Nexus 8 hub feels stiff

For discussions about bikes and equipment.
Stroud Active
Posts: 8
Joined: 11 Jul 2020, 3:53pm

Nexus 8 hub feels stiff

Postby Stroud Active » 10 Jan 2021, 8:45pm

I've got 2 Orbea Katus - one with a 5 speed Sturmey Archer hub, the other with a Shimano Nexus 8 speed hub. The 8 speed bike is the better bike (disc brakes and hub dynamo) which I bought after liking the 5 speed bike so much.

I've found the effort to ride the 8 speed bike seemed to have gone up recently. I did the classic test of spinning the back wheel round and found that it did come to a stop far quicker than I would have expected. I compared the two bikes and the 5 speed is smooth when the cranks are rotated backwards by hand but the 8 speed seems to have far more resistance. Rotating the sprocket by hand (with the chain removed) then it feels a bit notchy and not as smooth as I would expect.

The hub came on a nearly new bike so I doubt it has covered many miles in the year or two since manufacture.

I think there's something wrong with the hub. Are there any known issues? Someone told me to adjust the bearing cone preload. I could do that but I'm minded to hand the whole hub over to a local bike shop and ask for an opinion.

Brucey
Posts: 42102
Joined: 4 Jan 2012, 6:25pm

Re: Nexus 8 hub feels stiff

Postby Brucey » 10 Jan 2021, 9:15pm

Nexus 8 is grease-lubricated and after a couple of years (regardless of mileage) water can have got into the seals/bearings and/or the grease will have dried out. Either can and does make the seals on the RHS or the roller clutches inside the hub somewhat draggy.

Usually a good first step is to remove the LH cone only, tip about 30cc of gear oil into the hub, then put it back together. This oil will wet the seals and turn dried grease into something that is less like cheese and more like a lubricant.

It is (IME) highly unlikely that bearing adjustment will be at fault so as to be the sole cause the problems you are seeing. Note also that nexus 8 hubs don't allow such free freewheeling as SA hubs do; the roller clutches inside Nexus 8 hubs see to that. It is possible that your hub is 'perfectly normal; 'normal' for a N8 hub, that is. It isn't a bad idea to compare your N8 hub with others. Note that shimano roller brakes can be slightly draggy too.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

ElCani
Posts: 335
Joined: 5 Mar 2015, 11:24am

Re: Nexus 8 hub feels stiff

Postby ElCani » 10 Jan 2021, 10:14pm

My Alfine 8 speed IGH is similar, and it has the bearings adjusted correctly and is well lubricated with Land Rover front swivel SFG. None of it feels particularly smooth or 'free' on the work stand, but in use it's fine.

Stroud Active
Posts: 8
Joined: 11 Jul 2020, 3:53pm

Re: Nexus 8 hub feels stiff

Postby Stroud Active » 11 Jan 2021, 11:57am

Thanks for the info. I've been riding my 5 speed bike this weekend and it feels faster/easier to pedal to me. It did cross my mind that basic cleaning and lubrication of the seals might bring some benefit. I'll take a look at it.

I'll try to do some coast down timings on a flat road to see if it is measurable or just a suspicion.

Brucey
Posts: 42102
Joined: 4 Jan 2012, 6:25pm

Re: Nexus 8 hub feels stiff

Postby Brucey » 11 Jan 2021, 1:18pm

SA 5s hubs are not noted for being super-efficient in drive (except in the middle gear) but they are not that bad by comparison with some other hubs and (like all SA hubs) they do coast well.

Nexus 8 hubs have draggy roller clutches, draggy roller brakes, potentially draggy seals too. So at best they don't ever coast as well as a SA hub. In addition to that they are (except for 'premium' versions with roller bearings on the planet pinions) inherently lossy in drive, most noticeably in 4th gear.

One further observation/complaint is that the roller clutches inside a Nexus 8 hub (as well as Alfine hubs) impart a noticeably 'spongy' feeling to the drive. The Nexus 7 is not an efficient hub either (using two gear trains in series in the most-used gears) but it arguably feels nicer to ride because it has pawls (like an SA hub) rather than roller clutches, which feels a lot nicer when you are pedalling. Test data for bicycle transmissions is nearly always generated under constant torque, which means that losses from 'spongy' transmissions (roller clutches, belt drives etc) are not measured, and are in addition to whatever losses are measured.

IMHO in a nexus 8 hub, the parasitic losses in seal drag etc are small by comparison to the amount of effort that is lost between your legs and the wheel because of transmission inefficiency. IIRC standard nexus 8 hubs lose about 25% in 4th gear, and that doesn't allow for sponginess. They are better in 5th gear but there are still spongy roller clutches in the drive path.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Stroud Active
Posts: 8
Joined: 11 Jul 2020, 3:53pm

Re: Nexus 8 hub feels stiff

Postby Stroud Active » 11 Jan 2021, 8:24pm

I've noticed that the lowest gear is quite rough and lossy on both hubs. My opinion of the SA hub has gone up since this happened.

I took the left hand cone off this evening and poured about 50ml of gear oil into the hub. That seemed to make turning it by hand a lot easier and a quick ride suggested some improvement. I did a rough and ready coast down test on the road with both bikes and they were identical.

Essentially you seem to be saying that hub gears are a bit lossy. They're quite heavy too. Having said that, I was quite surprised how fast my Katu 40 (5 speed SA) was when I first got it so it can't be that bad. Hopefully I'll come to appreciate the benefits of them being sealed with time, though I'm starting to rediscover my love of derailleurs.

Brucey
Posts: 42102
Joined: 4 Jan 2012, 6:25pm

Re: Nexus 8 hub feels stiff

Postby Brucey » 11 Jan 2021, 8:45pm

I guess I have used IGHs for at least six-figure mileages so I suppose I must like them. However if derailleur gears offered the same low maintenance overhead, I'd have used them more than I have done (I own and ride bikes with derailleurs too). No such thing as a 'perfect bike', mainly just horses for courses.

FWIW I'd rate variations in tyres more significant than variations in transmission are likely to be.

SA gears (eventually) 'run in' to some extent, but even in 5s hubs which have seen high mileages, you can usually still feel the difference in bottom gear.

BTW all IGHs are lossy in some gear ratios. However most (but not all) also have efficient direct drive ratios too. So gear 3 in a SA 5s hub and gear 5 in a Nexus 8 are considerably efficient than the other ratios. If your riding style/terrain allows you to mainly use those gears, then the fact that there are also lossy ratios in the gearbox may have little overall impact. So for example if you use one gear 90% of the time and another gear just 1% of the time, then the 1% gear ratio would have to be about 90 times as lossy in order to have the same overall impact on your total losses.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Stroud Active
Posts: 8
Joined: 11 Jul 2020, 3:53pm

Re: Nexus 8 hub feels stiff

Postby Stroud Active » 12 Jan 2021, 1:30pm

I've read how they work now: https://www.sheldonbrown.com/internal-gear-theory.html
Very interesting it is too. I'm surprised at how cheap the gearboxes are.

The unity ratio is usually a direct link from input to output so minimal losses. Some of the Shimano units go down and back up to give ratios near unity so they're pretty lossy. I've been happy with the bike's performance in general and I bought a hub geared bike so I could find what they're like.

Brucey
Posts: 42102
Joined: 4 Jan 2012, 6:25pm

Re: Nexus 8 hub feels stiff

Postby Brucey » 12 Jan 2021, 2:30pm

'gearing down and back up again' is precisely how 4th gear works in a nexus 8 hub, hence my earlier comments about efficiency.

This leads to the most efficient gear (#5) being adjacent to the least efficient gear (#4). The difference in efficiency in a standard ( i.e. non-premium) hub is comparable to the difference in gear ratio. The result of this can be that you change down to fourth and there is no difference in the effort required, but you are mysteriously going ~20% slower than you were before.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Stroud Active
Posts: 8
Joined: 11 Jul 2020, 3:53pm

Re: Nexus 8 hub feels stiff

Postby Stroud Active » 12 Jan 2021, 9:04pm

Tipping oil in seems to have worked. I have regained a few km/h that seemed to have gone missing even though I was pedalling hard. I'm surprised how cheap the hubs are considering their internal complexity.