Internal gear hub, slipping sprocket, replacement?

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mjr
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Internal gear hub, slipping sprocket, replacement?

Postby mjr » 8 Apr 2017, 9:58pm

So... I was using a Sturmey Archer sprocket on my Nexus 3 coaster brake hub (SG-3C41) and it's started slipping. Reading other posts on here, I wonder if it's because I've got it dished outwards and the lugs aren't engaging snugly, exacerbated by the backpedalling to brake. So...

Would a genuine Nexus 21T sprocket behave better? It looks like they might have slightly different lugs.

Or is there a source for flat 21T ⅛" sprockets?

Do I need a new snap ring too?

Thanks for any tips, as ever.

(Edited to correct part number after checking.)
Last edited by mjr on 10 Apr 2017, 9:45am, edited 1 time in total.
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Gattonero
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Re: Internal gear hub, slipping sprocket, replacement?

Postby Gattonero » 9 Apr 2017, 10:24am

Put a ruler on the chainring and see where it ends on the sprocket, you want them to be on the same line.
IIRC, Sturmey does flat sprockets? Check if the teeth do math well with the chain, too, as I've see some 1/8" Sturmey sprockets not working well with chains like Sram PC1
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Re: Internal gear hub, slipping sprocket, replacement?

Postby mjr » 9 Apr 2017, 10:46am

I'm not sure if it was clear in my earlier post: the sprocket is slipping on the driver (visibly, with the bike on the stand and pushing pedals by hand). The chain's fine.
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Re: Internal gear hub, slipping sprocket, replacement?

Postby Gattonero » 9 Apr 2017, 10:49am

The sprocket is held by three round tabs.
How does do fit on the driver, is it a very loose fit? Can you engage the circlip snug over the sprocket, or there is still play? Or the circlip won't fully seat in the groove, perhaps because of a thick sprocket/spacer used?
If the sprocket is secured, it simply cannot slip on the driver, perhaps there is a problem with the pawls in the hub driver itself?
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fausto99
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Re: Internal gear hub, slipping sprocket, replacement?

Postby fausto99 » 9 Apr 2017, 4:37pm

mjr wrote:..the sprocket is slipping on the driver...


It can't, unless the sprocket tabs are not engaged in the driver grooves?!?

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Re: Internal gear hub, slipping sprocket, replacement?

Postby mjr » 9 Apr 2017, 6:03pm

Gattonero wrote:The sprocket is held by three round tabs.
How does do fit on the driver, is it a very loose fit? Can you engage the circlip snug over the sprocket, or there is still play? Or the circlip won't fully seat in the groove, perhaps because of a thick sprocket/spacer used?
If the sprocket is secured, it simply cannot slip on the driver, perhaps there is a problem with the pawls in the hub driver itself?

It's a snug fit horizontally but a loose fit rotationally.
The circlip is snug and there is no play in the direction of the clip.
The only spacer is the thin plastic one on the inside which I think is more of a shield.
Turning it by hand with the clip removed (see pictures) confirms that the sprocket lugs are not quite as wide as the grooves.
Strangely, the sprockets on the Shimano parts list for SG-3C41 are black while SG-3C41DX are silvery. Are the lugs on Shimano black sprockets slightly wider? Have I worn the grooves on my hub? How much should I worry about this? It feels flipping awful!
I tried two other sprockets (one SA 3/32" and one unbranded black that I suspect is an old SA compatible from 1990) with similar results in hand testing.


(Edited to correct part number after checking.)
Attachments
IMG_20170409_172306-2.jpg
Backpedal to brake and lug slips back
IMG_20170409_172315-1.jpg
Lug engaged
Last edited by mjr on 10 Apr 2017, 9:46am, edited 1 time in total.
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Re: Internal gear hub, slipping sprocket, replacement?

Postby Mick F » 9 Apr 2017, 6:14pm

I wonder if the sprocket is rotating through the circlip grove?
If the sprocket isn't snug on the driver, it could possibly rotate.
As stuff wears and then slips, it will wear more and slip more.

Is there a spacer behind the sprocket?
What me and my mate did back in the mid 60s, was to fit two different sized sprockets to our 3sp SA hubs. In order to get two on there, we had to remove the existing spacers and fit a thinner one between the two sprockets. We put a standard spacer on the flag stones, and ground it away under our shoe heels. Trial and error allowed the spacer to be the correct thickness to space the two sprockets correctly and allow the circlip to click into place firmly.

Maybe because yours is worn, you need a thinner spacer behind?
Mick F. Cornwall

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Re: Internal gear hub, slipping sprocket, replacement?

Postby mjr » 9 Apr 2017, 6:42pm

It only slips a few mm. It's definitely not fully in the clip groove. Just enough movement to feel when moving from braking to driving.

There's a thin disc behind the sprocket. I'm not sure if it's a spacer or merely a cover. Reassembling without it doesn't make the sprocket sit any more snugly.
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Re: Internal gear hub, slipping sprocket, replacement?

Postby thirdcrank » 9 Apr 2017, 7:17pm

You seem convinced from your visual inspection that the sprocket is slipping yet your pics show the three dogs (if that's the right word) to be properly located. Are you sure that it's not play somewhere else in the system? I've a Nexus 7 with all sort of waggle, but the sprocket is rock-solid on the hub.

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Re: Internal gear hub, slipping sprocket, replacement?

Postby mjr » 9 Apr 2017, 7:57pm

The backpedalling pic should show a small dark gap just above the lug on the lower right. I'm convinced the play I feel is the sprocket on the driver. I'm mainly unsure if it's wear, incorrect fitting somehow (but the clip really is engaged) or if Sturmey Archer sprockets aren't quite compatible with Shimano but it's close enough to work when there's no coaster brake.
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Re: Internal gear hub, slipping sprocket, replacement?

Postby squeaker » 10 Apr 2017, 9:53am

mjr wrote:The backpedalling pic should show a small dark gap just above the lug on the lower right. I'm convinced the play I feel is the sprocket on the driver. I'm mainly unsure if it's wear, incorrect fitting somehow (but the clip really is engaged) or if Sturmey Archer sprockets aren't quite compatible with Shimano but it's close enough to work when there's no coaster brake.
I can well believe how annoying that must feel through the pedals. Do you not have the Shimano sprocket that came with the hub to check the fit? If the driver is worn I would expect that wear to be visible if you removed the sprocket.
"42"

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Re: Internal gear hub, slipping sprocket, replacement?

Postby mjr » 10 Apr 2017, 10:01am

squeaker wrote:Do you not have the Shimano sprocket that came with the hub to check the fit? If the driver is worn I would expect that wear to be visible if you removed the sprocket.

I have a black sprocket that I believe may be the Shimano sprocket that came with the hub, but I have two hub-geared bikes, I've lowered the gearing on both and that sprocket is unbranded so I can't be absolutely sure although I also can't see any branding on pictures of replacement Shimano black sprockets. That sprocket also slips.

I couldn't see any wear in sunlight but I've just thought that I should check it with the inspection lamp after work today.
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Re: Internal gear hub, slipping sprocket, replacement?

Postby hamster » 10 Apr 2017, 11:33am

Stupid question I know, but are you sure it isn't just a worn chain?

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Re: Internal gear hub, slipping sprocket, replacement?

Postby mjr » 10 Apr 2017, 11:39am

hamster wrote:Stupid question I know, but are you sure it isn't just a worn chain?

There are no stupid questions, but I can slip the sprocket back and forth by hand with the chain off too.

Thanks for all the help exploring this, everyone!
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Re: Internal gear hub, slipping sprocket, replacement?

Postby hamster » 10 Apr 2017, 11:54am

Have a look here...near the bottom seems the same issue!
viewtopic.php?f=5&t=111354