JonOnWheels

I no longer have the old cassette, so I can't do any tests.

Look, we got off track debating whether the cassette was actually worn out or not. Let's assume it was worn out. I believe it was worn out, because I trust my LBS owner and that's what he told me. It does me no good to second-guess him. (Anyway, why in the world would the shopkeeper lie about the old cassette and then replace it at no cost to me? It makes no sense. There would be nothing in it for him to do that, and nobody who pulls stunts like that would stay in business for very long is such a competitive market.)

So assuming that what the LBS told me is correct, that brings me back to the question I originally posted here: Why would the three smallest cogs on a cassette on a 3-month old bike require replacement after 400 miles? What could cause that?

AnkleWork

You want to know why something that didn't happen, did happen?! I just have no idea at all.

Your photos are empirical evidence; the three smallest cogs show no wear. Whatever assumed hypothetical problem/cause wrt warranties, the LBS, the mechanic, Shimano, the bike brand, trust, faith, etc. is academic speculation and guess work. Best wishes.

jideta

The OP never said why he thought the cassette was worn, only that the LBS told him so.

Chain skip, wonky shifting etc. are signs of a worn cassette. The edges of the gears are usually soft and sometimes are shark fin shaped. The wear is OBVIOUS.

this is a worn cassette:



The gears have started to 'hook' where it contacts the chain and the teeth are obviously smaller. There is also some 'rounding' of the other teeth; the edges are no longer sharp and the ramps are all soft.
If I remember correctly, I probably had 3000 miles on this cassette, but I ride near a lot of sand.

I think the question is not 'why would' the cassette wear out, but IS the cassette worn out? Why would the LBS, upon you taking your bicycle in for a crank problem, come up with a worn cassette problem? the two are not related.
Was your cassette skipping or shifting incorrectly?
If you weren't experiencing any problems, what else lead you to believe the cassette was worn? The LBS said was it was 'badly worn?'

rm -rf

With modern cassettes, it's very hard to tell if a cassette is worn out just by looking at it. Perhaps the most obvious sign is the softened, rounded tips from the above post. Those two circled teeth don't look a lot more worn than my 17 cog below -- there's some extreme sloping of some teeth on new cassettes. See the same cog near the left edge of the photo -- that tooth doesn't look too worn. Perhaps comparing directly to a new cassette cog would help. But chains skipping / jumping under load is probably the best way to tell.

~~~

I looked at my 11-28 cassette, in my parts box after I swapped to a different cassette. I think it has a few thousand miles on it. I get 15000 miles or more on a cassette. And the smallest cogs aren't in use nearly as much as the middle ones.

Here's the 11 and 12 cogs. The arrow is the chain pulling direction. So the cassette teeth pull the chain using the right side of the teeth. EDIT -- It's the left side of the teeth -- the chain is pulling the cassette clockwise.

It's interesting that they look very "hooked", and "worn out", even "broken off". But they are designed that way, to improve shifting performance. Note that the unusual shapes repeat around the circle.

The middle cogs don't have this extreme teeth profile.




The same cassette, the 17 cog, also with minimal wear.
Note that the 12 o'clock and 6 o'clock teeth look extremely worn. Compare them to the 3 o'clock and 9 o'clock teeth. These tooth patterns are purposely designed to help shifting performance.

samkl 

That looks weird. Definitely something wrong with that cassette.

TimothyH

I am going to second guess the mechanic.

It is extremely unlikely given a new bicycle, that three sprockets are worn out in only 400. The only scenario where this is really possible is if the chain were completely worn out from the start and the worn chain began tearing up the cassette from the beginning.

You can trust the mechanic if you want but I ride fixed gear with only one sprocket and get thousands of miles out of it. Millions of single speed riders get thousands of miles out of their gears.

Something isn't right. If the cassette is worn after 400 miles then either the chain or cassette or both were not new or you rode it at the beach and the chain was completely covered with sand.


-Tim-

rm -rf

No, it looks like it's barely used. That's how cassettes are machined at the factory. Lots of strange tooth profiles.

oldtrekguy

Okay, I am an amateur mechanic, and I do almost all of my own work. I have no idea if I am a good mechanic. I guess I'm good enough that I have no complaints about any of my bikes. However, bicycles are forgiving machines, so sometimes "close enough" works.

The OP has stated that the LBS has replaced the cassette under warranty, and replaced the chain at no charge. I see no possibility that the LBS is running some kind of scam on him. Business people would rather be selling bikes and selling services, not fighting with the supply chain over warranty claims. I assume his mechanic has consulted with his colleagues about this, as we all agree the stated facts are unusual. We are all trying to second-guess the work of professionals who have seen the problem up close and personal.

I personally have zero confidence that I could look at a cassette, and determine if it was worn out. I have less than zero confidence that I could look at a picture of a cassette, and come to any conclusions. As others have suggested, I would need to have an issue with the drive train (poor shifting, ghost shifting, noise, etc.) before it would occur to me to diagnose a problem on a nearly-new bike.

I am also going to rule out the possibility that the OP took the bike to a car wash, and blasted off all the lube with a high-pressure spray. My arm-chair SWAG (Scientific Wild-A**ed Guess) is that there could be a chain line issue. If the OP types in a query in the ? box, he will get detailed instructions on how to measure chain line. Failing a precise measurement, shift the bike to the suspect cogs, and observe the chain line from above. Is the chain wildly skewed when the drive train is in the suspect gear? How skewed is it in a middle gear?

Was the original chain measured for wear? I would expect a chain to wear out before a cassette. If the old chain is still available, I would examine it for unusual wear and/or chain stretch.

My $.02. I'm looking forward to the OPs next post on the creaking BB, so I can pontificate on the evils of PF BBs. And, if his drivetrain is a 1X, I can grouse about that, too.

Iride01 

Nothing could cause a cassette to wear that quickly. Even the least worthy manufacturers of cassettes make cassettes that last much for longer than many will use their bike.

So if the pics you posted are the actual cassette in question, then I can't see any definitive wear. There are a couple places I'd like to have seen in a better shot at a different angle. However wear is not the issue from what I can see. I would have liked to have seen the parts cleaned up and photographed again. There still is a question in my mind if that is truly a Shimano cassette. Perhaps it's just the lighting, dirt, etc., but it looks a little rougher finish than I've seen on my Shimano parts. Counterfeit is a real thing, but I hate throwing that out for us keyboard quarterbacks. That is something I'd have insisted be shipped to Shimano and have them answer for me.

As for those few that think it's worn..... where? The only significant thing that caught my eye first as being odd is actually something you can find on a picture of a CS-HG700 cassette I got off Shimano's site.

fronesis

This is a really odd and interesting thread. The OP wants to A) insist the cassette was worn, and then B) insist that people answer the question of WHY it was worn.

But:
– the OP had no problem with the chain, cassette, or shifting prior to taking it in.
– the cassette doesn't LOOK worn to basically anyone here who has studied the pics.

So the OP's assumption is wrong. The LBS was WRONG when they said he needed a new cassette. Whether that was because the mechanic was an idiot, or the LBS wanted to sell him a new cassette...I have no idea, as that's all speculation.

But it's a ridiculous exercise to try to figure out why a cassette wore out in 400 miles, when the fact is that it DID NOT wear out in 400 miles. Instead, someone misdiagnosed it (sincerely or not) as having worn out.

For what it's worth, I agree Iride 01, the OP's original cassette finish looks "funny," not like the dozens of 11s shimano cassettes I've seen and installed. The finish just looks OFF.

Racing Dan

"Does that really happen?"

No it does not. Im 99% sure you either misunderstood or they are taking you for a fool. Why was the bike in the shop?

jimincalif

I have to agree nothing short of an angle grinder would wear out cassette cogs that quickly. I have two back wheels for my road bike, both with 10s Ultegra cassettes. One cassette has at least 16,000 miles, I rotated it to my backup wheel and put a new cassette on my primary wheel. They still both shift perfectly, I cannot tell the difference when riding. The “new” cassette is probably nearing 10,000 miles now. I do keep up with chain wear but I’ve never had a chain go less than 3500 miles. My 11s bike same thing, I have at least 3,000 miles on one cassette and 1,000 on another. IMO it’s just not possible to wear out a cog in 400 miles assuming the chain was new at the same time.

It it will be interesting to see if Shimano actually examines the returned cassette and whether the OP ever finds out what they say. As to why the LBS would say it’s worn out, I have no idea. Seems pretty clear they aren’t trying to screw over the OP.

Oso Polar

Yeah, cassette should have no any noticeable wear after 400 miles. But I'd not go that far as in this quote above - on a bike I once bought at Target for my son cassette simply disintegrated after couple of weeks of very light use (probably way less than 100 miles total) - teeth started to break off from the cogs, then few cogs cracked, then they broke in parts and fell off.

HerrKaLeun

If you really only used those 3 smallest cogs, I also would have your knees checked for low cadence damage.

400 miles doesn't even wear a chain. If the chain didn't skip and it shifted fine, cassette was OK.

rm -rf

Top: An 18 sprocket with somewhere between 12000 and 15000 miles on it. I don't know exactly when I installed it.
Bottom: An unused 18.

I accidentally let my chain get close to 1% wear, so the cassette should show some excessive wear. But there's no obvious wear on the teeth profiles. Just chain plate scuffing on the tooth sides and perhaps a slight rounding of the edges. The chain pulls on the right side of the teeth in this view.

I don't see how a shop mechanic could eyeball an installed cassette and decide it needs replacing.



Different tooth shapes for optimal shifting to different adjacent sprockets.
The worn one is from a 14-28. Coded 18D. It includes ... 16 17 18 19 20 ...
The new one is from an 11-32. Coded 18C. It has ... 16 18 20 ...

It's interesting how different the two are. They are rotated, have different ramps and much different tooth shapes. Note the pointy teeth and only 1 ramp on the left sprocket.