Ok guys - I've had my bit of excitement and sat down to count my lucky stars since; but yesterday I snapped my chain about halfway through an intersection, just levelling out at the bottom of a downhill. Of course instant lock up of the back wheel & cranks. Luckily for me the road was pretty wet and this enabled me to skid reasonably long, gather my wits, get to the kerb and unclipped for a pretty well controlled stop.
I guess now is the post mortem time for the chain. Just revisiting what I think I might done, or rather not done, to let this happen. I'm keen to get back on the fixie, but obviously keen to prevent this happening again.
The chain itself itself was a 3/32" KMC "Narrow" Z chain about 4 or 5 months old with about 600km on it. The running gear was the stock kit removed from my project Steamroller; ie. 17t Surly cog, Sugino 48t Ring, Sugino RD crank. I had built up the mid 80s MBK road frame as a conversion with pretty near perfect chainline, using the straight-edge method of checking.
I had been riding this quite succesfully since the build - with no funny noises, rubbing or grinding. I've been negotiating up & downhills no worries and mastering gradually more graceful trackstands.
My biggest suspicion is that I might have let the chain slack. I hadn't really checked it since the initial buildup, but noticed recently just by eye it appeared to sag a little more. Is my own laziness in not doing something about this most likely to be the culprit here?
But otherwise... what are the most common causes of chain break on fixed gears. I appreciate that there must be some different stresses in the backpedalling motion/force that a freewheel chain wouldn't experience - which is why I believe the straw that broke the camels back in my case was me controlling my descent through the downhill.
I have some photos of the deceased chain at the break - if they're of any use I can post.