Chain broken. What caused this?

[I like Skol]

Ooh, that's interesting. That thread has a selection of links all showing similar breaks. Can you recall which direction your chain was travelling?

Luckily there is just enough background to allow the chain direction to be deduced. The picture is taken from the transmission side and is looking down towards the opposite chain stay. The brake hose for the rear brake is visible.
This means the picture shows the outer face of the chain and it is travelling from the cassette that is out of shot on the left and going towards the top of the chain rings on the right. The cracked side plate has possibly been notched by one of the shifting pins during a clumsy gear change (we all do it occasionally ).

 

[Yellow Saddle]

Luckily there is just enough background to allow the chain direction to be deducted. The picture is taken from the transmission side and is looking down towards the opposite chain stay. The brake hose for the rear brake is visible.
This means the picture shows the outer face of the chain and it is travelling from the cassette that is out of shot on the left and going towards the top of the chain rings on the right. The cracked side plate has possibly been notched by one of the shifting pins during a clumsy gear change (we all do it occasionally ).

Chain direction has nothing to do with it, nor has a "nick" in the plate. The crack starts from inside the hole and grows outwards. The chain components have been heat treated and are extremely hard, a shift pin won't make a scratch on it.

 

[cyberknight]

I was going to say it the lads awesome powah .

 

[subaqua]

I was going to say it the lads awesome powah .

I was also going to say let the lad have his moment of glory for massive Quad power . Before dashing him down with metallurgical/ mechanical reality .

 

[Tim Hall]

I was going to say it the lads awesome powah .

I was also going to say let the lad have his moment of glory for massive Quad power . Before dashing him down with metallurgical/ mechanical reality .

Comment over dinner from The Boy: "I may have to register, just so I can like those last two posts."

 

[raleighnut]

Comment over dinner from The Boy: "I may have to register, just so I can like those last two posts."

 

[gbb]

Quite extraordinary the similarity between the two links...yet actually, it makes sense when you have the facts.
After seeing a few Shimano chains that were failing a while ago I took to a good visual check occasionally, link by link after wiping my chain, before oiling it. I suspect OPs damage was possibly visible a while before it actually failed in this case.
5 minutes checking once I a while could save a whole lot of pain and injury.