Nexus hub gear slipping

[Brucey]

...
Now here's the thing; the SG-C6000-8D EV techdoc also shows the hub internals as being a 'B' fit for SG-S7000-8. ...

not any longer in EV C6001(following C6000). Therein it is consistent again

indeed. I wonder if the 'B' in the SG-C6000-8D techdoc should have been one column leftwards...?

cheers

[vandjq]

'Sorry it took me so long to get back, I've been laid up with a chest infection. I checked the cable and cleaned the cassette and pulley groove with alcohol.

Brucey suggested moving the yellow mark slightly, but that caused slipping in gear 3 as well as 4.

So, with a heavy heart I took the bike back to the shop I bought it from. They rang me a couple of days later and said the chain was very loose, which was a surprise as I tightened the bolts to 42nm as recommended by Shimano. I wonder if they slipped?

Of course they could not replicate the fault, as it only happens a couple of times each day on my commute (I don't use 4th very often). I can't imagine they rode the bike for 10 miles. They told me that if the fault persists they will return it to Shimano.

They were very nice, but I don't think they know much about IGH. I will collect the bike on Saturday and post again a bit quicker this time.

[Brucey]

a slack chain ought not cause slipping in one gear but not others, not unless you pedal very differently in different gears.

The track nuts can back out, (even if they are tight enough to start with), if the NTW tangs are a slack fit in the dropouts, or the axle is a slack fit in the NTW. Either thing can cause the axle to turn (in alternate directions depending on whether you are using high or low gears) and this can cause the whole shooting match to work loose. If the frame is aluminium, it is possible for extensive damage to the frame to occur in this way.

[edit; if the track nuts are not 100% tight, the axle moves on one side (making the chain slack), and the frame is stiff enough, the axle can be slightly flexed out of shape, i.e. bent. This can inhibit the action of the gearshift within the hub, and make slippage in some gears likely. Usually when the wheel is remounted correctly, the axle goes back straight (i.e. it hasn't been permanently deformed) and the gears can mysteriously start working again.

If wheel slippage is persistent, one approach is to use some chain tugs to help prevent it. Batavus (amongst others) use some neat double-sided stainless steel chain tugs on their Nexus-equipped machines; these are not the strongest design, but they do have little cutouts in them for the NTW tangs; very many chain tugs simply won't work with Nexus hubs because they lack these features. The alternative would be a single-sided chain tug, but this would be thicker and have to go between the hub and the dropout; not all bikes will accept this.]

cheers

[vandjq]

Well, 4th gear was slipping again. This time the shop sent the hub back to Shimano, who said it was OK inside. They directed the shop to a tech doc which states that the distance between the end of the cable housing and the centre of the inner cable fixing bolt must be 101mm. Since then it's been generally OK, with just a few minor slips which I can live with. But today the chain fell off. I used a chain wear tool which shows 1mm wear in the chain, which apparently means I need a new chain and a new rear sprocket. I have ordered a new chain, KMC Z51, which is a straight replacement for the old one. In the meantime I thought I'd tighten up the old chain, which is very loose. But the drop-outs are vertical on this bike, so there's no way to pull the chain tighter.

Any ideas?

BTW I'm not very pleased that a bike shop didn't warn me about a worn chain and sprocket. Is this a DIY job, and what if I need a new chain ring?

[Brucey]

you probably have an eccentric BB to adjust your chain tension.

Unless I missed it, you didn't change your cable housing (or cable) yet? This is the first thing I'd do with a hub that was misbehaving.

FWIW if you change your chain when it is less than 1% worn, you normally only need to change the chain; the sprocket and chainring won't be worn out, that is whole point.

cheers

[jb]

For what its worth horizontal drops aren't much good on aluminium bikes as the N.T. washers make a niche to sit in and if you try adjusting them slightly it simply finds its way back to the original position no matter how tight they are. So eccentric adjustment of the bottom bracket makes more sense, and is a lot easier to do providing you've not allowed it to corrode.

[vandjq]

Thanks for the replies.

When I was adjusting the cable I alternated from 1-4 and 1-5 then back to 4. Sometimes it lined up, sometimes not, which suggests a dragging cable, as Brucey mentioned earlier. In an earlier post Roadster suggested using Jagwire cables and housing.

When I measured the chain wear I had the bike upside down. When I turned it back upright the chain wear tool showed wear of less than 0.75mm, so not worn. Presumably the upper part of the chain is under more tension when the bike is upright?

As mentioned earlier the bike shop noticed the chain was loose but did not tighten it up. How do I adjust an eccentric bottom bracket? Do I need special tools?

[jb]

There is usually a pinch bolt(s) under neath but types differ. - post a photo of yours.

[vandjq]

Well, I managed to install the new chain by loosening off the two allen bolts underneath the eccentric bottom bracket. I have never touched a bottom bracket before, preferring to let the LBS do it. But as it was just to install a new chain, I thought I'd have a go. I bought a Park Tool pin spanner to do the adjustment, but was surprised to find the pins were not long enough to reach the two holes in the BB, which was recessed into the frame, not like the one in this picture:




I assumed the BB was correctly positioned until I read Sheldon Brown's article about chain alignment. He states that (for IGH bikes) the centre of the front chain-ring should be 42mm from the centre line of the seat post.





When I moved the BB to that position I was easily able to use the pin spanner to adjust the new chain, which now has about 3/4" slack.

I am waiting for new Jagwire housing and cable to arrive.

[vandjq]

The gear change is working smoothly now I have replaced the cable/housing, but I have a different problem now. Following Sheldon Brown's advice on the position of the bottom bracket means the chain is now running at an angle because the rear sprocket is further out than the front chain-ring. Any ideas on how to fix this? Or should I ignore it?

[Brucey]

a 42mm chainline for IGH is not a universal truth.

However you can adjust the chainline at the hub by using a different sprocket. Nexus hubs will normally accept a sprocket that can be dished inwards, flat or dished outwards. However not all dished sprockets fit in all sizes.

If you presently have a sprocket that is dished outwards, you could try flipping it over.

cheers

[KenoshaKid]

2) if the above checks out OK, try adjusting the gears so that the yellow marks are not quite lined up (by about 1mm) in 4th gear. There's no way to know which way will be an improvement, so try both ways. Again it isn't uncommon for Nexus 8 hubs and cassette joints to be made imperfectly so that the optimum gear adjustment isn't quite where the yellow marks might suggest.

If 2) works OK for you, experiment to find the optimum setting and then use that when adjusting the hub in future. If you want to determine the correct setting by some other method than trial and error, this will require that the axle assembly is removed from the hub internal. Once connected to the cassette joint and shifter, it is possible to observe the pawl lifts directly. IME the 'window' for correct adjustment of gear 4 is very small indeed, hence any error in tolerances causes problems of the sort that you are seeing.

BTW I have encountered some Alfine and Nexus 8s hubs that were badly made such that if gear 4 was set perfectly (as determined by pawl lift) the (separate) sliding clutch that engages gear 5 wasn't fully home when gear 5 was selected. These hubs can be repaired by disassembling the sliding clutch actuating cam and reprofiling it. However this isn't an easy job at all; I wouldn't wish it on anyone.

I realize this is an old thread, but I was very happy to find it. My Nexus 8 was slipping in 5th, but I adjusted the alignment mark (first too much so that 4th gear skipped, but then found the sweet spot). Now the flat part of my commute is much happier, so thank you, Brucey!

[SilverTurfer]

Hi Guys,
I know this is an old thread but some of you guys seem to know a lot about this hub.

I've got a Shimano Nexus 8 Speed, the markings are SG-8R31
its mounted in a Gazelle Innergy Orange e-bike .. I'm not sure of the age, its a very nice bike but:
I've bought it second hand from eBay, and I think I've bought a bit of a pup

It was ok for about 5 minutes but it starting messing around in the gears.
I have it set up as it should be .. yellow spot to yellow line. (and I've twiddled it a whole lot to find a sweet spot)

The problem is that gears 1,2 & 3 are all the same .. 1 I imagine,
4 usually just does not engage i.e. slips.
5,6, 7 & 8 are all the same, seems like 5.

That was bad enough, but now when I shift up to 5 it just jams

It looks like this nexus box has had it or needs to be repaired by someone who knows what they are doing.

Can anyone advise me ?
I live near Cambridge

[mjr]

Bicycle Ambulance in Cambridge used to service Nexus hubs. Might be worth a go?

[SilverTurfer]

Cheers Mjr,

I'll see if I can find them.
Ah yes, I've found them, I'll give them a call tomorrow.

Thank you!