When to replace cassette?

[Vantage]

I change my chains around 3000 miles and a new cassette every third chain. By that stage, gear shifts are getting pretty sloppy.

 

[Yellow Saddle]

I change my chains around 3000 miles and a new cassette every third chain. By that stage, gear shifts are getting pretty sloppy.

Sloppy gear changes have nothing to do with cassette or chain health. I acknowledge that you may have mentioned gears and chains in one post because three chains may indicate time for a service, but it reads as if you attribute the one to the other.

 

[Yellow Saddle]

On my 8spd I usually get 2-3 chains per cassette. I change the chain when worn and replace the cassette when the new chain skips on it.

Last easter I replaced a chain and it skipped, however i was unable to get a new cassette for a few weeks so I persevered after about 150miles it stopped skipping and was fine for the rest of the (usual) chain life. I have no idea what the mechanism for that was. But I am sure YS will be along with a sensible answer.

Raleighnut has given a good hint towards the process. However, there are other reasons as well. A chain is just an articulated, riveted lap joint. In such joints, rivets have to give a bit before the stress distribution around it and any holes it goes through are balanced and settled. In other words, rivets never go through undersize holes for good reason. Therefore there will be movement between plate and rivet as soon as tension is put onto the chain. That sees to it that the chain very quickly elongates after fitment. Thereafter it stabilises and only elongates through wear.
Obviously wear starts to happen the second the chain is used, but I'll argue that the initial elongation is settlement rather than wear.
Obsessive-compulsive chain measurers like @User9609 may have noticed that a brand new chain is slightly shorter than spec and very soon after fitment it "snaps" into spec and then slowly elongates through wear. guess the chain settles by about 0.005% elongation soon after fitment and then slowly elongates by another 0.5% in the next 2500 kms.
One can hear this process as well as measure it. A brand new chain makes a different noise - slightly grindy, I would describe it - than a chain that's been on there for a while.

 

[Yellow Saddle]

Even Rudyard Kipling understood rivet creep.

"I've got one fraction of an inch of play, at any rate," said the garboardstrake, triumphantly.So he had, and all the bottom of the ship felt easier for it.
"Then we're no good," sobbed the bottom rivets. "we were ordered - we were ordered - never to give; and we've given, and the sea will come in, and we'll all go tot he bottom together! first we're blamed for everything unpleasant, and now we haven't the consolation of having done our work."
"Don't say I told you, whispered the Steam, consolingly; "but between you and me and the last cloud I came from, it was bound to happen sooner or later. You had to give a fraction, and you've given without knowing it. Now hold on, as before."

The Ship that Found Herself.

 

[aferris2]

We seem to have slipped back to the topic of chains, but my question was about the cassette.
Is there something that specific to be looking for to say that it needs replacing? So far I can see that the plates have got narrower at the tips of the teeth and there is a change in shape between a couple of teeth on one ring (as mentioned above). I'm guessing it's the width of the teeth that eventually tell you that it's time for a replacement. I suppose this would also coincide with a reduction in life of the chain too.
I think far too often cycle shops will suggest a new cassette to go with a new chain because there's less chance of the bike coming back with tales of the chain jumping. It might not be too much of an issue with a £5 8-speed cassette, but the 11-speed ones are a bit more expensive!

 

[Racing roadkill]

Well, to whet @derrick 's appetite further, I decided to leave the old cassette on and there's no skipping on the short test run around the village. I compared new and old cassettes and there's hardly any difference in the shape of the teeth (or rather the gaps between). Except for the thickness of the plates, the 25T was the only one that was noticeably different, but only in 2 places.
I hadn't realised before looking closely today that every tooth is a different shape...

That's to aid shifting, they are designed with some nifty CAD software.

 

[Racing roadkill]

We seem to have slipped back to the topic of chains, but my question was about the cassette.
Is there something that specific to be looking for to say that it needs replacing? So far I can see that the plates have got narrower at the tips of the teeth and there is a change in shape between a couple of teeth on one ring (as mentioned above). I'm guessing it's the width of the teeth that eventually tell you that it's time for a replacement. I suppose this would also coincide with a reduction in life of the chain too.
I think far too often cycle shops will suggest a new cassette to go with a new chain because there's less chance of the bike coming back with tales of the chain jumping. It might not be too much of an issue with a £5 8-speed cassette, but the 11-speed ones are a bit more expensive!

Specifically look for 'shark toothing' if any teeth on the sprocket are really sharply pointed, or if any teeth are missing, the cassette really needs replacing.

 

[Vantage]

Sloppy gear changes have nothing to do with cassette or chain health. I acknowledge that you may have mentioned gears and chains in one post because three chains may indicate time for a service, but it reads as if you attribute the one to the other.

It's a little off topic so my apologies to the op, but in my own personal experience, as the chain wears, its side to side movement increases and gear shifts become more 'sloppy'. It takes a little longer for the chain to snap into place. Feels that way anyway to me at least and I'm no cycling virgin.

 

[Yellow Saddle]

We seem to have slipped back to the topic of chains, but my question was about the cassette.
Is there something that specific to be looking for to say that it needs replacing? So far I can see that the plates have got narrower at the tips of the teeth and there is a change in shape between a couple of teeth on one ring (as mentioned above). I'm guessing it's the width of the teeth that eventually tell you that it's time for a replacement. I suppose this would also coincide with a reduction in life of the chain too.
I think far too often cycle shops will suggest a new cassette to go with a new chain because there's less chance of the bike coming back with tales of the chain jumping. It might not be too much of an issue with a £5 8-speed cassette, but the 11-speed ones are a bit more expensive!

You cannot inspect a cassette to see if it will make chains skate or not. It is possible to see wear but it is extremely difficult to predict whether the cassette will skate. The cassette teeth do NOT become narrower with wear. Wear is purely on the trailing edge of each cog and square to the plane of the sprocket. Wear only happens in the bottom of the cog valleys and not at the tips. An experienced mechanic will be able to see which cogs are worn by recognising an elongated valley shape but won't be able to predict malfunction. A worn cassette also displays the wear by ultra-smooth pressure faces on the trailing edges when compared to lesser-used sprockets. A new cassette has a rough texture on this face and a slightly rounded edge, whereas worn edges are perfectly square with a sharp edge. The only confirmation for malfunction is malfunction itself. In other words, each new chain gets tested on the old cassette until the next chain skates over the cassette.
I have explained the process here before, so a search with me as the author and suitable keywords such as "new chain on used cassette." Unfortunately there is no FAQ on this site and I don't catalogue these things.

You are partly correct on guessing why shops do this however, it is very easy to test a new chain on an old cassette but it pays them to just bamboozle the customer.

 

[Yellow Saddle]

Specifically look for 'shark toothing' if any teeth on the sprocket are really sharply pointed, or if any teeth are missing, the cassette really needs replacing.

It has been pointed out to you before that cassette teeth do not become sharp-pointed or shark-toothed as you claim. Further, the term shark-toothed is completely wrong. The original term was shark-finned and referred to the teeth on driving sprockets being worn on the roots of the pressure faces and the tips of the trailing edges, creating a shark-fin look.
Cassettes with broken cogs are so rare, to mention them is ridiculous. Cassette sprockets are case-hardened only, still soft in the centre, and thus don't break in brittle fractures.
Please stop presenting your theories as fact in technical discussions. You may well have me on ignore due to previous discussions, therefore let this serve as a cautionary to others interested in this topic.

 

[BrynCP]

11200 miles on a single cassette here and 5 chains; have a new one waiting, but despite the odd occasion coming up I think I need a new cassette, it's always another problem.

 

[Yellow Saddle]

It's a little off topic so my apologies to the op, but in my own personal experience, as the chain wears, its side to side movement increases and gear shifts become more 'sloppy'. It takes a little longer for the chain to snap into place. Feels that way anyway to me at least and I'm no cycling virgin.

All chains designed for multi-sprocket cassettes have built in side-to-side play that straight out of the box exceeds by far the distance a chain is bent sideways during gear changes. A bit of extra flexibility is therefore not "seen" by the derailleur since it never uses available flex to the limit.

 

[Ajax Bay]

the current chain has just reached 2500 and I recon it's time to be changed, so should I change the cassette as well or keep the original

Buy a new chain and cassette; fit the new chain, try it and if it skates, fit the cassette. This can be done in minutes few. If it doesn't skate, keep using the cassette. At the next chain change the skating will tell you the cassette's done its life's work, and you have a new cassette ready to fit. As YS says this is the only practical way to determine if you need a new cassette.
Edit: One other thing: since one cannot tell how much wear a cassette has had, never buy cassettes second hand. They may look pristine but it's a b***** when one finds it (well one or more of the sprockets) is worn and skating occurs and one can't remember who/where it came from.