Brompton chain issue.

[silva]

But the chain only jumps off "wildly" in a certain gear, while chain tension remains the same in all gears, so tension as a first cause, can be eliminated then, no?

 

[Sharky]

Have you tried, derailing the chain, so not connected to either the sprocket or chainring.

Then with the wheel still place and lifted off the floor, turn the sprocket anti clockwise in each of the gear positions.

Is there any difference in resistance when in the gear in question, compared to the other gears?

 

[88robb]

Doesn’t have a derailleur limit adjustment screw, there are just two sprockets which I don’t change, the issue happens just when I move to the 3rd internal gear from 2nd, I don’t change sprockets at all but the issue still happens.

Sounds like your hub gear is out of adjustment. The cable tension is likely off. When you click the shifter to 3rd, it's pulling the chain too far, making it rub. Try turning the cable barrel adjuster a half-turn counter-clockwise to loosen the cable slightly. Test and adjust until the rubbing stops in all three gears.

 

[88robb]

but you got me thinking…the 2nd gear in the hub is actually direct drive, whereby the 3rd gear is classed as overdrive and this is when the issue happens when the hub does its thing so looks like it could be a hub issue or more likely I need to remove the hub chain screw then refit, that might fix the issue…Ive removed this a few times as it needs removing when removing the rear wheel, never had an issue previously, but maybe this time it was an issue?

Yes, that's a very likely cause. If the hub's chain screw wasn't perfectly straight and tight when you refitted it, it can pull the whole axle assembly out of alignment. This would cause the chain to rub in the overdrive gear. Remove it, make sure it goes back in straight, and tighten it firmly. That should fix the rubbing in 3rd gear.

 

[Willam]

Does the tensioner appear to be working OK / reassembled correctly from changing the chain?

Chain going slack and bouncing sounds like a chain tensioner issue to me?


*If the selector is selecting all 3 gears with no gear slippage, then the selector is correctly adjusted.

Think this might be it…tried removing the gear chain and I still had the issue, not previously noticed but the chain tension arm is what is causing the chain to jump, so I’m guessing it either a fitting error on my part, tho it looks correct or maybe cleaning it may fix the issue? tho I am still struggling to understand why this is only happening in 3rd gear as the chain never enters the Hub or the length of the chain doesn’t alter, so why the tension arm is being affected beats me.

 

[Willam]

Have you tried, derailing the chain, so not connected to either the sprocket or chainring.

Then with the wheel still place and lifted off the floor, turn the sprocket anti clockwise in each of the gear positions.

Is there any difference in resistance when in the gear in question, compared to the other gears?

Will try this tonight.