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Pitch (i.e. the distance between two links/two cogs) is a major issue in drivetrain wear, which explains why waiting too long to replace a chain may lead to premature cassette replacement.

This being said, three questions:

1) What is the maximum chain elongation tolerated by a new cassette? 1.5%? 3%? Can you explain in some amount of detail? (intuitively I would think that the answer involves taking into account the difference in sprockets' radius at the bottom and near the top of a cog -- as chain pitch increases, it rides higher and higher on the cogs, up to the point where it is no longer gripped by the cog).

2) What is the maximum chain elongation at which a chain will skip on sprockets that have not been changed during the life of the chain? (intuitively I would think that the answer would be the same as above, or a tad lower - worn sprockets have deformed cogs, but the same principles apply).

3) Large sprockets last longer than smaller ones. Why? (intuitively I would think that wear is a function of the number of revolutions -- or distance/speed)

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    Most people don't use the large sprockets as much as the smaller ones. Generally, the sprockets you use the most will wear out first. – Batman Dec 15 '16 at 15:26
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    Actually, a worn sprocket will often work better with the corresponding worn chain than will a new chain on a worn sprocket or a worn chain on a new sprocket. Though there are are points in the wear process where the new chain, in particular, will work better than the old one, on a worn sprocket. It's really a sort of toss-up. – Daniel R Hicks Dec 15 '16 at 22:16
  • As to chain elongation, I regard 0.8% to be the replacement point, with 1.0% being the absolute limit (except in emergency, of course). As to what elongation a new sprocket will "tolerate", it depends on the size of the cog -- small cogs will probably tolerate 3%, while large ones less than 1.5%. – Daniel R Hicks Dec 15 '16 at 22:19
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According to Park Tools:

A worn chain shifts poorly and wears sprockets at an accelerated rate. The CC-3.2 is a go/no-go gauge designed to accurately indicate when a chain reaches .5% and .75%, the points at which most chain manufacturers suggest replacement. For 9 and 10-speed chains, replace chain just as the gauge fits the 0.75% side fits flat into the chain. For 11 and 12-speed chain, replace as the 0.5% side fits. The CC-3.2 is long, accurate and features permanent measurement markings.

The 0.5% - 0.75% answers when you should replace the chain but does not answer the max stretch before skipping.

Sheldon Brown's site ( http://www.sheldonbrown.com/brandt/chain-care.html ) indicates things get dramatically worse once the elongation reaches 1.0% because at that point it is every other link that is load bearing... so I would assume >= 1.0% is the answer with the variance being attributed to the condition of the cog. Checkout the link and you will learn more than you can imaging on chain-cog engagement.

The best way to determine whether a chain is worn is by measuring its length. A new half inch pitch chain will have a pin at exactly every half inch. As the pins and sleeves wear, this spacing increases, concentrating more load on the last tooth of engagement as the chain rolls off the sprocket, thus changing the tooth profile. When chain pitch grows over one half percent, it is time for a new chain. At one percent, sprocket wear progresses rapidly because this length change occurs only between pin and sleeve so that it is concentrated on every second pitch; the pitch of the inner link containing the rollers remaining constant.

  • Thanks for your input. I am familiar with Jobst Brandt post. If you read it, you'll notice that he describes the process without stating an elongation threshold. I'd be curious to read the engineering math. (or intuitive approximation) – user3127882 Dec 15 '16 at 17:53
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    @user3127882 different drivetrain (e.g., 6, 7, 8, 9, 10, 11 speed) have different tolerances and can tolerate different chain elongation. Radius (I.e., gear size) and forces also will factor into the amount of elongation that can be tolerated. Hence no single answer. – Rider_X Dec 16 '16 at 3:08
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Regarding your second question about exactly how worn a chain has to be before it skips on sprockets that are also worn, I don't believe that a nice clean engineering analysis is possible, because you would need a specific wear model for both the sprocket and the chain. There are far too many variables: alloy composition, heat treatment, and surface finish of the various parts; size distribution, composition, and amount of dirt particles; how often the chain was lubricated; and so on. Instead the industry has devised a practical rule-of-thumb measurement that works very well: the elongation percentage. @dafew's answer describes that well.

On a personal note, let me add that chain skipping isn't the only danger of an elongated chain. Once I had a BSO with a badly-worn chain. I jumped on and started pedaling hard; the chain skipped and then caught, ripping several teeth off the cog, as I flew over the handlebars. I ended up with some impressive bruises and road rash on my forearms.

As for why smaller cogs wear faster than larger cogs, consider a road bike with a 50-tooth front ring and an 11–32 cassette on the back. On the 11, every turn of the crank will use each tooth 4.5 times. On the 32, each turn of the crank will use each tooth 1.6 times. The smaller cogs wear faster because they rotate more often.

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    I agree that the second question is riddled with uncertainty. – user3127882 Dec 15 '16 at 19:54
  • Thanks for your kind words in chat. At present I'm still reading Italian and Latin (I don't know enough to post) so I do get to see any comments. I never say never, but it's time to take at least a long break. – andy256 Mar 19 '17 at 0:23
  • @andy256 Thanks for all your contributions to the site and the community, and we'll miss you. – rclocher3 Mar 19 '17 at 15:50
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1) 0%. New cassette means new chain unless its got no more than a week's riding or 100 km. Whichever is less.

2) Too variable to answer. I've seen chains at well over 2% elongated still working "okay" because rider was a granny and had little power.

3) Other answers cover this perfectly well.

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