Originally Posted by
FBinNY
Just speculating so factor that.
With the understanding that modern chains have plates thinned to the point of having minimal safety margin, we can expect greater instances of breakage owing to variables.
There's always a tolerance in manufacturing precision, alloy formula, heat treating, etc, so there's always a weakest link. With little margin for error, what used to be OK no longer is.
However, I'm not convinced that the failure near the master link is coincidence. 66:1 odds (either side of connector in 122 link chain) argue against that.
So consider that each inner plate carries half the load roughly equally, and can't carry much more.
The dimensional precision of the connector is more complicated than the normal plates, so it's conceivable that one side is a bit off. That would alter the 50/50 load sharing of the adjacent inner plates, taking the harder working one above the fatigue limit, and eventually to failure.
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BTW I noticed some nicking of the outer plate in the photo. This is usually caused by shifting under load. Not that it caused the break, but it is hard on chains and not a good thing. If you see much of this type of damage, take a hint and try to time your shifts better.
Every time the bike industry adds another cog, we hear the same old fears, "Man, these chains are REALLY THIN! I don't trust 'em!" (And I admit, I wonder as well.)
But then you find that 12spd. are regularly used on tandems without issue.* A single rider will never achieve the pedaling force two riders impart on a tandem main chain (and everything else in the drive train). Double the power is one thing, but the clincher is ZERO rear wheel traction loss or front end lift. This is what makes mountain tandems rear hub destroyers. I worry about blowing up my seventh hub. I've attempted Slickrock twice on the tandem and have a zero success rate. First hub failure was, not surprisingly, a DT/Swiss from '97. But the second was splitting a Phil Wood tandem FSC hub in half. And subsequent to that, we've mangled two more sets of PW tandem hub internals. Still on my "to-do" list, but fear I may need to fit a King or Rohloff to avoid my third catastrophic failure on Slickrock. But it pisses me off that I still haven't checked that off my list after 30 years!
So I have yet to encounter epidemic chain failures because they've "made 'em too thin." Despite the hub failures, in 30 years, we've never suffered a main chain failure. Our ONLY chain issue was while touring on the mountain tandem when a rock was kicked up into and breaking the timing chain. Even e-bikes may not be able to achieve tandem torque loads because of wheel traction loss and front wheel lift. But I may be wrong; instantaneous torque electric motors produce might impart sudden high torque loads before the side-effects occur. The ultimate hub killer? An e-mountain tandem. Ay, ye poor freehubs! This is a pretty strong argument for a gearbox.
*Apt analogy. Whenever someone sees a bicycle equipped with S&S couplers, they oftentimes ask, "Wow, are those things strong enough?" Well, they use them on tandems, triples, quads and more. Yup, they're effing strong!