have to be the one to make the decision. It may be seen as the right opportunity to replace & upgrade, or it may be the sound of the death knell, and you may move on to another bike entirely. Or, you may find that your rings still have a lot of life left in them. A lot of it depends on the worth of the bike itself.
I'll assume we're asking this question because we have a bike that we love & wish to preserve. But a lesser ride may not be worth the investment in new components. The point I hope to make overall is that it's not usually an easy thing to determine, and you need to use your best judgment. These are most of the guidelines I've used in my 47 years of bike repairs. Hope this helps.
Cassettes are harder to measure. For example they adjust spacing for IG (Interactive Glide).
Chain is something you can easily measure.
And it is the cheapest and easiest component of the drive train to replace.
A stretched chain will increase the wear on cassette and chain ring.
- Measure chain
- Replace chain when stretched
- For sure replace cassette when chain jumps on a new chain
Or a visual inspection of the cassette - compare it to picture or if you have a replacement on hand compare it to the new.
- Chain ring
Visual inspection is a good guide. If the teeth are pointed it is worn.
Cheap it (optional)
At most you are gong to get 3 chains.
In the 3rd chain wait for it to skip and replace chain, cassette, and chain ring. As Hick commented a chain ring will typically last two cassettes so only cheap it when you know the chain ring is also on its last leg.