Wednesday, December 21, 2011

Sturmey Archer GH6 Dynohub magnetic keeper ring

The Sturmey Archer Dynohub 'Keeper Ring' has become the stuff of legends. Frequently mentioned in discussions about servicing the Dynohub but never described, it is as rare as hens teeth.  It is, apparently, essential to maintain the GH6 ring magnet's field strength during circumstances in which the armature needs to removed.   A google search did turn up one on auction back in February, which even included a photograph:

The only photograph of a Dynohub keeper ring I could find on the internet

Update: just after posting this I found a wealth of information on the keeper ring in this thread on bike forums, including dimensions and part number!

Other than infrequently turning up on eBay, they are unavailable. Gentleman Cyclist does, however, mention on their parts page that they will soon have keeper rings for the Dynohub (you should take a look at the other interesting bits and bobs they carry for the 3-speed enthusiast).

Well, I had an old Dynohub from '75 that was rusty as all heck. It came from a trashed ladies Raleigh Superbe that was languishing in the basement of Polly's Recycle in the east end.  The armature could barely spin within the ring magnet and the whole assembly was covered in a coat of rusty grit.  Usually a Dynohub can be serviced without the need for removing the armature from the ring magnet, but in this particular hub's case, the armature need a good going over with a wire brush.  So, I had my Internet Material Synthesizer make one out of mild steel #45.  Now, I think my Dynohub magnet upgrade makes the original magnet kind of obsolete, but I was getting a bunch of other parts made so I thought I'd get a keeper ring made as well.  It arrived today:

Machined Dynohub Keeper Ring

It has a 2.700" outer diameter, the same as the armature, and is 0.75" thick.

Dynohub Alnico ring magnet with armature.  Don't remove that magnet without a keeper!

Dynohub ring magnet with keeper

Does it work? I don't know.  I pushed the armature out with the keeper, so the field wasn't disrupted.  The magnet feels about as strong as it did before the armature was removed.  The design is very similar to the original, so I expect it's doing its job.

Do you want one?  I can easily get more made.  They would cost between $15-$30 each, depending on number I get made in one go.  I'm also thinking of offering a magnet upgrade for the Dynohub (this is still a work in progress though...)

If you're interested send me an email (find the address in my profile).


MT cyclist said...

Great idea. I hope it works.
I, too, have a GH6 that came with a 1970 Raleigh Twenty Shopper that I bought off of ebay a couple years ago. It was completely seized up, and full of rusty gunk. But after taking out the spindle and spraying down the magnet and armature with WD-40, the whole thing started to turn ever so slightly. I kept cranking on on it and cleaning out the gunk, and pretty soon the armature started spinning inside the magnet. I cleaned the rust out of the shell, replaced the bearings, greased it up, adjusted the cones and lock nuts and hooked up a bare bulb with some wire. Boy was I thrilled when it actually started generating electricity. The hub was still missing an axle nut, and I couldn't find the right size anywhere. Then I thought about the trashed rear wheel off of my wife's Raleigh Sprite. Sure enough, the two axle nuts were the right size, 3/8, not the 13/32 size that comes on most Sturmey Archer hubs. My next step will be to build the hub into a new wheel.
Pictures of my dynohub in pieces are in the gallery at the Raleigh Twenty blog:

Anonymous said...


Great sight and interesting lights. I would be interested to know if your keeper ring worked. I have access to a machine shop work at my place of employment and looking at your ring was planning to make one myself.

I recently used Reflectalight LED's in some 50's style Sturmey Archer Lamps, they should be putting up my review soon,a pre-view image can be seen here:

Thank you.

James (

Anonymous said...

I am currently trying to service an SA GH6 from a ladies superbe. The Hub spins fine, but the armature spins with the rest of the hub. I suppose it is seized to somehow and I'm a little hesitant to whack it out with a mallet as outlined in the SA literature on servicing this hub. I don't really understand what keeps the armature stationary while the rest of the wheel spins. Should it be fastened to the axel with a locknut to prevent it's turning. May be missing a couple of spacers between the locknut and the cone, but I don't see what affect that would have on isolating the armature. Would you please explain how this works for me.

Perhaps I will try some WD40. See if that frees up the hub and lets the armature and magnet turn independently. Please help if you have any advice or insight.