Well, now I've found a real one. Alan kindly sent me an S2C Duomatic to play with that he had rejected. As soon as I got it out of the box I could see why; with the sprocket fitted, turning it forwards took both strength and a masochistic streak as the teeth dug in, the force was that great.
So I set to as promised and started to strip it down. My first action was to question the parentage of the designer for his choice of 22.6mm AF locknuts on the left side of the hub, and no tab washer. This makes adjustment of the hub rather tricky as you need two of the piggin' spanners to adjust the hub bearing in situ.
However having undone the locknuts the brake plate comes off, then the centre pulls out in one piece from the right side, brake shoes and all.
The SA hub has a screw-in right side ballrace, but this can stay put unless it is worn; there is no need to disturb it to remove the centre.
The right side cone and locknut also don't have a tab washer which may lead people to suppose that this is where you should adjust the hub bearing. This would be a mistake. There is typically about 0.5mm 'float' in the centre parts, so the traditional SA scheme of snugging the right side (with the hub dead loose on the left), backing off half a turn, then locking these and leaving them alone for ever is the right one here, too. Unless you are dismantling the entire centre, the only gain to be had by removing the right side cone is an inspection of the bearings; it affords no further view inside and is not a good place to add lubricant.
The hub bearings adjust on the left side; as usual the locknut will preload the bearing if care is not taken with the adjustment. Unless you happen to have two thin 22.6mm spanners the bearing adjustment is probably best done with the axle clamped in a bench vice.
As supplied, the hub cannot be fitted with tab washers to facilitate cone adjustment and locking, because the axle flats are (amazingly) about 3mm too short on both sides. I plan to grind the flats a bit longer and fit tab washers. Fortunately the axle threading is the traditional 13/32" size long favoured by SA for three-speed hubs etc so AW tab washers and locknuts can be fitted OK. This may mean I can even avoid having to make an extra-thin 22.6mm AF cone spanner... we'll see...
Inside there is a fair amount of grease and everything looks quite substantial. In the pictures you can see some of the hub's inner workings. The planet ring is in constant mesh and turns 138% of the sprocket speed. It is fitted with eight detents; four shallow 'low gear' detents and four deeper 'high gear' detents. In the low gear, the high gear pawls are defeated by the shallow detents and drive is transferred by the low gear pawls. In the high gear, the high gear pawls are free to engage with the hub shell and transmit drive; the low gear pawls are overrun backwards and will emit the classic sturmey-archer 'tick-tick' sound.
The planet cage is a bolt-up affair; I didn't disturb it but I would assume that the bolts are loctited in for the duration. I'm not usually a fan of this arrangement, but this looked OK to me, pretty robust. The gear meshing seemed snug and confidence inspiring, but it may have just been an illusion created by the grease they use which is rather sticky.
A small turn backwards on the pedals will engage the coaster brake via a roller/ramp mechanism. This is helped to disengage via a drag spring labelled 'DS2' in the pictures. The brake shoes are held together and thus away from the hub shell by a spring clip 'DS3'. This allows the hub shell at least to turn very freely when coasting. A short backpedal will also change gear, although it takes a slightly different length backpedal going from high to low than low to high. This means that whenever you brake, you will also change gear.
It turns out that when stationary with the hub in question, it should change from low to high, but not high to low. To make the high to low shift, the hub shell needs to be turning forwards (as well as the pedals backwards) i.e. it seems that you need to be moving.This is the exact reverse of what is desired; if pedalling in low gear, working the brake shifts into high; if you then stop without a second burst of braking you will stop in high gear, not low gear, and you cannot change again until you are moving forwards once more. It may be that once the brake shoes have worn a little, the length of backpedal possible when stationary (which is limited by the brake coming on) will lengthen slightly and allow a stationary high-to-low shift. Until then it may be rather annoying. [edit; after more fiddling, it now does the high-low-shift when stationary, but not every time. I am somewhat optimistic that it will settle down in this respect]
[With the standard S2 hub, this is not going to happen in the same way; there is no reason to backpedal whilst slowing down, and a gear change (or several by pedalling backwards further) can be made whilst stationary. The 'click' of the high gear pawls is different depending on which gear is engaging, so it is quite possible that you will be able to select the 'gear by ear' as it were.]
I stripped down the centre to its main parts, then removed and reworked the three drag springs. All were too tight and needed various forms of reshaping. I guess these should ease with use anyway, but they were stupidly, way-too-tight. The drag force seemed worse in the high gear, but this may have been illusory.[edit; I have since learned that this high drag has been 'explained' in some cases as 'bearings shipped adjusted tighter than normal to avoid damage in transit'. What a crock...]
DS1 effectively preloads and therefore allows the planet ring not to chatter when it isn't being used for drive. This helps to keep the chosen gear in when pedalling forwards. I'm not sure if it has a great effect on the brake operation or not. I made it about half as draggy.
DS2 is the brake drag spring, and needs a slightly higher force I believe. It is stiffer and thicker than DS1. I again made it about half the force.
DS3 is the brake shoe retainer. I'm not even sure it is really necessary, but the axial float in the shoes seems quite large, so maybe they can go skew without this spring and cause the hub to jam. This I'm not sure of, but what I do know is that they could have made it so that the shoes are held against one another but away from the both the inner and outer part, but they didn't. They made it so that the shoes are held tight by DS3 against the roller assy, and this creates yet further drag whenever the hub is driven. I again eased the tension in this spring.
I put everything back together again with a load more semi-fluid grease, and was pleased to note that I can now turn the hub drive without my fingers bleeding from the effort. Only time will tell if this is a good fix or not. There is a good spot for adding a lube port in time, where the Al shell is the only thing that needs drilling through. I noted that the planet gears are not easy to lubricate per se, but should water get past the right side ring bearing, it will head straight in there.
[edit; the axle on my hub is 160mm long. I reckon this is long enough for a 130mm frame, provided you put thin (say1.5mm) washers on the outside and use the NTWs on the inside of the dropouts. My hub has a serial ID of JFA8, and the internals are of rather different design to those you will see in the McCraw blog (link in a following post). I don't know if these internals are likely to suffer problems similar to those seen by others. I guess only time will tell...]
Comparisons with the SRAM A2 Automatix coaster brake are inevitable. Despite its steel shell, the SRAM is lighter (by 7 Oz, ~200g), less draggy, and easier to use. Whilst I'm a sceptic for automatic gear changing in general (especially electronic, and it'll be here soon, trust me....) with two gears I can live with it, it really is a no-brainer for an urban bike.
By comparison the SA S2C Duomatic is heavier, more draggy and fussy in use, especially in this coaster brake form. It does have a nice aluminium shell which can be had in pretty colours, and it does look a little more robust inside (18 balls in the main bearings rather than 12, bigger pawls, very chunky anti-turn washers etc). Also, it will hold the gear you are in regardless of road speed if you want it to.
Sealing and lubricant retention looks about the same for both hubs, they both have about the same gear ratios, and both came with a 19T sprocket (suggesting 36x19 gearing on a 700c wheel maybe). Long term reliability of either hub is not exactly proven; thus far the S2C has had a fair amount of bad press but maybe they have sold a load more of them, had teething troubles etc..... the centre on both can be replaced easily, and in theory the S2C is more repairable because of it's screw-in RHS ball race.
Overall however the SRAM does look to be the easier to live with, even assuming that you can get the S2C to work without the massive drag penalty that my example was saddled with out of the box.
[edit the tooth counts are as follows; 17T planets, 21T sun, 57T ring gear. This gives a ratio of 1.3684.]
Thus far I've not ridden the S2C, only bench tested it, but it has been quite educational. As with the notes re. the SRAM A2 hub I prepared last week, the usual disclaimers apply; none of this is endorsed by SA, you may void your warranty, you carry out such at your own risk etc. However I hope folk find it interesting/useful, anyway.
cheers
i'll follow it at the weekend 
Your post might encourage me to get it down and see if I can make things better!