Shimano Nexus 7; how it works. (long)

For discussions about bikes and equipment.
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Shimano Nexus 7; how it works. (long)

Post by Brucey »

Time for a little update on this.

I recently found this;

http://mmi.tudelft.nl/~wouter/publications/pasman12a.pdf

which very helpfully shows how to disassemble the shift control mechanism in an N7 hub. Thank you Mr Pasman!

In recent times I have encountered a few more failure modes in N7 hubs;

1) cassette joint slipping; once worn it is possible for the cassette joint parts to slip over the castellations in the engagement on the hub, resulting in bad shifting and/or no spring return. Unlesss wear is catastrophic, it should be possible to recover the situation by adding packing washers.

2) Gross wear in the main ring bearing. I found a hub that was so worn (through water ingress) that 3/16" balls would no longer work in the main ring bearing. The owner had been riding it like this even though there was only one working gear! All the others were unable to work because the grease inside the hub had turned to something the consistency of toffee. I am currently experimenting with using 7/32" balls instead. I found out that if a 3/16" ball escapes into the workings it can break stuff....

3) Broken shift control. I received a hub that someone else had 'taken apart'. I quickly found that the first part of the shift control mechanism was broken. Specifically the small pressed steel part which has four legs (two long legs facing outwards and two short legs facing inwards) had broken; the long legs had snapped off and were floating around inside the hub.

Since you cannot buy this part separately, I figured it was worth trying to repair this by welding. I think I have succeeded in this, but I don't know how long it will last. My repair leaves the part dressed smooth, fully fused, and leaves the surface shot-peened for good fatigue resistance too. Who knows, it might be stronger than the original... :wink:

The main problem then was that the shift control mechanism had been misassembled. I've taken this apart before now and had no trouble because I've started with it in the right way and put it back together the same way again; no problem. But this time things were different.... Eventually I figured out that it is possible to assemble the main return spring collar three different ways onto the main shift sleeve (plus further permutations of spring preload) of which there is only one 'right way'. In addition the smaller shift control spring assembly (SCSA) can also be assembled three different ways, of which only one is correct. Further to this when installing the SCSA to the rest of the hub it is also possible to do this several different ways, and again there is only one 'right way'.

After several hours during which time I explored many different combinations of misassembly and realised how it was that

a) you could give an N7 something it normally doesn't have, a direct drive gear- but unfortunately no others... and
b) you could leave it such that a hard tug on the shifter would break the shift control mechanism...

I eventually found Mr Pasman's illustrations and worked out how to do it properly. My alternative would have been to have disassembled a working hub and to have looked at that. I intend to take some additional photos which will help to make the correct assembly clearer yet.

[In the meantime, in words, the main return spring can drive the main shift sleeve directly on downshifts (CW) but not upshifts (ACW). The SCSA outer sleeve is driven directly in a CW direction but only by the shift control spring in the ACW direction. The SCSA outer ring is coupled directly to the main shift sleeve. The four legged shift sleeve is also engaged directly with the main return spring collar.]

An additional comment is that almost any of the pawls in this hub can be dislodged during reassembly; if this happens the hub will not work and may even break if you try it. My suggestion is to do a trial assembly of each stage, then disassemble it again and make sure that you did it without dislodging any pawls. Then do the same thing again.

Mr Pasman has a good tip which is to trial assemble (make a 'toy assembly' he says) with the shift control mechanism but without the main return spring. This will allow the shift control to be worked more or less normally so you can check the hub for major faults. However I would only do this if the main return spring is apart anyway. I would suggest that you only disassemble the main return spring if you absolutely must; it is pretty much a major PITA to get back together again.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Shimano Nexus 7; how it works. (long)

Post by Brucey »

time for another little update:

Shimano are now manufacturing some new Nexus 7 models which are being used in the European market. Some are even manufactured in Europe, according to the stickers on them.

There is a coaster brake hub SG-C3000-7C-DX for which the EV techdoc is here;

http://si.shimano.com/php/download.php?file=pdf/ev/EV-SG-C3000-7C_7C-DX-3724.pdf

There is a rim brake/roller brake hub (with an alloy shell) SG-C3000-7R for which the EV techdoc is here;

http://si.shimano.com/php/download.php?file=pdf/ev/EV-SG-C3000-7R-3723.pdf

The new models look very similar to the older ones and looking at the techdocs shows that (as is usual for Nexus 7) the new internals will interchange into older hubshells. All of which may lead anyone to say 'well so what?'. Fair question.

Well, although the gear ratios are the same, the internals will swap en masse, and several of the individual parts are also interchangeable, the new internals are completely different.

The old N7 hubs had locking sun pinions with internal pawls that were sprung inwards. Each sun had four pawls and this gave a degree of redundancy as well as considerable strength; the pawls engaged two at a time, bearing against shoulders that were raised above an otherwise solid axle.

However the new N7 axle looks a lot like the N8 axle; now there are simpler sun pinions which are locked by a single pawl per sun pinion. The pawls sit in a deep grooves in the axle, just like the N8 arrangement.

Now having ridden hubs with both styles of shift control, I'd argue that the original N7 arrangement is in many ways the better one. The axle is stronger (no deep grooves in it), the pawls are strong enough, and the sun pinions are kept concentric to the axle when under load. The new N7/N8 arrangement is arguably weaker, and is more likely to wear badly (you can't buy new pawls) and is more likely to skip if experience with the N8 is anything to go by.

The new N7 axle is certainly a lot weaker, and to date I've seen more trouble with sun locking in N8 hubs than in N7 hubs.

However it isn't all bad; with an N8 style axle you can test the shift control of the hub by looking at how the sun locking pawls move on a bare axle as the shifter is moved. Although you can do the same thing with the N7 original axle, it isn't so easy to see exactly how the motion of the shift control sleeve relates to the sun locking.

So one has to ask 'why did they change it?' ; -well, it could be that they consider the N8 arrangement to be better in some way. However I (perhaps cynically) suppose that the true reason may be that the N8 arrangement is simply a lot cheaper to manufacture; all those little pawls must be fiddly to make and fit into the original style N7 sun pinions....

Needless to say none of the changes they have made address any of the criticisms of the previous N7 models; the planet pinions still have plain bushings, there is (I think, judging from the operation of the reduction gear clutch) still no direct drive gear, there is still no lube port, they still use the same white grease (that does not inhibit corrosion very well) and the sealing is similar to the previous SG-7R50 model. So at best it is the same, slightly draggy, slightly utilitarian hub that it has always been, only now they have perhaps added some new failure modes...

So I'd argue it isn't a greatly improved hub... and the best that can be hoped for it is that it isn't much worse than the previous models. In this respect disappointment may lie ahead; I've already seen several examples in which the bearings were clearly set too tight (something that cannot be corrected without removing the wheel from the frame in the 'R' model) and those chickens are now coming home to roost; today I saw a very low mileage hub in which the LH bearing had disintegrated. The broken pieces of ball bearing made it into both the carrier units and (despite the slacker clearances in the gears, presumably to allow for the now off-centre-when-locked sun pinions) both gear trains were damaged beyond repair.

Hey ho.... :roll:

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
spinachcharm
Posts: 6
Joined: 24 Feb 2019, 3:46pm

Re: Shimano Nexus 7; how it works. (long)

Post by spinachcharm »

hi all,

sorry to jump on this thread, but I have the SG-C3000-7R Nexus hub mentioned above on my Gazelle Cityzen. I am having some issues with changing gear, the change is really stiff when changing up, but easy when changing down. The instructions downloaded from the Shimano website don't help. I tried applying some oil on the mechanism but that hasn't helped either.

Any advice welcome, many thanks in advance!

cheers Chris
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Shimano Nexus 7; how it works. (long)

Post by Brucey »

if the cable starts to get draggy, a high shifting force at the shifter is the very first symptom.

First check that the cassette joint moves freely in both directions (if it doesn't there is a fault inside the hub or more likely a load of gunge caked around the cassette joint).

If the cassette joint pulley moves freely then the next thing to check is the cable. It too should move freely in both directions; if it doesn't it needs cleaning and lubing or (more likely if it is really bad) a replacement cable is required.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
spinachcharm
Posts: 6
Joined: 24 Feb 2019, 3:46pm

Re: Shimano Nexus 7; how it works. (long)

Post by spinachcharm »

Thanks Brucey

Apologies for being slow replying. Everything moves as it should, it's just the stiffness when changing up. This is temperamental, it can get stiffer or less stiff. I will certainly think about putting a new cable in.

Generally, regarding maintenance, what is the best thing? Regular oiling? I sometimes feel oiling is making the problem worse. Maybe using the wrong oil, which attracts gunk.

It doesn't help that I'm a complete newcomer to hub gears. I used to have a derailleur bike.

thanks,

Chris
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Shimano Nexus 7; how it works. (long)

Post by Brucey »

spinachcharm wrote:Thanks Brucey

Apologies for being slow replying. Everything moves as it should, it's just the stiffness when changing up. This is temperamental, it can get stiffer or less stiff. I will certainly think about putting a new cable in.

Generally, regarding maintenance, what is the best thing? Regular oiling? I sometimes feel oiling is making the problem worse. Maybe using the wrong oil, which attracts gunk.

It doesn't help that I'm a complete newcomer to hub gears. I used to have a derailleur bike.

thanks,

Chris


some oils soften the liner in some cable hosuings. However, that aside, you are usually better off with oil than without. Shimano IGH cable runs tend to fill up with water and go bindy; it is a very common problem.

FWIW if the problem is water entering at the bottom, you can buy a more weatherproof cassette joint.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
spinachcharm
Posts: 6
Joined: 24 Feb 2019, 3:46pm

Re: Shimano Nexus 7; how it works. (long)

Post by spinachcharm »

thanks again. So is there a particular oil you would recommend for this purpose? And should I oil the cable before threading it through the sheath?
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Shimano Nexus 7; how it works. (long)

Post by Brucey »

spinachcharm wrote:thanks again. So is there a particular oil you would recommend for this purpose? And should I oil the cable before threading it through the sheath?


IME almost any oil is better than none; it seems to be pot luck whether the oil softens the cable liner or not (since both oils and cable liners vary). With oil (unlike grease) you can't really overdo it; any surplus just runs out of the cable at the bottom, which is fairly harmless. Yes, oiling the cable on the way in is a good idea.

FWIW with greases, they tend to need an amount of shear force required before there is any movement in the cable; this varies with the type of grease but there is almost always some stickiness rather than none.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
spinachcharm
Posts: 6
Joined: 24 Feb 2019, 3:46pm

Re: Shimano Nexus 7; how it works. (long)

Post by spinachcharm »

so oil better than grease then?
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Shimano Nexus 7; how it works. (long)

Post by Brucey »

spinachcharm wrote:so oil better than grease then?


usually, yes.

I will happily use a thin smear of certain greases with certain liners, but with a random/unknown liner, cable, and grease on offer, oil is a safer bet.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
spinachcharm
Posts: 6
Joined: 24 Feb 2019, 3:46pm

Re: Shimano Nexus 7; how it works. (long)

Post by spinachcharm »

great thanks Brucey. And sorry one last question, I don't understand why it's stiff changing up but easy changing down. If it was lack of lube it would surely be the same? Or is there a torque in the system that pulls toward the lower gears?
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Shimano Nexus 7; how it works. (long)

Post by Brucey »

in this case the whole system is spring loaded towards low gears. Without cable friction, the force you feel would be the same going both ways (which is more or less the case if you work the cassette joint pulley by hand). With cable friction however, cable tension begats more friction, and more friction necessitates more cable tension to overcome the friction, and so forth. This means that upshifts are usually a lot stiffer than downshifts, once the cable goes bad.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
spinachcharm
Posts: 6
Joined: 24 Feb 2019, 3:46pm

Re: Shimano Nexus 7; how it works. (long)

Post by spinachcharm »

Ah that makes sense. (Sorry, thought I'd replied to this. Thanks so much for your help).
Whizz
Posts: 5
Joined: 18 Apr 2019, 8:03pm

Re: Shimano Nexus 7; how it works. (long)

Post by Whizz »

Sorry to bump this old thread, but I think it is good to keep the information in one place.

It looks like last year or a bit earlier Shimano introduced a new version of Nexus 7 - the SG-C3001-* series. They make claims such as "internal gear engagement that allows for a smoother ride", "smooth shifting performance", "improved performance for both E-BIKE and non-E-BIKE usage".

The part lists for all 4 models can be found here:
https://si.shimano.com/#/en/EV/SG-C3001-7C
https://si.shimano.com/#/en/EV/SG-C3001-7C-DX
https://si.shimano.com/#/en/EV/SG-C3001-7D
https://si.shimano.com/#/en/EV/SG-C3001-7R

A few parts have been changed in comparison to the SG-C3000-* series including sun gears and one of the carrier units. I wonder what kind of improvement (if any) there really is. Also, there are two models with a coaster brake: a "deluxe" (-DX) one and a normal one. I couldn't find any good explanation of the differences except for the fact that the DX model is available with 184 mm axle.

I have a bike with the SG-C3001-7D hub and I plan to service it with the Land Rover SFG in a few months. I can take pictures of the internals, but I do not plan to do any deep disassembly of them.
zeluzel
Posts: 28
Joined: 27 Jan 2020, 11:38am
Location: Wrocław, PL

Re: Shimano Nexus 7; how it works. (long)

Post by zeluzel »

Whizz wrote:there are two models with a coaster brake: a "deluxe" (-DX) one and a normal one. I couldn't find any good explanation of the differences except for the fact that the DX model is available with 184 mm axle.


It's about corrosion resistance. Techdoc for inter 3 says "The DX version features a clear coated finish and/or stainless steel material". I bet it's only about hubshell.

Whizz wrote:I have a bike with the SG-C3001-7D hub and I plan to service it with the Land Rover SFG in a few months. I can take pictures of the internals, but I do not plan to do any deep disassembly of them.


Please do!
Post Reply