Author Topic: Chain tensioner / Nexus 7 issue  (Read 311 times)

Chain tensioner / Nexus 7 issue
« on: November 19, 2020, 10:20:18 pm »
I have a Tern Vektron with a Nexus 7 hub.

In general it has been great bike, but I have had chain tensioner trouble twice. The first time was not long after I bought the bike. I was going down a slight incline, transitioning from freewheeling to pedalling as the slope levelled out, went over a bump, and the chain got caught up with the chain tensioner being pulled back and up. I was able to ride it after repositioning, but it happened again soon after. There was no clear indication of why this had occurred. Alignment etc was checked, tension seemed ok, but the problem occurred again soon afterwards. The chain tensioner was replaced with the same Shimano model and all was good for several thousand km.

A few days ago, it happened again. Exactly the same cicumstances - downhill, over a bump just as I was beginning to pedal after freewheeling. I was able to ride it home. I can see that the tension is not what it should be - my impression is that once the tensioner is pulled up and back like that, even if one stops pedalling immediately (not that there is much option), something alters and the tensioner will not exert the proper tension again. It feels like a chain with one too many links in it.

Has anyone experienced this?

I am thinking about abandoning the Shimano tensioner and trying a Surly Singulator.

I’m not sure if it is relevant or not, but it has always been impossible to backpedal with this hub - while it doesn’t push the chain forward when freewheeling, and indeed doesn’t seem to have too much drag when freewheeling, there has always been a lot of resistance when turning the pedals backwards, to the extent that I have always avoided doing it on this bike unless I’m fettling it.

Any thoughts? To be frank I’ve always thought that needing a chain tensioner negates some of the simplicity of having a hub gear and kind of wish I’d got a derailleur version. (Edited to add: because then I’d be on familiar territory).

Re: Chain tensioner / Nexus 7 issue
« Reply #1 on: November 19, 2020, 10:46:28 pm »
like this?




Re: Chain tensioner / Nexus 7 issue
« Reply #2 on: November 19, 2020, 10:50:04 pm »
Yes, that’s the one.

At present, post-banjax, I think both parts are more vertical than they ought to be. There isn’t enough tension in the lower pivot; the top pivot is trying to compensate but it isn’t enough.

Re: Chain tensioner / Nexus 7 issue
« Reply #3 on: November 19, 2020, 11:13:11 pm »
shimano also do a single-pulley tensioner, which isn't spring loaded but is fixed in position. This needs to be adjusted from time to time but if you keep it adjusted it is practically impossible for the chain to fall off. However I'm not sure that the chain would be pushed low enough to clear the low chainstay on that frame design.

BTW it occurs to me that the tensioner you have doesn't usually move so may have at least partially seized; the pivots in a spring-loaded tensioner need to be free moving (just like those on a derailleur) otherwise the chain isn't always retensioned immediately should you go over a bump.

cheers

Re: Chain tensioner / Nexus 7 issue
« Reply #4 on: November 20, 2020, 08:29:31 am »
Thanks, Brucey.

Both pivots seem to be moving freely, although it is possible that one could have been seized or partially seized just before the incident occurred and were freed up when I repositioned it to get me home.

I’m going to bring it to the LBS who is luckily a Tern dealer and has a P7i there for tension comparison purposes.

I can’t see any way to restore tension in the existing one, as the springs all seem to be properly located, and I am leaning towards trying an alternative this time. Maybe the Alfine tensioner. There’s a single pulley version of the Alfine, which may be the one you are thinking of, but I agree that it probably wouldn’t work with that chainstay. The Singulator might work, and appeals if only for the fact that there is only one pivot to be concerned about.

Re: Chain tensioner / Nexus 7 issue
« Reply #5 on: November 26, 2020, 09:04:15 pm »
I'll +1 for the singleator on my Sturmey-Archer IGH. Withstood my pretty lax maintenance regime for 5yrs and sold on no worries just recently. And that was with probably 1 or 2 too many links in the chain for the first 4 yrs. It'll also tension up or down depending on how you set the spring.
Cruzbike V2k, S40