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This answer suggests that having B-tension adjusted so that the chain sits far from the cogs increases wear. Can anyone cite a source that says that? Experimental evidence would be preferable, but a statement from an expert would suffice.

Sheldon notably doesn't mention it, instead saying that proper B-screw adjustment leads to quicker shifting, so I'm wondering if it's really the case.

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    Its an excellent question. I have no proof, but consider an elongated chain accelerates cassette wear by having fewer points in contact, so one tooth bears all the load in the worst case – Criggie♦ Jul 27 '17 at 0:21
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    Not an answer as OP requires evidence. One thing anyone answering will find hard to separate is wear vs shifting performance. Its known that worn chains affect shifting (citation required), and its also known (another citation required) that high B tension reduces shifting performance. Its entirely possible to jump to the conclusion B tension increases wear, as shifting performance is a standard measure of wear (citation required). A wild guess is the extra wear cause by high B Tension is statistically insignificant in real world conditions. – mattnz Jul 27 '17 at 1:19
  • @mattnz I wouldn't be surprised at all if your wild guess is actually the case. As far as my behavior goes, the answer doesn't really matter: I'm going to keep setting my B tension the same way regardless to maintain good shifting performance. – T.C. Proctor Jul 27 '17 at 1:31
  • Yes - the sole point of increasing B tension is to hold the top jockey wheel down a bit further to clear a large-tooth-count big-cog. I suspect mountain derailleurs would traverse a better "path" across the cassette. – Criggie♦ Jul 27 '17 at 1:52
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FWIW, the mechanical engineering analysis that I have read about the mechanics of chains states that almost all the load is spread across the first 3 links that are in contact with the sprocket. So as long as you have 3 links in contact with the sprocket, you aren't increasing the load.

But this is all largely arguing in a vacuum. Set your bike up so that it shifts well, check the chain wear frequently and replace the chain when it starts to show signs of wear. Gears don't "wear" significantly until the chain starts to wear and elongate and increase stress on the gears. If you always have a "fresh" chain, your gear cluster will last a very long time, regardless of how the B-tension is adjusted.

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  • I feel like this is beating a dead horse, but... I think the way I've set out the parameters of the question, a citation of that mechanical engineering analysis would be necessary for an accepted answer. I'm also really interested to read it! – T.C. Proctor Jul 27 '17 at 19:34
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In the comments, it looks like Criggie gives you a very helpful rundown of the Physics of why that causes more wear. Sheldon Brown is a great resource for a lot of things, but in the long run, he was just one mechanic, sharing info with the world, and didn't get to put everything there or be completely comprehensive. I can tell you from my experience as a mechanic, you want enough b-tension to keep your pulleys off your cogs, and that's about it, too much more, and as Criggie said in that other post, your chain will have less contact with your smaller cogs, and will wear them faster.

As for a citable source, PDF of Shimano general instructions Page 20 tells you how to set proper B-limit screw tension for several systems, even though it doesn't give a specific reason for why it's like that.

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    Criggie's "very helpful rundown of the Physics of why that causes more wear" is really just a justification for why it might be reasonable. As a physicist, a justification like that is insufficient. To be convinced by a theoretical explanation like that, I'd need to see some good math tying it significantly to some metric that has been experimentally shown to cause wear (Maybe pressure on the cog? I'm not sure). Experimental evidence would be much better. – T.C. Proctor Jul 26 '17 at 16:08
  • Also, Sheldon does have an explanation for why you set B-tension the way you do - for quick shifting. It's not just Sheldon - in all the Google searches I could come up with to target the issue, I couldn't find anything that even mentioned that as a possible problem. – T.C. Proctor Jul 26 '17 at 16:14
  • Unfortunately, there aren't many entities with the resources and interest to study wear properly - Shimano is happily making money replacing worn parts, and I'd imagine that most big racing teams replace parts so often that it isn't worth it to study. – T.C. Proctor Jul 26 '17 at 16:17
  • As you said in your first comment, it's a reasonable justification for why you set it like that. If that's not enough for you, maybe do an independent study yourself. But it isn't going to take a lot to prove that having more contact on a cog with a chain is going to spread a pressure load across more teeth, reducing the pressure to each tooth and thus the wear to each. – CardMechanic Jul 26 '17 at 17:08
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    @Criggie I don't actually know for sure. I'd imagine that it's both. Sheldon says: "The major cause of chain "stretch" is wearing away of the metal where the link pin rotates inside of the bushing (or the "bushing" part of the inside plate) as the chain links flex and straighten as the chain goes onto and off of the sprockets." – T.C. Proctor Jul 28 '17 at 17:51

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