Converting SA 3 peed to 6 speed

[bertgrower]

What are the barriers to converting SA 3 peed to 6 speed as per Brompton to be used on Dawes Kingpin 1970 version?

[Brucey]

none really; unlike later hubs a 1970s AW hub has a driver that is long enough to accept two 1/8" sprockets, or two 3/32" sprockets. You can fit a standard rear mech. Older rear mechs (hurets and the like) will accept a 1/8" chain.

Some care is needed with selecting the sprocket sizes; the ratio intervals in that hub are +33%, which means that you can

a) half-step the gears with a ~+15% interval in the sprocket sizes (say 17,20T),
http://www.gear-calculator.com/?GR=SAAW&KB=52&RZ=17,20&UF=1540&TF=90&SL=2.6&UN=MPH
or
b) 1-1/2 step the gears (eg 17,26T) or
c) have two sequential ranges (eg 16, 37T)

A small issue is that the SA sprockets are not designed to derail, so the shifting is a bit clunky.

The recent Brompton hubs use a different driver that uses a slightly different spline, and the sprockets are designed more for derailleur use so shift better. The BWR driver will also fit into later (NIG type) SA hubs, which otherwise struggle to support multiple sprockets, because they have a shorter driver spline than the older ones.

cheers

[speedsixdave]

A small issue is that the SA sprockets are not designed to derail, so the shifting is a bit clunky.

cheers

If you have an old Shimano cassette to hand you can file all but three of the inner splines off an individual Shimano cog and then shape the remaining three to slide onto the SA driver. They fit surprisingly well.

[Mick F]

Me and a mate fitted two cogs on our 3sp bikes back when we woz young teenagers circa 1966.
Can't remember my ratios.

I do know that I worked out - and I still remember the sequence even 50years later - on the big cog it was 1st, 2nd, 4th on the trigger shifter, then 3rd, 5th, 6th on the smaller cog. I've a feeling they were 14t and 20t or 22t ....... but I could be wrong.

Wot we did woz .......
Took the standard cog off and fitted the bigger one "backwards" ie dished inwards, after removing all the spacers and the dust cover.
Took one of the spacers, and ground it thinner under our heels on the flagstones outside.
Trial and error of the thickness until the smaller cog could go on dished outwards and allow the circlip to bed in nicely.

Main issues were that the gear-change was quite crunchy, and every now and again, the circlip would spring off due to the gear change struggling and forcing against the bigger cog. This wouldn't have been an issue if I'd have picked cogs with less of a difference.

My bike was like that from then in 1966 right through to when I gave it away to our next-door neighbour's son in 1982.

[bertgrower]

Update

I am thinking of do this but in additon is OK to fit a front derailurer so I could make the 3 speed Kingpin 1970 version into 12 speed?

[rjb]

My original Kingpin came with a 1/8 " chain but a 3/32" chainset.
I had a pair of them, this one i converted to 406 size wheels and fitted a 7 speed block with the a 3/32" chain and home made drop bolts to lower the brakes to fit, and fitted an ahead stem.

Best of luck there are several possabilities you could use, try searching on kingpin for them on this forum

heres your starter for ten, but there are lots of others, viewtopic.php?f=5&t=60322&p=582170&hilit=kingpin#p582170

[Brucey]

front mech is fair game but you may find that the clamp size around the seat tube is non-standard. There are workarounds for this though, if it is the case.

Suggestion; 30,50 chainrings and 14, 16T sprockets;

http://www.gear-calculator.com/?GR=SAAW&KB=30,50&RZ=14,16&UF=1540&TF=90&SL=2.6&UN=MPH

cheers