1/8 onto 3/32 wont go

[border-rider]

Hmm.  Conventional wisdom (and my experience) is that it's fine to run a 1/8 chain on a 3/32 sprocket

I just converted the Brommie from 3 speed to SS, with a 3/32 sprocket.  And the new 1/8 chain won't work. Is it because the sprocket is very small (12 teeth I think) ?  But this is the same as the one that came off - and that was 1/8. 

a 3/32 chain works perfectly. 

[Ian H]

What happens?

[border-rider]

It jumps. The chain rides upon the teeth of the sprocket and  - jump, jump, jump every few links.

[rogerzilla]

Are you sure the chainline is correct?  Is the sprocket in line with the tensioner pulleys?  A 1/8" with full bushings may be less forgiving than a 3/32" (which is designed to flex sideways for derailleur systems).

[border-rider]

Yep.  I've been tweaking that.  It's not a sideways issue  - it's up and down.

[rogerzilla]

It's just that a chain tends to ride up on the teeth of a derailleur cluster when the indexing is out.

[border-rider]

Sure, but that's because it's pushing sideways against the teeth.  This isn't.  It almost looks as if the pitches of he sprocket and chain differ a smidge - impossible though that is.

I suspect it's a combination of the teeth profile and the very tight radius of the sprocket

[Ian H]

It would work if the chain was tight - no tensioner. So a fixed, or even a normal single freewheel wouldn't be a problem. Puzzling, though.

[Zoidburg]

The brommie uses big chainring does it not? I  am thinking that maybe the short but steep amount of rise from sprocket to ring is creating a situation not unlike a worn sprocket, the chain is being lifted high enough that you are not getting enough engagement of teeth at the rear. You need deeper sprocket teeth or a bigger sprocket.

[LittleWheelsandBig]

I have a couple of times found a problem with mismatches of radii between chains (diameter of roller) and cogs (recesses for rollers).  Sedis had this problem with a minor change in roller diameter many years ago.  It is pretty rare.

[Chris N]

I think Zoiders and Ian have it - I had a similar problem on a SS MTB conversion where the tensioner wasn't wrapping enough chain around the sprocket and it'd slip.  Taking a link out of the chain tightened everything up a treat.

[border-rider]

LwB's idea looks feasible - as does ZB's

There's loads of chain wrapped round the sprocket though.  It's just not sitting down properly.

[LittleWheelsandBig]

The Brommie chain tensioner wraps the chain about 180 degrees around the cog.

[Zoidburg]

Are you using a sprung tensioner or chain tension from the drop out?  eccentric movement of the BB might be at fault or a badly machined chain ring. A biopace ring works because of its even distribution of teeth over the ring, eccentric motion of an otherwise circular drive ring is going to throw the ratio of tooth engagement out in the same manner as chain stretch

[LittleWheelsandBig]

Bromptons have to have a sprung chain tensioner, to wrap the chain when folded.

[border-rider]

eccentric movement of the BB might be at fault or a badly machined chain ring. A biopace ring works because of its even distribution of teeth over the ring, eccentric motion of an otherwise circular drive ring is going to throw the ratio of tooth engagement out in the same manner as chain stretch

But it works fine with a 3/32 8-speed chain...

looking at it again, I think it is as you and LWaB said - it's a combination of a very tight-radius sprocket with shallow teeth, and a chain with fat rollers

[giropaul]

I have no hesitation in using a 1/32 ring with an 1/8 chain, but I would always use a 1/8 sprocket.

12 is a very small sprocket. It won't run as well as a larger one. I only ever used a 12 for roller record attempts, and it was critical to get the line right if the chain wasn't likely to try to climb off. As has been identified, the angle between the teeth is much greater on a small sprocket. The chain will also have to go through more movement, which adds wear and is less efficient.

[border-rider]

I have no hesitation in using a 1/32 ring with an 1/8 chain, but I would always use a 1/8 sprocket.

I've used 3/32 sprockets with 1/8 chain for years.  It's not been a problem before.

12 is a very small sprocket. It won't run as well as a larger one. I only ever used a 12 for roller record attempts, and it was critical to get the line right if the chain wasn't likely to try to climb off. As has been identified, the angle between the teeth is much greater on a small sprocket. The chain will also have to go through more movement, which adds wear and is less efficient.

Sure, but it's the size Brommies use.  It had a 12 sprocket on there before and that worked fine.  Different tooth profile though...

[Ian H]

I've used 3/32 sprockets with 1/8 chain for years.  It's not been a problem before.

But perhaps not with a sprung tensioner?

I did peer at the chains on my fixed and geared bikes last night, but I couldn't find calipers to measure the exact roller diameter.