Brompton chain issue.

[Willam]

Just changed the chain on my 6 speed Brompton, used an 8 speed chain, the chain jumps violently off when I change the gear to 3 on the internal hub, but only if I stop pedalling, if the cranks are moving the chain runs fine, it runs normally in 1 and 2 setting, any suggestions why this is happening?

 

[Gunk]

8 speed chain should be fine, are you sure it’s the correct length?

 

[Sharky]

Is it jumping off the chain ring or the sprocket?

 

[Willam]

8 speed chain should be fine, are you sure it’s the correct length?

Measured from the previous chain that came new on the bike, even doubled checked that a 50 tooth chain ring requires a 100 link chain.

 

[Willam]

Is it jumping off the chain ring or the sprocket?

It jumps violently from the top middle of the chain, up and down a few times then comes off the chainring, very weird as I was thinking the free wheel as this only happens when I stop pedalling but doesn’t do this when I’m in a lower gear and stop pedalling?
I’m thinking the sprockets are not the issues as I don’t change those, this is only when changing the internal hub gears.

 

[88robb]

Just changed the chain on my 6 speed Brompton, used an 8 speed chain, the chain jumps violently off when I change the gear to 3 on the internal hub, but only if I stop pedalling, if the cranks are moving the chain runs fine, it runs normally in 1 and 2 setting, any suggestions why this is happening?

The chain is likely too wide. An 8-speed chain can be slightly too thick for the internal hub's sprocket. When you stop pedaling in gear 3, the chain catches on the hub casing and jams. Fit a correct 6/7/8-speed chain to solve it.

 

[Willam]

The chain is likely too wide. An 8-speed chain can be slightly too thick for the internal hub's sprocket. When you stop pedaling in gear 3, the chain catches on the hub casing and jams. Fit a correct 6/7/8-speed chain to solve it.

it was a KMC 8 speed, the description says it’s compatible with 6/7/8 speeds?

The chain doesnt actually go into the hub so not sure what it could be catching on?

 

[Sharky]

Something I had, which is not dissimilar was ...

When I stopped pedaling, the freewheel did not "free", so the sprocket kept turning and the chain kept moving to the chainring and derailed.

 

[Gunk]

How old is your Brompton?

Reason I ask is that I’ve had a similar issue and solved it by changing the rear sprocket.

The earlier Bromptons had a different chain and sprocket. A newer 8 speed chain doesn’t fit and will skip

Your bike may be fitted with an original 1/8" sprocket that was discontinued (in 2017) To replace the chain you need the 3/32" sprocket which was introduced in 2012