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I have a 3x9 gearing installed and I've noticed that the chain broke, by failure of master link, twice within 25 km. Interestingly, the failure was perpendicular to the direction of motion:Chain break

What am I doing wrong?

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The upper one in the picture looks like it is bent. Is this a perspective thing or is it really bent? –  Benedikt Bauer Jan 14 '13 at 23:21
    
My guess is either wrong size link or you're being really hard on your chain when you shift. –  Daniel R Hicks Jan 15 '13 at 0:14
    
@BenediktBauer Yes, it bent as a result of failure. –  moshbear Jan 15 '13 at 1:15
    
It would be worthwhile to mention the brand of these links. I don't believe that Shimano makes any keyhole links, so I presume they're SRAM? –  Daniel R Hicks Jan 15 '13 at 4:27
    
Though on closer inspection they more resemble KMC "Missing Link" links. –  Daniel R Hicks Jan 15 '13 at 4:44
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2 Answers 2

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Given the fact that the outer plates are deformed in the same direction, I believe you're probably cranking on the pedals way too hard when you're shifting.

Are you waiting too long to downshift in the front when starting up an incline? Are you standing while shifting? Are your forcing the front derailleur by putting excessive pressure on the shift lever? Any one of these factors are going to increase your chances of busting a chain. The bigger person you are weight wise, the more you have to be careful of these things, although you can weigh under a buck and still trash chains if you don't give them some love.

Keep in mind that when shifting you're putting side load on the chain links and really torquing them in a way they don't appreciate. You need to let off a little when you make any shift, up or down, front or rear. Beyond saving your chain from breakage, it will also save your drivetrain from premature and excessive wear.

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Due to powerlifting, my legs are used to applying large amounts of force, so I'm likely torquing too hard while shifting. It also looks like I'm going to go through 2-3 48T chainrings per year. –  moshbear Jan 16 '13 at 22:38
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There were some threads on Internet cycling forums about two years ago regarding Shimano's CN-6700 vs. CN-6701 (the 10 speed Ultegra chains) being a tacit admission that CN-6700-- which people reported breaking-- was defective. A search reveals several such threads. Clearly, sometimes a run of components can have problems.

In this case, I don't use the type of master link you've pictured, so I can't answer your question directly from my experience. The top link in your picture does appear to be bent; was that done when you removed it, or when it broke?

If the master link is a KMC, are you sure it was fully engaged?

Personally I use Wippermann master links, and I've never broken one.

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It was bent when it broke, and yes, it was fully engaged. –  moshbear Jan 15 '13 at 1:21
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