Chain bouncing off

[Phileas]

I commute on a single speed bike and I’m getting this issue since I put the current chain on - KMC S1. I realise I could keep the chain tighter but I’d rather be able to run it with a little bit of slack.

I think what is happening is that sometimes going over a bump (rough tarmac) the chain shakes laterally and derails.

Previously I was using a bushed chain which I’m guessing had more lateral stiffness so was less affected.

Previous to that I was using a Hebie Chainguard which prevented derailing but it’s a bit tight on a fat chain.

So is my only option to go back to narrow chains and use the Hebie Chainguard?

[Brucey]

if your chainring doesn't have full height teeth (because it is really meant for a derailleur system) then that is likely where your troubles lie IME. I suggest getting a chainring with full height teeth if you don't have one already.

BTW a KMC B1 chain is fully bushed and is less flexible laterally. This may improve matters. IME this chain runs OK on a 1/8" sprocket and a 3/32" chainring (with full height teeth).

Also make sure that your chainline is good; a lot of SS bikes are deficient in this respect (even if they are built in a factory not a shed... )

cheers

[LinusR]

I commute on a single speed bike and I’m getting this issue since I put the current chain on - KMC S1. I realise I could keep the chain tighter but I’d rather be able to run it with a little bit of slack.

You may have adjusted it with too much slack when you put the new chain on. You should have about 12mm of vertical movement mid-way along the chain. It should feel free but not bind when you pedal (by hand). It can be a fiddle adjusting a singlespeed to get it just right and getting the rear wheel running central.

I run a single speed fixed as a hack bike and leave it outside in all weathers and rarely oil the chain. I use a cheap KMC BMX chain and a standard front narrow chainring and I've never in many years had it bounce off even when kerb hopping. The only thing I do is keep the vertical play under control. I'm assuming yours is bouncing off the chainring not the rear cog?.

[mig]

I commute on a single speed bike and I’m getting this issue since I put the current chain on - KMC S1. I realise I could keep the chain tighter but I’d rather be able to run it with a little bit of slack.

You may have adjusted it with too much slack when you put the new chain on. You should have about 12mm of vertical movement mid-way along the chain. It should feel free but not bind when you pedal (by hand). It can be a fiddle adjusting a singlespeed to get it just right and getting the rear wheel running central.

I run a single speed fixed as a hack bike and leave it outside in all weathers and rarely oil the chain. I use a cheap KMC BMX chain and a standard front narrow chainring and I've never in many years had it bounce off even when kerb hopping. The only thing I do is keep the vertical play under control. I'm assuming yours is bouncing off the chainring not the rear cog?.

how does it run? doesn't it squeak like blazes?

[LinusR]

how does it run? doesn't it squeak like blazes?

Sometimes, and its quite noisy. But it's a hack bike I've had for years. It just keeps going.

[Phileas]

if your chainring doesn't have full height teeth (because it is really meant for a derailleur system) then that is likely where your troubles lie IME. I suggest getting a chainring with full height teeth if you don't have one already.

BTW a KMC B1 chain is fully bushed and is less flexible laterally. This may improve matters. IME this chain runs OK on a 1/8" sprocket and a 3/32" chainring (with full height teeth).

Also make sure that your chainline is good; a lot of SS bikes are deficient in this respect (even if they are built in a factory not a shed... )

cheers

Thanks.

The chainline is pretty much perfect.

I might still go back to a narrower chain and use the Chainglider, unless I can find reasonably cheap full height 5 arm 130pcd 42t chainrings.

[Phileas]

I commute on a single speed bike and I’m getting this issue since I put the current chain on - KMC S1. I realise I could keep the chain tighter but I’d rather be able to run it with a little bit of slack.

You may have adjusted it with too much slack when you put the new chain on. You should have about 12mm of vertical movement mid-way along the chain. It should feel free but not bind when you pedal (by hand). It can be a fiddle adjusting a singlespeed to get it just right and getting the rear wheel running central.

Like I said, I prefer to run it slack if possible and even a relatively small amount of play still allows the chain to bounce off.

Also, it’s not so much the initial adjustment but the need to keep readjusting the chain which I want to avoid since I’m using an eccentric BB which is more fiddly to adjust than the wheel position in a track frame.

[peetee]

It's not uncommon for chainrings or sprocket/freewheel to be machined 'out of round' and for there to be a tight or slack point for the chain as it rotates. If this is the case for your bike and you checked the chain tension at the tight point then when the slack comes round it might be enough to allow the chain to unshipped on a bump - especially if the discrepancy is on both chainring and sprocket and they both engage at once.

[Bez]

I'm not sure why you're determined to run the chain slack. It means the chain falls off. Am I missing something?

Anyway, as above, you should be using full-tooth (ie singlspeed specific) ring and sprocket, the chainline should be good, and you should check the seating of the ring on the crank to make sure it's concentric with the axle; it's easy to be off-centre with some ring/crank combinations. And, for what it's worth, IME singlespeed/BMX chains tend to come off more easily; they're not designed with the lateral play of derailleur chains so they're less tolerant of imperfections in chainline.

But anyway, run the chain reasonably taut

[Phileas]

I'm not sure why you're determined to run the chain slack. It means the chain falls off. Am I missing something?

It’s not particularly slack which is why I found it surprising that the chain bounces off. As I said, the previous chain was fully bushed and was much less prone to derailing.

[Bez]

Fair enough, there's slack and there's slack, I guess, but I can't help thinking that if it's what you'd actually call "slack" then the obvious step is to fix that.

[Phileas]

Fair enough, there's slack and there's slack, I guess, but I can't help thinking that if it's what you'd actually call "slack" then the obvious step is to fix that.

I can fix it but I prefer not to have to fiddle with the bike on a weekly basis.

[Bez]

If it's going slack on a weekly basis then maybe the answer is a chain tensioner. Obviously depending on your frame this may not be a trivial solution

[Phileas]

If it's going slack on a weekly basis then maybe the answer is a chain tensioner. Obviously depending on your frame this may not be a trivial solution

Well, it’s not really going slack, or at least probably not by your meaning of the word.

I might take a picture later to try to indicate the level of slackness I’m talking about (although I adjusted the tension this morning).

[Brucey]

the KMC S1 chain is of a bushingless design and it is flexible enough (laterally) that it will run in some derailleur gear setups. Whether the chain comes off when it is slack depends on the shape of the inner side plates, the tooth height on the chainring, and the quality of the vibrations in the chain.

To my great surprise I found on one bike (with an IGH fitted) a 3/32" chainring with full height teeth allowed a bushingless KMC 1/8" chain to be remarkably resistant to unshipping, even when the lower run had enough slack in it to flap up and down about 4"....

However the chainline was perfect, and the bike had a perfect track too. This meant the chain only ever flapped up and down vertically, more or less. I did manage to unship the chain about once a year; this happened when pushing the bike on bumpy ground, and when I freewheeled over bumps in the road whilst 'leant off the bike' . Both these things caused the chain to flap laterally in way that it didn't normally, and then it could unship.

Any small error in the chainline, the track of the bike, or the weight distribution will make the chain flap laterally over every bump. So if (say) you run with one pannier on, that'd do it.

A quick test of all this is to

a) ride no hands; are you exactly looking down the steerer, or do you have to lean slightly to one side?
b) ride no hands through a puddle on an otherwise dry road; if the front and rear tyre tracks line up exactly the track of the bike is good
c) doublecheck the chainline, via both centreline measurements and using a straight edge against the chainring. Ideally the chainline needs to be within 1mm by both measures, Very often the chainline is not good enough. Even having the rear wheel slightly skew in the frame can be enough to throw things out of kilter.

cheers