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My bike mechanic told me today that my front chainrings are starting to wear out and I should consider replacing it soon (with my next chain replacement). The cassette is still in good condition and does not show any signs of needing replacement. The bike has 3000 to 4000 miles on it and all parts except the chain are original.

The specific reason for the visit to the mechanic was due to the chain dropping to the inside. The drivetrain is all SRAM Rival. There have been no other issues with the drive train. The mechanic seemed to think it was a bit unusual for the front chainring to wear out before the cassette, though it was not unheard of.

Are there any common causes (ex: lack of specific maintenance?) that would cause the chainring to wear out so much faster than the cassette?

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    Nothing I can think of that would cause premature wear of the front ring, other than badly worn chain or some really gonzo riding. But how did he judge it to be worn -- did he stick a gauge on it, or is he looking at the tooth profile or what?
    – Daniel R Hicks
    Jun 24, 2012 at 20:23
  • He said it looked worn based based on the tooth profile. He described them as getting "pointy". Compared to my other bicycle (SRAM Apex), the teeth clearly have a different shape, particularly the outer chainring.
    – James Schek
    Jun 24, 2012 at 21:00
  • If it were a badly worn chain, would you expect to see the same sort of wear on the cassette? (I've checked the chain with a tool and had previously replaced it when it reached the "replace" on the tool)
    – James Schek
    Jun 24, 2012 at 21:01
  • Yeah, you'd expect to see similar wear on the cassette. Normally 2-3 cassettes per main chainring. But note that this is a "compact" set, so the chainring will wear a bit faster.
    – Daniel R Hicks
    Jun 24, 2012 at 22:26

2 Answers 2

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Why do you wear chainrings quickly?

  • Cross Chaining - If you tend to be in the big ring on the back and the big ring on the front at the same time, you're probably doing it wrong. Same with running the small ring on the front with the small ring on the back. This can cause uneven wear on the front chainrings due to the awkward angles the chain must bend. This can wear the sides of the front chainring teeth and make them more pointy.
  • You are riding dirty or don't lube up first- If you ride offroad especially, your chain can get dirty. If your chain is not lubed properly, it can cause extra friction. Keep the chain clean or it will work like sandpaper, wearing down your chainrings and make sure you use the proper type of lube for your environment. Chain wear tends to deform the inside surface of the gap between teeth, making an uneven 'U' shape between teeth.
  • Riding with an already "stretched" chain - If your chain is worn, It has a different shape, which is going to cause extra wear on your chainrings. If it's worn too much, you can only really replace it. Running with a "stretched" chain is probably worse than running with a worn chainring.

    If it's a pretty new bike and the chain is well maintained, I'd probably blame it on cross chaining. Be aware of your riding style and make sure you're not overextending the range of your front chainrings.

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    Generally, one drops a chain during a down shift. Sometimes it happens only when you shift quickly or under heavy load, but it does happen. There are a few things that can cause a chain to miss the inner chainring: A maladjusted front derailleur or shifter, a very worn or dirty drivetrain, a bent or loose chainring, or a maladjusted chainline (the line which centers your chainrings and cassette parallel to your frame.)

    Since your chainrings are also wearing out prematurely, it could be that everything is really dirty or your derailleur is out of whack, but these would be obvious and easy to fix (cleaning and lubing your chain infrequently should get you 2-3,000 miles of life, fastidious cleaning might get you 5k. I have had good luck with wet lubes-specifically ProGold.) Additionally, make sure that any chain that gets installed is specifically for 10 speed systems, otherwise the wider plates may rub against adjacent cogs or rings.

    A less obvious cause of premature-wear-and-tear-and-drop would be a bent or loose chainring, but you or your LBS will probably notice this before it totally wears out your rings.

    The last issue is one that I think may be worth investigating, and that is a chainline that puts your chainrings too far out. Downshifts will tend to pull the chain to the inside, especially when you are on your inside cog. This could also exacerbate wear from crossing your gears (which, on a compact 2x10 isn't a huge deal.) Any bike that is set up by a pro will spec parts that work (and even a home builder should be in good enough shape. Big problems arise for single and fixed gears, though.) All told, I can't imagine a scenario where a road frame built up with Sram Rival components would have a chainline so far out that it would wear out a pair of rings in 3-4,000 miles. This could be an issue when mixing mountain and road components. Given these instructions, I could also see someone potentially messing up a BB/crank installation.

    In any case, a chain keeper-a device which prevents your chain from dropping to the inside-is a small but valuable investment-though in your case not a solution.

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