Nexus 7 hub failure?

[Mark Berry]

Hi,

I've been using, or to be honest abusing, a Nexus 7 speed hub on my commuting bike for about four years. It has occasionally objected audibly to a rushed or loaded gear change. The only servicing has been a small dribble of motor oil via the left hand cone, once a year or so.

Yesterday's ride home was uneventful, including the last ascent up my road in first or second gear. But today, no gear change, from the start. Just stuck in low (1st?) gear. Although dark, it was clear that the cassette joint thingy was in place and rotating as expected, and I could feel the tension of the cable.

I'm not going to see this bike in the light until the weekend (unless MrsB goes out!) so I'm keen to know the worst and start sorting a replacement ASAP: commuting via car was grim.

Is this a common failure mode for the Nexus 7?

Is user-repair feasible? (I've rebuilt Sturmey 3 speeds but I guess this is in a different league).

This hub owes me nothing: it's been much more reliable than the Nexus 8 it replaced. If I need to renew, what hub would you recommend? I'm tempted by the same again.

Thanks.

[Brucey]

Occasionally the cassette joint will slip on its splines (through wear, external to the hub) so that the gear selection doesn't work any more. However when this happens it is rare to lose all gears bar first and for the cassette joint to move normally i.e. under spring pressure through the full range.

In order for this to happen, the various clutches inside the hub have to stop working. To my mind a likely cause for this is the recent wet then cold temperatures we have had; if there is any water in the grease or the grease is a bit sticky anyway, various parts of the hub can start to gunge up rather quickly.

All the clutches in an N7 hub rely upon pawls of various kinds that engage under spring pressure. The springs are made from a corrosion resistant alloy, so when they stop working it isn't usually a problem with the springs themselves.

If you can dismantle an SA 3s hub you can at least take a look at the N7 internals. Just undo the LH cone, brake (or cover) and locknut assy. Then use a soft hammer to knock the internal out; the dust cover on the RH side is just a push fit into the hubshell.

My bet is that you will find that there is sticky dried grease and/or corrosion present. If so, dousing the whole thing in a thin, penetrating lubricant is a good idea. In fact if you don't fancy removing the internal, just remove the LH cone and add as much oil to the hub from that side as you can manage, then reassemble the LH cone (with a little grease on the bearings) and work the hub some.

For this purpose ATF is pretty good; it isn't the best lube for a hub that is in good shape (a nice gear oil would be better) but it has a high detergent value and that makes it a good choice for hubs that are dancing in the last chance saloon, like yours is.

On the gear centre there are two sets of external pawls that drive the hubshell; those on the far left of the internal (which give you first gear only) and those on the middle of the internal, which give you all the other gears. My guess is that the latter set have simply gone sticky; if so the rest of the hub could be fine, just two sticky pawls can lose six gears!

If the hub comes back to life, some of the excess oil will find its way out of the hub over the coming weeks. This will make a bit of a mess but isn't necessarily a bad thing, since it will carry the contaminants away with it. In the long run the hub may work best for you if you run it in oil. Adding oil (tablespoonsful not teaspoonsful) via the LH cone once every six months will probably keep the hub sweet for years.

hth

cheers

[Mark Berry]

Dear Brucey,

That is very helpful indeed, thank you.

It was pretty cold last night (to us soft Southerners anyway), it's been very wet this Autumn, and the bike does live outside. I have added oil (teaspoons, not tablespoons) in the past and never noticed any coming out, and I was therefore assuming that the hub is watertight. I've probably been deluding myself.

I'll wait 'till the weekend, open it up, and report back. Fingers crossed....

[RacingGreen]

Hi, I gave my Nexus 8 an oil bath(in ATF) last year and never noticed an oil leak (the oil is red so i'm sure I would have). Anyway and I just topped it up today, just in case it was low, and because the bike got quite wet (in heavy rainfall) a few times recently and I thought a few squirts of oil were in order.

I simply undid the cone on the left hand side and squirted in some ATF. I would say I put in about 30 to 40ml in total (squirt, wait, squirt, wait, you know what I mean). Then I greased the bearings (which should also help to keep the oil in) and did it back up.

Hope this is helpful to some

[Brucey]

that's pretty much what I had in mind for the OP too.

BTW the likelihood of leakage varies with the vintage of the hub (the RH seals vary in their construction). Even the seals that are meant to seal well sometimes don't, and seals that don't always seal well sometimes do, because they are filled with dried grease.

cheers

[Mark Berry]

Brucey, your diagnosis was right - the hub was full of a black emulsion and I guess that this had gummed up overnight in the cold weather. Luckily there wasn't rust. The bike has been used virtually every day for four years and maybe this is enough to prevent rusting even if there is only mayonnaise circulating.

I bought this hub as 'NOS - removed from unused wheel' and I peeped inside when it arrived to confirm. The grease was then spotless white. The gunge that I've now cleaned out was black as ink! Happily no visible metallic particles.

What does concern me is what I take to be wear in the pinion pins or cage. The pins visibly move in the cage, radially, when the internal is rocked back and forth. So maybe I really am' dancing in the last chance saloon'!

Oddly the lock ring which holds the cassette joint on wouldn't click convincingly in place and came adrift when I went for a test ride. The whole thing had been filthy and gritty but snug before. Maybe I should have left it that way. Do these things wear? Anyway, there was a cheap one on ebay so I'll know in a day or two.

PS. I put a Sturmey AW 3-speed wheel into a 1970's gas-pipe ten-speed-racer for my commute this week and guess what? It flies! I may have answered my own question about a hub replacement when needed. But the Nexus is in a 1990's Gary Fisher mountain bike (originally 21 speed) and this makes a very nice safe, weather-proof load-carrying commuter. Do Sturmey Archers come with axles wide enough for that frame?

Cheers!

[Brucey]

well that all makes sense to me; four years with little added lube means some wear is likely. If the pins are worn where they go through the LH planet cage this is a sure-fire sign that the lube wasn't adequate; these gears are used in 6/7 gears and the planet gears should rotate on the pins; if the pins themselves start to turn this is normally because the lube is bad.

A little wear is not unusual and normally isn't very harmful.

The cassette joints do wear on these hubs; sometimes you can pack them out with a plastic washer (eg the sort that come with gripshifts) but a new lockring is usually a better answer.

The Nexus 7 is a good hub but.... super-efficient it ain't. I reckon when it is in good nick you are losing between 5 and 10% in the middle three gears (because there are two gear trains in use; there is no direct drive); if the lube is bad it could be a lot less efficient than that.

By contrast an SA 3s has a nice efficient middle gear and the other gears are all more efficient than those in an N7. If the hub goes a bit draggy a squirt of oil should make it good again. If you were flying on an SA 3s, it could be that you had a tailwind but then again maybe the better efficiency shone through; it is not for nothing that such hubs have been utility stalwarts for as long as they have.....

If you have a 135mm steel frame with dropouts that are not to thick and you use skinny NTWs and a bit of springing you can just about squeeze an SA 3s (with the right axle) in there, but otherwise I think you are going to struggle. IIRC the longest axle you can get is about 163mm or something. SJS have most of the axle types in stock; just be sure to get the right one for your hub- they do vary.

If you were happy with the N7 before, and the wear isn't too bad maybe fitting a lube port (and regularly adding gear oil through it) will keep you going for another four or five years?

cheers