Snapped chain on hub geared bike - what causes it

[alicat]

A couple of days ago, I snapped the chain on a hub geared bike. I am wondering if I could have done anything to prevent it.

I took a short cut over some soggy grass and was giving it as much welly as I could in order to keep going and not sink into the grass. I may have been changing gear rather hurriedly in a desperate bid not to 'stall'.

When I replaced the chain, I noticed that the new chain was quite taut. Was it possible that the high power and hitting a high spot make it snap? Or the fact that it was probably a cheap chain? Or was I just unlucky?

I can't remember how old the chain was - probably three or four years old at least.

I would be grateful for any enlightenment on what makes chains break on a hub-geared bike.

 

[subaqua]

Any of them things could cause a chain to snap.

I would just walk round with a smug grin thinking it was my powerful legs that did it rather than mech failure.

Question is - should the chain be taut ?

 

[rogerzilla]

A chain on a hub gear shouldn't be waving about in the breeze but it doesn't have to be as tight as on a fixie. For one thing, the consequences of it coming off are not as serious. Also, some hub gears don't much like a drum-tight chain (where the top and bottom runs are both tight) as it can speed up internal wear. Minimal visible slack and absolutely no binding is what I'd go for.

What's your chainline like, and what sort of master link did you use? Did it break at the master link? Had the chain ever been joined without a master link or special pin? Most modern chains have mushroomed pins which are prone to working their way out if removed and refitted, because they enlarge the holes in the side plates as they come out.

 

[Yellow Saddle]

Define "Snap".

I'm asking because chains don't snap, i.e. break in tension. They simply separate at the rivets. This separation is 99% of the time caused by something that's out of line and then the application of force at a critical point. An example is when the chain is between two gears - one link is still in 5 and the next one is already in 6. If you then apply power just as the crossover comes into the 11 o'clock position on the sprockets, there is a strong sideways force on the chain and it opens up.

You don't say if your hub geared bike has one or more chainrings up front. Such a cross-over can happen there even though it is hub-geared.

There is always the possibility of a poorly-installed link and knowledge of the chain type (and speed) as well as installation history would help. Poor installations reveal themselves rather quickly, so if the chain has been on for 1000kms, then it almost certainly isn't an installation problem.

Photos?

 

[Ian H]

In the past I have had two cheap 1/8th chains break. In both cases a sideplate split across a pin, and not where I had joined them. The second time the chain didn't part and I managed to repair it with an old-fashioned spring link. The first occasion dumped me on the verge, but there was enough length of drop-out (just) to allow for an inch off the chain and rejoin.

Since spending more money on my fixed chains I have not had a repeat.

 

[Deleted member 1258]

In the past I have had two cheap 1/8th chains break. In both cases a sideplate split across a pin, and not where I had joined them. The second time the chain didn't part and I managed to repair it with an old-fashioned spring link. The first occasion dumped me on the verge, but there was enough length of drop-out (just) to allow for an inch off the chain and rejoin.

Since spending more money on my fixed chains I have not had a repeat.

The only time I've snapped a chain on a fixed was a long time ago, late 1980's, and it was a very old chain and badly worn, it failed under hard acceleration wrapping itself round the hub. These days I change them more often.

 

[Yellow Saddle]

The only time I've snapped a chain on a fixed was a long time ago, late 1980's, and it was a very old chain and badly worn, it failed under hard acceleration wrapping itself round the hub. These days I change them more often.

I can sort of see why people think that frequent replacement will prevent failure, but it doesn't. The place where a chain separates receives no wear whatsoever and will break with exactly the same force when it has done 10000 kms as when it has done one.

In fact, the most likely time for a chain to break is early on in its life when manufacturing or installation errors come to light.

 

[Deleted member 1258]

I can sort of see why people think that frequent replacement will prevent failure, but it doesn't. The place where a chain separates receives no wear whatsoever and will break with exactly the same force when it has done 10000 kms as when it has done one.

In fact, the most likely time for a chain to break is early on in its life when manufacturing or installation errors come to light.

That may be the case but I've not had a failure since I started changing them frequently rather than leaving them on the bike till they failed.

 

[rogerzilla]

It's possible for chain tension with a smallish ring to approach 50% of the UTS for a cheap chain (I've seen figures of only just over 800kgf for one major branded chain). That could be over the fatigue limit, depending on the grade of steel used, so it will eventually weaken and snap.

 

[alicat]

Thanks for all the replies so far.

You're right, @Yellow Saddle. The chain broke at a link rather than snapping in the middle of a link. Here are some pics showing the bowed outer link.

What is UTS @rogerzilla?

The bike has one 36T chainring. The chain was a no brand 1/8th inch jobby. The chainline is slightly out (1mm or so). I had to do that to buy a bottom bracket that would accept a chaincase.

 

[Yellow Saddle]

It's possible for chain tension with a smallish ring to approach 50% of the UTS for a cheap chain (I've seen figures of only just over 800kgf for one major branded chain). That could be over the fatigue limit, depending on the grade of steel used, so it will eventually weaken and snap.

No, the fatigue limit for heat-treated high-carbon steel is way, way above what a leg can apply. I'll go so far as to say not one bicycle chain in the world has ever broken in fatigue.

 

[Yellow Saddle]

That may be the case but I've not had a failure since I started changing them frequently rather than leaving them on the bike till they failed.

Correlation is not causation.

 

[Yellow Saddle]

Thanks for all the replies so far.

You're right, @Yellow Saddle. The chain broke at a link rather than snapping in the middle of a link. Here are some pics showing the bowed outer link.

What is UTS @rogerzilla?

The bike has one 36T chainring. The chain was a no brand 1/8th inch jobby. The chainline is slightly out (1mm or so). I had to do that to buy a bottom bracket that would accept a chaincase. View attachment 441550 View attachment 441551

Bad assembly, either by the factory or...ahem....you.

 

[alicat]

Okay, thanks @Yellow Saddle. Luckily I have fitted a masterlink to the replacement!

 

[Deleted member 1258]

Correlation is not causation.

The plan was not to have failure's I've not had a failure yet, I'm happy with that.