Problems with a Nexus 8 - help!

[Ivorcadaver]

Shimano Alfine tensioner fitted last night and no jumping and the problem appears to have been solved - my 13 mile commute into Manchester achieved this morning with no problems transmission wise. The Shimano design gives a much greater chain wrap on the rear sprocket compared to the Surley. I got to admit I am surprised at how simple the solution was... Once again thanks for all the helpful suggestions.

[Sum]

I'm glad to hear this and I hope it stays this way - well done! The problem with the spring-loaded single-pulley tensioner is that if the chain has too much slack then the tensioner sometimes doesn't provide enough chain wrap and tension to stop the chain skipping. If you still want to replace the Alfine tensioner then, as the tension of the Surley Singulator is adjustable, it may have been possible to get it to work by increasing the tension. Or the next time you're replacing the drivetrain try for a 'magic ratio' of chainring and rear sprocket that eliminates the chain slack as much as possible so you can use the Singulator in push-up mode.

[Brucey]

the fact that it was jumping probably indicates a slightly worn sprocket and a slightly newer chain; the chain can jump the same way as it does with similarly worn derailleur gears with this combination.

Since a new SA sprocket and a new chain can be bought for about £5 all in there is no need to run around with worn parts here if that is the problem.

cheers

[Sum]

A combination of a new chain & worn sprocket can be a cause of chain skipping although in this case it appears the OP had bought both a new chain and sprocket together.

[Ivorcadaver]

The whole of the drive chain (chain, chainring and rear sprocket) was brand new. The geometry of the Surley meant that a max of 7 teeth could be engaged on an 18t sprocket in push down mode. Push up would have been preferable and given more wrap but the chain would have fouled the gear change mechanism.

[Brucey]

I can't help but wonder if there isn't something slightly odd about the setup; if 7T of wrap plus a little tension isn't enough to stop skipping then there are plenty of derailleur systems which would never work. I wonder if there is something different or peculiar about the tooth geometry or the chain?

BTW if you would prefer to use the singulator, a 1/8" chain can be fitted with a half-link. I'd guess that this might well fit with the amount of slack you have and inhibit jumping nicely.

cheers

[Sum]

I can't help but wonder if there isn't something slightly odd about the setup; if 7T of wrap plus a little tension isn't enough to stop skipping then there are plenty of derailleur systems which would never work. I wonder if there is something different or peculiar about the tooth geometry or the chain?

There may have been something odd about the set up. The tension in the Surley Singulator is adjustable and has to be set correctly when installing the tensioner; It’s possible that there wasn't enough tension in the chain. Straight out of the box the Singulator should only be used with a 3/32" chain but can be used with a 1/8" chain if both chain guide plates are turned 180 degrees so the beveled edges are facing the pulley. The chain might have a stiff link if the chain was joined by a chain tool. I guess the OP would have checked for these though.

The problem with the spring-loaded single-pulley tensioners is that the greater the chain slack then the more the tensioner has to unwind to take up the slack which in turn reduces the chain wrap and chain tension. My experience of bodging single speed conversions is that the chain can sometimes skip under load if there’s more than a links worth (i.e. one inner or one outer link) of slack. IIRC the installation instructions for the On One Doofer (used to) say that the tensioner was not suitable for use with chains that are slack by more than one link. The Doofer was effectively non-adjustable though, hence I was wondering if the Singulator would have worked if set up with sufficient tension.

[Ivorcadaver]

I think this will remain mystery. The spring in the Singulator seems a pretty feeble affair and even when adjusted to its fullest extent with cone spanner as per fitting instructions gives much less tension compared to the Shimano. Plus the double sprocket design of the Shimano gives a geometry that allows for more wrap on the drive sprocket. I guess the lesson is beware of the Singulator (probably and other single sprocket designs); set up seems to be quite sensitive. On the plus side I have learnt that there is little to fear from disassembly of the Nexus hub and it looked in great condition after 1700 miles