Fixed chain snap... yowch.
 

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.

Sweet. I have the same chain...

i have had chains break from bad chain alignment from the front chainring to the rear cog, but it sounds like your's was pretty good, right? i remember a mechanic once say that everytime you break a chain (with a chain breaker) and put it back together it is weaker. has that chain been broken many times?

I busted a chain from having it too tight. I won't take any guesses about the cause of your chain failure, because all of my other guesses about stuff today have been wrong.

Keep it properly tensioned, properly lubed, and ride with a brake.

most common cause of chain failure.....improper installation

there isn't more stress on a fixed gear chain. putting backpressure on is no different than pedaling forwards. It's still just chain tension.
There is probably less stress on a fixed gear chain since the chainline is straight. No lateral pressure from cross-chaining across 10 gears.

good point, if your front chainring was bent even a little, and your chain was very tight, then the stress put on your chain by the chainring turning would be extremely great (in the pejorative sense of the word)

If anything my chain was slack.

I was concerned that my chainring might have been bent as from the resulting tangle/skid/wedge. Once I got it all home and pulled it apart I noticed that the entire rotation of the chainring evenly rubbed against the chain stay. Though this confused me for a minute but this was due to the chainstay having bendt out slightly. I bent it back loosely and the chain ring ran pretty true. whilst I am not sure of the exact point of rotation where the chain broke - it seemed to throw the remaining chain off and around the base of the driveside crank arm and wedge and stress things from there.

But I guess more immediate concern will be breaking out the string and alligning the chain & seat stays before I worry about the small componet replacement

are you joking? what about the teeth bashing back against the bushings when you backpedal or resist

well.. they bash against the bushings going forward as well.. and they are circular... so I take baxtefer's point to some degree

(why do I get that can-of-worms-opening type feeling....)

My line of thinking is that more sudden staccato like movement of say, a heavier, guy like myself (just shy of 105kg or ~230lb) practising his trackstands :D with lots of rapid, to and fro movement might have also placed undue stress and weakened the chain as well.

While this is true in many situations, how many people do you know that can break traction pedaling forward on a geared bike (the equivalent tension of skidding)?

Never... only the time I removed a couple links when it was new out of the box to get the right length. I bought a masterlink at the same time and used that upon installation. I hadn't even used that again since.

Not that it's probably of any significance, more coincidence; the break was only one link from the master.

Id like to see a pic of how it broke.

Those situations are not equivalent. When skidding, one unweights the rear wheel to initiate the skid. When people pedal forward, they do not do this.

Some cyclists can deliver 1800 Watts. Their chains don't break. It's highly unlikely that a broken chain is actually due to stresses caused by typical urban fixed gear riding. It's really hard (i.e. impossible) to break a properly installed chain with no manufacturing defects.

I think the original suspicion of too much slack is probably correct. Originally I suspected the OP reused a pin on installation (don't do that with 9/10-speed chains), but he used a Masterlink.

Chain breaks =

1) Improper installation
2) manufacturing defect

Glad to hear you're ok.

One possibility is that you threw your chain first, then broke it.
If your chain is wound around the rear hub instead of the cog, then you will exert a bunch of sideways stress on the chain, probably enough to break it.

Please post pics of the chain.

From your assertion that you had just bottomed out from a downhill + slack chain perhaps equals thrown chain quickly followed by rear cog wrap & break with bonus skid through intersection.

Glad you're OK, though.

the chain is not the problem. Either the slack in the chain allowed it to derail slightly and the inertia of your forward movement broke the chain or the chain separated where you pushed the pin back in when installing the chain. Either way I say get another Z chain and put it on with no slack.

I would tend to agree with him throwing the chain first, then breaking it. It wouldn't make sense to me for the chain to just snap, and that would somehow lock up the wheel.

I don't know if it's of any significance, but the OP was running an odd-number cog on an even chainring. Jedi Master Sheldon contraindicates this, as it contributes to uneven chain wear (if I understand him correctly, which isn't certain). 48/17 is a pretty beefy ratio, especially with a 230lb guy mashing down suddenly-- could uneven chain wear combined with such a monster load have caused the cahin to fail?

I''m not asserting anything here, just asking, so I'd appreciate not being jumped on if I'm way off the mark...

If I unweigh the rear wheel because I'm standing up like a gopher in traffic, I can break traction. It's not hard. :rolleyes:

I *think* Sheldon is refering to uneven wear in the event that you want to replace the cog/chainring or something. If you have even numbered gearing you can always get the teeth to line up with the previous worn spots.

could the masterlink be at fault?

i don't use them because they scare me.

That would be my guess too. A slack chan can wiggle so that the top of a tooth bumps into a side plate. That can generate a LOT of force on a fixed gear.
Not clear that it's a "monster load." Indeed, it doesn't sound like it to me. The magnitude of the load is related to the size of the sprocket driving the chain, not the ratio.

Thus, the pedaling load is _less_ with a large ring like a 48 than it would be with, say, a 42.

If the force was being applied by the rear spocket, as when resisting to slow down, a 17 tooth sprocket would not pull as hard a a smaller one, say a 15, would.

Sheldon "Loose Chains Break" Brown

i would suspect:

masterlink
improper installation
chain dropping then breaking because it got stuck
manufacturer defect

i doubt 600 miles is enough to weaken a chain from just wear. i don't see how slack in the chain could've made it break. maybe it made it drop and then that force exerted on it after it got wedged in the hub made it break.

Good lord. It's spooky when he does that...amazing, but spooky.