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Here is a closeup of my triple: Triple Closeup

All three rings are Shimano Ultegra and have never been changed. They probably have at least 12000+ km on them. I've been pretty good about changing the chain regularly although at this point I'm pretty sure that the rear cassette is in need of changing.

If you compare the middle ring with the two outer ones, it looks fairly worn (shark fin pattern). Is it bad enough to need replacing? If I just change the chain and rear cassette will I be entering a world of pain?

UPDATE: Thanks for the advice so far. I have a new cassette and chain standing by but would need to order a new middle ring since it's a bit exotic (one LBS didn't know Shimano still made Ultegra triples). I know that there's a lot of km's on these rings (mostly on the middle one) and I'm also familiar with the "if it still skips, try changing it" philosophy, but I was hoping for a more objective answer. Something about the wear pattern or what to look for if I put a new chain on?

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6 Answers 6

up vote 5 down vote accepted

Judging by Sheldon's Guide, it definitely looks like you are in need of a new chainring. Your middle and larger sprockets in particular look like they have taken on a significant ramp-like profile, which will surely lead to lackluster shifting.

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Can you elaborate on that rather than linking to another site? (and I fully admit that Sheldon Brown's site is awesome) –  darkcanuck Sep 1 '10 at 23:10
    
I worry for all of us that some day Sheldon's website will go offline and a billion links will become shortcuts to 404. –  James Schek Aug 1 '12 at 4:55

In general, you only need to replace the front rings when they give you problems, either with skipping, chain suck, or shifting problems. Running with a worn front will not add appreciably to the wear of other components.

But a well-equipped bike shop should have a gauge for checking the rings.

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Interestingly enough, I just replaced my middle ring on almost the identical equpment that you have. It cost me $27 US dollars for a new 42 tooth Ultegra ring (from Amazon), and it took about 5 minutes to swap out. The shifting wasn't the problem, it was the horrendous "snap" I would get when pedaling more than idly when sitting on the 42... and mine isn't nearly as worn as yours!

(A few facts about chain-line wear:
1. The actual wear-out of almost any chain is the rollers against the pins. The rollers are just a little cylinder of metal, and it isn't as hard as the metal of the chain pins, so it wears first. As the roller gets thinner-walled, the effective length of the chain gets longer, although measuring pin-to-pin with a ruler barely changes at all!
2. Old chain/worn rings "works" because the chain has so much slop in it that it can get right over the teeth.
3. New chain/worn rings fails in this way: Picture the top of the chainring, just as the chain is feeding onto it. A link goes over a pin, and the roller "beds down" into the hollow of the sharktooth pattern. The next roller would do the same, but first it has to clear the tip of the next tooth, which it can't quite do under high tension, so it "sits on". As the chainring rotates, eventally the roller that it stuck on the tip of the tooth "drops in" - with a loud snap and a very disconcerting feeling of the pedal dropping out from under your foot. Ride it that way for very long and you'll wear out the chain VERY QUICKLY INDEED
4. Filing out the teeth of a worn ring to keep using it - yeah that oughta work, in a way, but the shifting performance won't be as nice. The reason the teeth are so strangely shaped is for smoother, quicker shifting. Also the teeth will be much weaker and the end-game of filing them down for "extended" life is that they will eventually start breaking under high load.)

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I use to successfully reshape chainrings like these using a round file (diameter more or less similar to the chain roller). This is a tried and tested method, and is very easy (much more than doing the same with the cogs, which I also do when needed). The only limitation is with skill and time, since it is a bit monotonous and require a bit of labor.

You just have to remove material in a way that it gets the same shape as the others, which means the teeth would be thinner looking from the side. Of course, it is necessary to remove the chainring from the crankset.

(and a final advice would be: change the chain often, before this happens again)

The picture below shows the regions that must be removed:

enter image description here

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Note how you're not filing anything on the opposite edge of the tooth, which has a decently shaped profile for meshing with the chain rollers. Maybe you could save yourself a ton of labor simply by reversing the worn ring left to right. –  Kaz Aug 17 at 14:49
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@Kaz you're right, but only if the chainring is symmetrical, which is usually not the case with indexed rings. Even "flat" chainrings usually have a lowered edge in one side of the holes. –  heltonbiker Aug 18 at 23:23

I would only replace if:

  1. The chain is skipping because of wear of the teeth
  2. You get chainsuck even when the chain is clean (small chainrings are more prone to chainsuck)

For cassettes I would not consider changing unless it skips when you have put on a new chain.

The best tactic is to change your chain often enough so that you minimise the wear in your cassette and chainrings.

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Presumably with 12K+ km, and never having bought new chain rings, you have a very high cadence. You should get a new chain every time you change your cassette, btw.

If you are doing the work yourself, I don't see any reason to change the chain ring immediately. Try the new cassette and chain and see how it works. If it isn't broke, don't fix it. If it doesn't ride well after changing the first set of parts then yes, change it.

On the other hand, if you are taking it to a shop and paying for the work, it's probably worth having them put on a new chain ring while they've got it in the stand. A good shop will test ride it and be able to tell you if it is needed before you buy.

12K is a lot of distance for the equipment, so I wouldn't hesitate to change it either way.

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You should get a new chain at least twice as often as you change your casette. If you do, you'll end up buying a lot fewer cassettes. –  Benson Mar 2 '11 at 12:35

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