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Old 08-18-11, 05:00 PM   #1
Airburst
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Sturmey-Archer dual-sprocket?

Is there any way to retrofit the dual sprocket setup seen on Brompton bikes to a standard Sturmey-Archer 3-speed? Does it require a new driver, or is there some sort of double sprocket assembly that attaches to the standard 3-spline one?

Also, how does the Brompton derailleur attach to the frame? Is it a standard hanger? If not, is there any reason a standard derailleur wouldn't work to shift between the two sprockets, if the limit screws were set right?

As you might imagine, I'm trying to turn a 3-speed into a 6-speed, just to see if I can. Any help from the various Sturmey-Archer gurus on here would be appreciated

Thanks in advance
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Old 08-18-11, 06:12 PM   #2
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Is there any way to retrofit the dual sprocket setup seen on Brompton bikes to a standard Sturmey-Archer 3-speed? Does it require a new driver, or is there some sort of double sprocket assembly that attaches to the standard 3-spline one?
the UK & TW hubs may not be able to cross breed in some ways.
the driver sub assembly is different, you would have to tear the matching type hub apart,
and swap that part..
probably want the longer hub's axle if not a very narrow frame rear spacing.

newer BWR use 1 kind of sprocket. I think the Shimano BMX cog is the new spline type.

prior were sourced from Sachs, and use 3 lobe spline sprockets ,
common to 3 speeds of many brands.
AFAIK, very few cog sizes made in 3/32 type thickness, most 1/8"
Quote:
Also, how does the Brompton derailleur attach to the frame? Is it a standard hanger?
No, they are completely unique, shifts by having a fork shove the pulley sideways.
there is a boss for that pivot, under the chainstay tube

You can also make a 3 into 6 speed by keeping 1 cog on the hub, a thin one,
so a 5.6.7 speed chain fits
and running a 2 chainring crank., and a front derailleur..

Rear derailleur/non shifting chain-tensioner takes up the slack when on the small chainring..

Seems a S3X will screw on a White industries Dos freewheel on,
and thus offer a 2 speed external pair , that way..

Last edited by fietsbob; 08-18-11 at 06:21 PM.
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Old 08-18-11, 06:20 PM   #3
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Is there any way to retrofit the dual sprocket setup seen on Brompton bikes to a standard Sturmey-Archer 3-speed? Does it require a new driver, or is there some sort of double sprocket assembly that attaches to the standard 3-spline one?
This is pretty high on the effort scale but I made one by brazing a cog taken from a multispeed freewheel to a standard gearhub cog. It works but if I were doing it again I'd be more careful about getting the cogs exactly centered (chain tension is uneven on the big cog.)


P9070043 by Saccade, on Flickr


P9070047 by Saccade, on Flickr
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Old 08-18-11, 06:24 PM   #4
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yea a chain tensioner would make up for 2nd cog not being quite on center..
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Old 08-18-11, 07:56 PM   #5
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Sturmey-Archer cogs are slightly dished. An old trick was to take two cogs with the concavities facing in opposite directions to run a two-cog set-up on a standard SA hub.
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Old 08-19-11, 10:39 AM   #6
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just !/8" chain does not make the best derailleur chain,
and most S-A cogs are for 1/8" .

other than that ..

Let us know how it comes out.

Last edited by fietsbob; 08-19-11 at 10:43 AM.
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Old 08-19-11, 11:34 AM   #7
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Sturmey-Archer cogs are slightly dished. An old trick was to take two cogs with the concavities facing in opposite directions to run a two-cog set-up on a standard SA hub.
Yep, riders have been doing this for over 75 years. Common half step cog pairs were 19-22 and 18-21.
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Old 08-19-11, 11:43 AM   #8
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Thanks guys! I've looked at SJS cycles' website, they have a selection of 3/32" SA sprockets, so I might go with a pair of those, as I can't seem to find the Brompton driver on it despite them selling Bromptons and spare SA parts. I'll just use an old rear mech to do the shifting, and I'll let you know how it turns out.
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Old 08-19-11, 12:09 PM   #9
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I did it with an old Sturmey-Archer AW hub- I don't know if the hub body is different now or not. I took a dremel grinder to the inner splines of a couple of 7-speed Shimano hyperglide cogs. I ground the 9 square splines down to 3 half-moon shaped splines, then with a 7-speed spacer, the cogs fit perfectly onto the AW hub body. Add a derailleur and voila!





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Old 08-19-11, 12:21 PM   #10
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Very clever, cycle_maven!
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Old 08-19-11, 01:38 PM   #11
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AH, I see ..
http://www.sjscycles.co.uk/sturmey-a...778-prod19337/

andhttp://www.sjscycles.co.uk/sturmey-a...858-prod19354/
would cover that..
theres a 3/32 17t too, but thats It. .. 3 out of 30.

SA has a US distributor in Cal, since they are made on the Pacific rim now.
so might be available thru the retail food chain..

Last edited by fietsbob; 08-19-11 at 01:46 PM.
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Old 08-19-11, 08:16 PM   #12
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Originally Posted by cycle_maven View Post
I don't know if the hub body is different now or not.
The cog interface on the driver has been the same since the early fifties, when S-A went to the three-spline format.
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Old 08-20-11, 08:07 AM   #13
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AH, I see ..
http://www.sjscycles.co.uk/sturmey-a...778-prod19337/

andhttp://www.sjscycles.co.uk/sturmey-a...858-prod19354/
would cover that..
theres a 3/32 17t too, but thats It. .. 3 out of 30.

SA has a US distributor in Cal, since they are made on the Pacific rim now.
so might be available thru the retail food chain..
There are 3rd party cogs for 3/32" chain that will work with SA hubs. And as noted above, a minor surgical procedure will allow common Shimano cogs to work on SA hubs as well.
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Old 08-20-11, 08:38 AM   #14
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Yea, AW3 spline is a de facto standard.. common for many makers of 3 speeds..
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Old 08-28-11, 01:06 PM   #15
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OK, it took me a little while, but I've done it!

Here are a few pics



The shift lever is a cheap plastic Shimano friction one, it's aleft-hand unit and it's mounted underneath my left brake lever. The derailleur is the Huret one that the bike had when it was given to me, which went in my shed when I switched to IGH.

Sorry about the bad-quality pics, if anyone wants better ones I can borrow a better camera.
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Old 08-29-11, 02:47 AM   #16
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What do you do about the circlip? Doesn't the assembly have to be as thin as a 1/8 cog?

I had to come up with three of these for Bikezilla...

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Old 08-29-11, 06:10 AM   #17
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There was a spacer underneath the sprocket, which I removed. The total sprocket thickness is 2 x 3/32", which is 3/16", or 1/16" thicker than a 1/8" cog. The spacer I removed was just under 1/16" thick, so the overall thickness is slightly greater than it was to begin with, but circlip just barely fits on.
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