Shimano Nexus 8 internal drag puzzle

bob prohaska

unread,

Mar 18, 2022, 10:34:13 PM3/18/22

to

I'm trying to sort out a problem of drag between sprocket and hub
on a Shimano Nexus 8 red band hub, marked SG-8R36. When coasting
the chain and pedals follow the hub quite strongly, with consequent
drag when not pedaling.

At first I thought the problem was a bent shield caused by having a
bungee cord wrap between sprocket and hub in a moment of inattention,
but removing the shield (which is clearly bent) didn't help. It looks
as if the hub is lubricated very generously with white grease.

A second bike with a Nexus red band hub marked SG-8R25 has much
less drag when coasting. The chain and crank don't turn with the
hub when coasting. It's lubricated with oil.

Might the grease be the source of the extra drag?

Thanks for reading,

bob prohaska

John B.

unread,

Mar 18, 2022, 11:29:17 PM3/18/22

to

I don't know whether it will help but Sheldon had a page on
internal-gear hubs
https://sheldonbrown.com/internal-gears.html#lubrication
--
Cheers,

John B.

Sepp Ruf

unread,

Mar 19, 2022, 4:31:47 AM3/19/22

to

I doubt it. Have you held the wheel freely by itself to check if the
drag is a constant, or a cyclical one?

"Loosen the axle nuts and re-check the freewheeling ability.
Recommend torque is 3-4.5kNcm"
<https://web.archive.org/web/20130925011325/http://www.shimano.com/publish/content/global_cycle/en/us/index/tech_support/tech_tips.download.-Par50lparsys-0018-downloadFile.html/11%29%208%20Speed%20Trouble%20Shooting%20Guide.pdf>

Shimano factory-greased all 8sp gear hubs, more or less. Is the -25's
oil from a service lubrication?
<https://www.2rad.nrw/project/shimano-nexus-8-gang-nabe-demontage/>
For lack of oil spray or reservoir, even after servicing 8sp gear units
in Shimano oil (or GL4 gear oil), one does still regrease their bearings.

AMuzi

unread,

Mar 19, 2022, 9:00:13 AM3/19/22

to

How old is the gearbox or how long since rebuild? Yes,
grease or any lubricant degrades over time. Possibly your cause.

I'd look at bearing adjustment first. With axle flats in a
vise, adjust to slight play then just take up that freeplay,
no tighter. Locknut torque 25~30nm. If there's no change in
drag then clean/rebuild/relubricate.

Contamination can cause some dramatic failures in those -
parts are smallish and there's little room for crud to be
pushed aside:

http://www.yellowjersey.org/photosfromthepast/alf8rust.jpg
http://www.yellowjersey.org/photosfromthepast/nx7brokn.jpg
http://www.astounding.org.uk/ian/hubgear/mangled.html
http://www.yellowjersey.org/failnx16.jpg

--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971

Andre Jute

unread,

Mar 19, 2022, 9:23:36 AM3/19/22

to

On Saturday, March 19, 2022 at 1:00:13 PM UTC, AMuzi wrote:
"Those of a mechanically sensitive nature should probably look no further..."
>
Would you mind putting some space between your warning and those sickening photographs of man's cruelty to inoffensive hub gearboxes? -- AJ

AMuzi

unread,

Mar 19, 2022, 10:06:50 AM3/19/22

to

meh.
Not any more ugly than a rod through an engine block:

https://toyota-club.net/files/faq/16-05-30_faq_3s-bolt/x11.jpg

Tom Kunich

unread,

Mar 19, 2022, 10:55:27 AM3/19/22

to

I agree, the first thing I would be suspicious of is that the axle nut is far overtightened.

Frank Krygowski

unread,

Mar 19, 2022, 1:13:49 PM3/19/22

to

On 3/19/2022 10:06 AM, AMuzi wrote:
> On 3/19/2022 8:23 AM, Andre Jute wrote:
>> On Saturday, March 19, 2022 at 1:00:13 PM UTC, AMuzi wrote:
>> "Those of a mechanically sensitive nature should probably look no
>> further..."
>>>
>> Would you mind putting some space between your warning and those
>> sickening photographs of man's cruelty to inoffensive hub gearboxes?
>> -- AJ
>>
>
> meh.
> Not any more ugly than a rod through an engine block:
>
> https://toyota-club.net/files/faq/16-05-30_faq_3s-bolt/x11.jpg

Yep, I've seen that. One of the kids thought an old Triumph Spitfire
would be fun. Instead, it became a mechanical education.

Quote: "I've changed an engine! Now I'm a real mechanic!"

(Um, no you're not...)

--
- Frank Krygowski

Frank Krygowski

unread,

Mar 19, 2022, 1:24:55 PM3/19/22

to

I'm wondering hoe much drag and if it's really abnormal. Long story follows:

I've worked on several classic Sturmey-Archer 3 speed hubs. But a few
years ago I was building up a kids bike (restoring an antique) and I had
no SA to install, so I used an old Shimano 3 speed hub I'd been given.
The hub felt a little draggy to me compared to a SA, so I ended up
dismantling it at least twice, but found no problems.

Eventually I decided to measure the drag, using a spring scale, etc. It
turned out to be just a couple Watts, and was likely "normal." A little
kid would never notice it. That bike has now been passed down to a
second kid and is still running great. (Well, the Shimano is touchier
about adjustment than a Sturmey Archer, but it's OK.)

So overall, make sure it's really a problem before making it a huge
project.

--
- Frank Krygowski

bob prohaska

unread,

Mar 19, 2022, 4:00:41 PM3/19/22

to

Sepp Ruf <inq...@safe-mail.net> wrote:
>
> Shimano factory-greased all 8sp gear hubs, more or less. Is the -25's
> oil from a service lubrication?
> <https://www.2rad.nrw/project/shimano-nexus-8-gang-nabe-demontage/>
> For lack of oil spray or reservoir, even after servicing 8sp gear units
> in Shimano oil (or GL4 gear oil), one does still regrease their bearings.

The -25 got a few squirts of GL-4 past one of the bearings when it seemed
a bit reluctant to shift. There was only a trace of grease near the dust
covers and it never had trouble freewheeling.

Thanks for replying!

bob prohaska

bob prohaska

unread,

Mar 19, 2022, 4:18:27 PM3/19/22

to

AMuzi <a...@yellowjersey.org> wrote:
>
> How old is the gearbox or how long since rebuild? Yes,
> grease or any lubricant degrades over time. Possibly your cause.
>

No idea. The bike has an LED headlight, so it's newer than my version.
Maybe 2013-ish? Looks like it's been used very little.

> I'd look at bearing adjustment first. With axle flats in a
> vise, adjust to slight play then just take up that freeplay,
> no tighter. Locknut torque 25~30nm. If there's no change in
> drag then clean/rebuild/relubricate.
>

There was grease oozing under the dust covers, so it clearly
wasn't under-lubricated. When I loosened up the cone to add
a bit of oil it did feel like the adjustment might be somewhat
tight. I added maybe a teaspoon of GL-4 through the bearing
opening and reassembled with the cone only finger tight before
setting the locknut. The idea was simply to thin the grease a bit.

On reassembly it rides noticeably better. Whether it's relaxing
the bearing preload or thinning the grease is unclear, but that
doesn't really matter. For the little use it's apt to get I'll
declare victory and retreat 8-)

One curious artifact is the troublesome hub freewheels silently.
No clickety-clack, at all. The "good" hub freewheels with all the
usual sounds.

Thanks for writing!

bob prohaska

Andre Jute

unread,

Mar 19, 2022, 10:21:24 PM3/19/22

to

I had a V12 engine out of an Jaguar XJ12 black-cromed and a sheet of armored glass bolted to it, and used it as a coffee table. It wasn't worth rebuilding because some idiot had bored a 5.3 liter block to way over 7 liters after everyone else already knew that seven liters even was the absolute limit for those engines. -- AJ