Modifying 1/8" Sturmey Archer sprockets for use with derailleurs

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SRV
Posts: 9
Joined: 3 Jul 2019, 2:21pm

Modifying 1/8" Sturmey Archer sprockets for use with derailleurs

Postby SRV » 3 Jul 2019, 2:52pm

This is my first post. I have just finished rebuilding an old Claud Butler Challenge. It now has an AW hub with a 21 and a 24 tooth 1/8" SA sprocket and a Huret Svelto derailleur.

I have two problems: the first being the reluctance of the chain to grab the larger sprocket and the second being it's propensity, once grabbed, to override the sprocket and go into the spokes.

No matter how I adjust the stop screw I cannot hit the sweet spot where the chain engages but does not overshoot. I have found no forums that cover the topic of sprocket modification.

I've tried a bit of grinding but to no avail. Can anyone help?

Brucey
Posts: 39367
Joined: 4 Jan 2012, 6:25pm

Re: Modifying 1/8" Sturmey Archer sprockets for use with derailleurs

Postby Brucey » 3 Jul 2019, 9:19pm

the conversion you are using is normally carried out using 3/32" sprockets (also available from SA/SRAM/Shimano) not 1/8" ones. Provided they are spaced correctly you can still use 1/8" chain on 3/32" sprockets if you like. There are a few fundamental issues with what you are trying to do

1) the derailleur does not place the guide pulley super-close to the sprockets
2) the chain is not designed for derailleur use (although it might have some derailleur chain parts in it; this varies with chain model)
3) the svelto mech usually has a smooth-faced (rather than toothed) guide pulley
4) many modern 1/8" chains are built bushingless and don't have inner side plates that stick up high enough for a smooth guide pulley to gain any traction on
5) a chain of any kind needs a fair amount of lateral clearance before a chain will derail on and off it easily.
6) the sprockets you are using lack any kind of ramps or gates.

Not that they are available in the right tooth counts for you, but it is worth looking at the SRAM sprockets that were made for 6s bromptons; these show how you need to modify a standard IGH sprocket so that a chain will derail on and off it easily. These features are not difficult to replicate on standard sprockets with a little grinding.

Image

What isn't so clear is that any 3/32" sprocket meant for derailleur use is normally about 2mm thickness, i.e. about 0.2mm narrower than a 3/32" sprocket meant for singlespeed use.

cheers
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SRV
Posts: 9
Joined: 3 Jul 2019, 2:21pm

Re: Modifying 1/8" Sturmey Archer sprockets for use with derailleurs

Postby SRV » 3 Jul 2019, 11:43pm

Thanks Brucey.

That was very interesting. I was considering going down the 3/32" route but believed that it was possible with 1/8". The facts that two dished SA sprockets back to back give an almost perfect clearance AND need no spacers and the flexibility and narrowness of the SRAM Pc-1 chain, were too tempting to pass up!

I understand the unsuitability of both sprockets and chain for this kind of duty but I had understood that it had been done before back in the mists of time.

I shall persevere for a bit longer before I adopt 3/32". The mark of a wise man is knowing when to give up. Wisdom is, however, a quality with which I am, sadly, underendowed.

Brucey
Posts: 39367
Joined: 4 Jan 2012, 6:25pm

Re: Modifying 1/8" Sturmey Archer sprockets for use with derailleurs

Postby Brucey » 4 Jul 2019, 12:20am

needless to say it isn't difficult to grind the teeth a bit thinner on a 1/8" sprocket.

SRAM PC-1 chain is bushingless but has inner side plates without cambers. IIRC most KMC 1/8" chains are bushingless but have cambered inner side plates; this means they behave differently on sprockets; I would imagine that they might better find their way onto the sprocket teeth.

I'd certainly try a different guide pulley/rear mech too.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~