Sturmey archer - dished vs flat sprocket

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Stroud Active
Posts: 8
Joined: 11 Jul 2020, 3:53pm

Sturmey archer - dished vs flat sprocket

Postby Stroud Active » 12 Jan 2021, 1:08pm

I want to gear down my 5 speed SA hub to give the same bottom ratio as mu Nexus 8 hub.
The hub ratios are 0.64 vs 0.53 so I need a 20% reduction. The bike has an 18 tooth sprocket so I need to reach 21 or 22 teeth.

The bike has a flat 18T sprocket, CSIRW type. The part numbers are listed in https://www.sturmey-archer.com/files/ca ... RX-RK5.pdf

The spec shows a 1.2 mm offset due to dished sprockets. https://www.sturmey-archer.com/files/ca ... %20C50.pdf

I've looked and the flat 22 T sprockets are not readily available online but there are loads of the dished version. Will it matter if I use a dished sprocket? On a derailleur bike the chain runs at an angle most of the time.

I've done some searching and it appears that this is not a new question! I need to check to see if there are spacers on the existing hub then. https://www.bikeforums.net/bicycle-mech ... stion.html

Brucey
Posts: 42208
Joined: 4 Jan 2012, 6:25pm

Re: Sturmey archer - dished vs flat sprocket

Postby Brucey » 12 Jan 2021, 1:43pm

there won't be sprocket spacers on a modern SA hub.

You should check the chainline anyway (it often isn't correct) and act accordingly. Often you can adjust the wheel spacing between the dropouts to improve the chainline, and if necessary adjust the wheel dish to suit.

RX series 5s hubs use a different sprocket fitting from other SA hubs; some available here

https://www.sjscycles.co.uk/sprockets-hub-gear/?type=sturmey+rotary+5spd

If the chain is bushingless (most are these days, even 1/8") it will usually run OK even if the chainline is a mm or two out. Best to sort it out in the long run though.

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

Stroud Active
Posts: 8
Joined: 11 Jul 2020, 3:53pm

Re: Sturmey archer - dished vs flat sprocket

Postby Stroud Active » 12 Jan 2021, 9:06pm

SJC can't supply the correct sprocket at the moment so I'll buy a dished one off Ebay and see what happens.