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Premature wear of crank ring?

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Premature wear of crank ring?

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Old 07-09-18, 06:08 PM
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PoeCo 
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Premature wear of crank ring?

I'm hoping some of you can offer some advice regarding crank wear. I have never in my life worn a chainring or crank on a single so quickly, but after riding our tandem for almost 1,000 miles, we noticed that about 4 of the teeth on the middle crank ring seem to be worn down (they are significantly shorter than the others). We **think** (honestly can't state this as fact, but we've assumed) that this has caused us some issues with "clunking" and the chain slipping from time to time when start pedaling from a stop. It's extremely unnerving and we both usually end up banging our shins or some body part every time it happens -- not to mention that it's made me (captain) a bit shaky starting from stops now because we're never quite sure when it's going to happen.

My questions/comments:
1) Is it possible to wear a crank ring this quickly? I realize there's more pressure on a tandem than a single, but this still seems strangely fast to me.
2) I believe the rings are made of aluminium. Do you think this is causing premature wear (as opposed to having steel or some other material)?
3) Is it possible that our lack of weekly maintenance has caused the wear? We do clean and lube, but we are not an every-ride or even ever-week sort of maintenance team (though that may need to change).

We ended up switching out the middle ring and will try the new one to see if that helps with the occasional clunking and losing-the-chain-when-starting cycle we've been experiencing, but if anyone has thoughts on something else that may be going on, we'd be most appreciative.
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Old 07-09-18, 07:49 PM
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It’s unlikely that you will wear only a few teeth. One should expect to wear all evenly. That said:

-the teeth of chainrings designed with profiles to assist in shifting will have different profiles making some look worn compared to others.

-it is possible to damage some teeth during shifting under power. This is more likely with softer aluminum rings. I have seen both bent and sheared off teeth as a result.

-if your chain is worn it can contribute to premature ring wear, but all teeth, not just a few.

i suspect that what you see is normal tooth profiles. Where is the clunking coming from?

Check you chain for wear and replace it if it is very near, or completely worm. If it is very worn you your cassette may be as well and should be replaced.
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Old 07-09-18, 08:25 PM
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Originally Posted by Alcanbrad View Post
i suspect that what you see is normal tooth profiles. Where is the clunking coming from?
The clunking comes from the chain slipping off the ring when we first start pedaling. It's as though it's missing the teeth and slipping off. The derailleur seems to be in line and the bike has been tuned, so I don't think that's the issue, but we are perplexed as to why it seems to happen randomly and without shifting gears. For example, we start off from the back of our home in a gear. When we get to the corner and stop, waiting to make a turn, we then lose the chain when starting to pedal. Then, it may happen again immediately after realigning everything, somewhere along the route, or it may not happen again for the entire ride.

Thank you for your response. It's helpful to try to eliminate possibilities or to know if we are working on the correct bits to eliminate this issue.
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Old 07-09-18, 09:48 PM
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You don't say whose rings they are. We have had good experiences with Shimano Ultegra chainrings.

Rear derailleur may not be providing appropriate chain tension due to who knows what. LBS take a look?
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Old 07-10-18, 04:23 AM
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Skipping when starting could be a worn chainring, bent teeth, bent chainring, broken chain (or Herculean power).

You could also try rotating the chainring on the crank so these 4 teeth are not engaged (or minimally engaged) during the part of the power stroke where the skipping occurs. This may impact shift performance a little but it would prove that it is the chainring.

I would inspect everything closely. If all you come up with are the 4 teeth you could try replacing the chainring.
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Old 07-10-18, 05:54 AM
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One other thought, We have broken the large cog spider of several cassettes (Seeking stronger cassette) which result in a few of the cogs not being parallel to the others - i.e. they appear to wobble compared to the other cogs. This also results in skipping or "clunking" when under power. You might want to check the cassette closely as well.
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Old 07-10-18, 07:39 AM
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A couple of things come to mind, not necessarily related to the problem at hand:
1. The shorter chainring teeth may be intentional features to aide shifting. This is nearly ubiquitous on modern chainrings. If so, there should be an alignment mark on the chainring pointing toward the crankarm. Verify that it's aligned with the crankarm.
2. Frames flex under load, especially at low cadences or from a standing stop. The tension from both chains causes the rear bottom bracket to twist clockwise (viewed from the top) and the boom tube and chainstays to bow. This sometimes changes the distance between cable stops, which allows the derailleurs to move uncommanded, which will result in ghost shifting or chain slipping. Generally, the frame compresses relative to the cable, so the rear derailleur moves temporarily toward a smaller cog. You should be able to replicate this on a stationary trainer.
3. Make sure the cassette, freehub body, and rear wheel are on tight.
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Old 07-10-18, 09:37 AM
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Right, the teeth may have been made that way. Show us a picture, and we can probably tell you.
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Old 07-10-18, 11:33 AM
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Measure the chain.

I'm curious to know how much chain "stretch" you have. When I was a young, strong, but inexperienced youth, I wore a small ring to the point where it did exactly what you describe: skipped under high load. The cause was excessive chain stretch; I didn't know it was a "thing" at the time, so the chain had gone was past its useful life.

So perhaps you've worn the chain to the extent that it caused premature chainring wear. (Even though it sounds like you're mindful of chain maintenance needs. Although you may be doing good maintenance to a very old chain!) Measure a 12" section of chain with a ruler (or buy a Park chain gauge at the LBS). A new chain measures exactly 12" pin to pin center (or edge, doesn't matter. Just be sure you use the same edge!). If the chain has elongated 1/16" or more in that 12" span, it's time to replace. If it has elongated that amount, or more, you may have to replace the rear cassettes as well. If you mate a new chain with a sufficiently-worn cassette, you'll get skipping under load, usually on the smaller cogs. And if chain stretch is so extreme, chainring skip can occur and replacement would be necessary.

So the rule is this: replace your chain before it affects the teeth spacing on your cassette (and rings). Chains are like tires; they wear out. People usually don't wear a chain enough to affect the chainrings, but it can happen.

So if you measure your chain and you have "excessive" stretch, than that's probably the cause of your ring wear. Replacing your chain sooner should prevent the chainring problems in the future, even if they are aluminum. I've used aluminum rings on my tandems for years and years and haven't worn one out yet. But I replace chains before they elongate much.

Let us know what you sort of "stretch" you measure. This is probably the issue.

Good luck!
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Old 07-11-18, 01:35 PM
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Thank you all for your advice and thoughts. It seems like we have a few things to check and hopefully this will get resolved quickly. When we figure out what exactly has been going on, I'll be sure to fill everyone in, but very much appreciate all your very valuable feedback.
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