idiot question - 1/8th chain on 3/32nd chainring/sprockets

[Attitudeless Badger]

Is this a good thing to have?  I am in the process of building up first ever fixie, finally got all the drivetrain in place and whilst chainline looks straight, it sounds clunky and doesn't appear to 'flow smoothly' over the cogs but is a bit jumpy.  Obviously I don't fancy riding it till this is sorted.

The chain I have is an SRAM 3 speed job, which is comparatively chunky compared to the 9/10 speed ones I am used to. Chainring is a standard one from an old double, and the sprocket is a 3/32nd.  To be perfectly honest (and this is the idiot question), I am not even sure what measurement the 3/32nd or 1/8th are supposed to represent.

Do I need to look at a new chain to stop this or just keep tinkering with chainline till it all works?  Should I expect any noise at all?  Any help for a fixie first timer much appreciated.

[Luke]

There's no reason your current setup shouldn't work.

You may find it slightly quieter/smoother with a 3/32nd chain tho'.

[Charlotte]

In general, 1/8th chain on 3/32nd rings is fine.  It might be a bit noisier, and then again, it might not be.  YMMV.

I ran a 1/8th chain made up of all the little bits of left over fixie chain on Margaret Hilda the tandem's timing chain.  Worked lovely, it did

Have you been to the Fixie FAQs, by the way?  Ver' good source of information for novice fixer and expert alike...

[bobajobrob]

My sprocket makes a fair old racket (EAI 3/32 with PC-58 3/32 chain) so I guess it's normal. 1/8 chains on 3/32 sprockets is doable, Sheldon Brown says it works OK so who am I to argue? You may get more noise from this setup. The thing I'd be concerned about is the "bit jumpy" bit. The chain should flow smoothly.

[mrcharly-YHT]

maybe tension is a tad too high?

[Charlotte]

We do like a taut chain round here, though 

[TimO]

...I am not even sure what measurement the 3/32nd or 1/8th are supposed to represent. ...

Chain width, you even see 5/64" sometimes.  Dérailleur gears tend to need narrow chains so that they change cleanly.

[Attitudeless Badger]

thanks all.

To my untrained eye, it looks like the sprocket teeth are catching on the side of the chain.  At first I thought this could be a due to the chain being too wide and rattling from side to side, but then also it could be due to a chainline prop? 

For chain tension, is there a simple guide, apart from "tight, but not too tight?"

[rogerzilla]

The measurement is the internal width, i.e. between the inner side plates. 

Generally, chainrings for 1/8" chain are 3mm thick (0.118") and chainrings for 3/32" chain are 2mm thick* (0.078").  This gives, in each case, a few thou clearance for smooth engagement and disengagement.  Therefore you can't use a 3/32" chain on 1/8" cogs, but you can use a 1/8" chain on 3/32" cogs.

Excuse the mixed units.

*if you're lucky.

[bobajobrob]

For chain tension, is there a simple guide, apart from "tight, but not too tight?"

It should be as tight as possible without binding. Rotate the pedals 1/8 turn at a time and check chain tension as there are usually tight spots. 1/2" - 1" of vertical slack at the loosest point is normal. You can run it much slacker than this and still be fine, but it's good to get it as tight as poss.

[mrcharly-YHT]

thanks all.

To my untrained eye, it looks like the sprocket teeth are catching on the side of the chain.  At first I thought this could be a due to the chain being too wide and rattling from side to side, but then also it could be due to a chainline prop? 

For chain tension, is there a simple guide, apart from "tight, but not too tight?"

What Bob said.

Too slack is dangerous. Try rotating your pedals whilst pushing the chain sideways with something, say a spanner handle.

You shouldn't be able to get the chain to come off.

Too tight results in tight spots, and it will be hard to rotate past these. You should be able to give the rear wheel a spin and watch it run forever, almost.

If you have a particular tight spot, try slackening your chainring bolts and minutely moving the ring in. Don't forget to tighten up the bolts afterwards.

[rogerzilla]

Half an inch of vertical slack is a lot.  I'd go for no more than 1/4".

[bobajobrob]

Half an inch of vertical slack is a lot.  I'd go for no more than 1/4".

That's fine if you have a very round chainring, but mine is more like 1/8" at the tightest spot and 1" at the loosest (TA chainring on an Ultegra crank) I've never had a problem. TA chainrings are known to be not very round.

[bobajobrob]

Too tight results in tight spots, and it will be hard to rotate past these. You should be able to give the rear wheel a spin and watch it run forever, almost.

Spinning the wheel on a fixed is dangerous - keep all appendages well away!

[rogerzilla]

The sure-fire way to check if your chain is tight enough is to try and push it off the chainring (or sprocket) while turning the pedals.  Use some kind of implement to do this, not your fingers.

Also, never use a chainring with cutouts and ramps; some cheap singlespeed and hub geared bikes do, because they're made from derailleur bike parts (stand up Halfords/Carrera), but you need full-height teeth all round on a fixed.

[giropaul]

I know I've posted this before, but I was taught to have a chain so that there is a little slack at the tightest point.

Then check that it can't come off by wizzing the pedals round and shaking the bike about horizontally. If it doesn't come off this way, it won't.

!/8 chains run fine on 3/32 rings, but 1/8 rear sprockets are a good idea. At least one Commonwealth Games Gold Medalist almost always used 3/32 rings with 1/8 chain and sprockets.

[Attitudeless Badger]

Thanks again all.  I tried a 3/32nd chain last night (botched together with a 1/8 chain link) all seems a lot better.  Test-rode it outside the house and no-one died.  Need to get a proper chain link and then hit the road for the weekend.....(probably literally as I forget not to freewheel and get thrown onto the tarmac....!)