Sturmey Archer Rear Sprockets.

[quiksilver]

Does anyone know why the larger sprockets are dished and the smaller not? Is it something to do with the larger ones flexing under load? Thanks

[LWaB]

The smallest sprockets don't have enough room between the hub and chain to allow the sprocket to be dished without overstressing the metal (cracking/tearing) when being formed, is my guess.

[MartinC]

Only the larger sprockets will clear the hub body if they're dished and facing inwards.

[Booty]

I have to admit to not knowing why the bigger Sturmey sprocket are dished. I agree that it would be difficult to dish smaller sprockets but that just begs the question why dish any sprocket; dishing seems to be an unnecessary manufacturing process adding cost. Hub clearance doesn't answer the question as all sprockets could be flat thereby maintaining a common chain-line. It isn't due to possible flexure as a correctly aligned chain put no side force on a flat sprocket, the dish increases side force. My guess is that it is to provide sprocket/chain clearance to the rear frame stays with the offset turned inwards - which obviously impacts the chain line. In the past I hand made flat Sturmey fit sprockets up to 32t for use on 'special' application bikes with no problem under extreme riding conditions.

[stevew]

Only the larger sprockets will clear the hub body if they're dished and facing inwards.

That's what I think too !

[MartinC]

all sprockets could be flat thereby maintaining a common chain-line

I've always assumed that they were dished to allow you a choice of 3 chainlines - dished in, flat or dished out - to match the chainset/BB you've got. The chainline on gear hubs varies with make and the number of speeds.

[hubgearfreak]

Only the larger sprockets will clear the hub body if they're dished and facing inwards.

That's what I think too !

almost. it's the chain that would foul the ball ring's dust cover if a small sprocket was dished inwards

[stevew]

I know it's an old thread but where can i get a flat 19 & 20 tooth one?

[Brucey]

might I enquire why you want a flat sprocket particularly? Also does it need to run 1/8" or 3/32" chain?

cheers

[stevew]

Thinking of changing the gearing on the Automatix on the Upstart E bike and it comes with a 18T flat so i would like to keep the same chain line. OTOH i'm sure the offset wont be too much of a prob. Nice to keep it 'right' though
3/32 chain i think (still haven't got the bike here yet)...........but that would work for either.

[Brucey]

really useful bikes have 20T flat 3/32" sprockets I think. They are a fair bit more cash than dished ones bought elsewhere; I'd suggest checking the present chainline carefully (who knows, it might improve with a dished sprocket...), or considering respacing the rear wheel a bit.

When time comes for a new chain I'd suggest getting a 1/8" fully bushed one, like a KMC B1; these last pretty well.

cheers