A few of us here have been trying out these hubs with the recent re-emergence of the old two speed auto and kick back hubs both with and without coaster brakes. Both SRAM and Sturmey Archer have now come to the market with new versions.
I initially came across the S/A, S2C kick back coaster brake version which I tried and hated. The change is inconsistent and there have been cases of it locking up. S/A's customer service is also appalling when you email them about this hub. They even removed the face book page which contained lots of complaints.
I have have recently fitted the new SRAM Automatix to my belt driven Moulton TSR2 and its a dream to use. That too has the S/A S2C fitted as standard.
I have toyed with the idea of fitting one to my Brompton for a while, but there are a few issues that initially put me off. The main being it was initially only available with 36h flanges which meant using different non stock Brompton rims or Brompton blank rims drilled with 36h which no longer seem to be available. Sun CR18s would suffice but I am not a fan of the 16" version with their shallow well making getting tires on and off difficult. The auto gear change point would also be quite low as the first A2s to appear were speced for 26"+ rims. Another problem is that the hub comes with an OLN of 120mm and spacers for up to 130mm. As we know the Brompton rear frame spacing is around 112mm
Anyway I noticed that 28h versions are becoming available in Germany initially without the coaster brake and specified for smaller wheel rims 20"-24" so the auto gear change point would be more appropriate.
Well my A2 turned up from Germany (good value at the moment with the weak Euro) and using some 148mm spokes I laced it up. The thin flanges require you use washers to get a good fit.
The next problem was the wider OLN. I spoke to some of the engineers at work and one reckoned the hub would be fine using only one of the two locking nuts as long as I "set" it when I tighten up the wheel nuts. He also took a mm or so off the single nut in the lathe bringing the OLN down to a healthy 115mm which meant it then just queezed into the rear 112-113mm Brompton spacing snugly.
The hub is specified for a 16T sprocket minimum which just fits in the frame without fouling the end of the chain stay. I guess you could use smaller sprockets but when I tried one you can't then get the dust cover on and the chain line is all wrong.
SRAM hubs tend to have their cog spacing/chain line slightly different to S/A. I wish these guys would standardise, though at least the sprockets are interchangeable. Using a smaller flat sprocket and different chain line (its about 2-3mm out) would also cause problems with the chain tension jockey wheels which would require washers to space them.
Because of the larger sprocket I swapped out my 50T main (which I was using for my S/A three speed hub) for a 54T which gives me a nice range of 56" and 76", better than than the stock Brompton two speed 56" and 74"
Here are some pics
The hub axle is a little long on the non drive side but can easily be cut/shortened.
I have been out for a ride and it changes up/down about 8-8.5 mph. I marked the locking nut and it is fine not moving once I set it to get the right pressure on the bearings (basically hand tight). The wheel nuts then clamp it up tight in the frame so nothing moves.
I live and cycle around Cambridge and it seems perfect for that, plus my Bromptons have added electric assist. Getting rid of a cable is nice and not even having to think about changing gear is good as well.
Weight wise there is hardly anything in it. The SRAM A2 is about 100g heavier than the S/A three speed, but by the time you remove cables, fittings and changer/trigger it balances out.
I will report back later if I get any problems. It might be worth trying the 28H SRAM A2 WITH coaster brake when it comes out later this summer, but again the tensioner can then be problematic.