jimlews wrote:...Would that "about 20k" be in spite of any fastidious lubrication/ servicing regimen, or could that milage be extended significantly with such.
and
Does the design exhibit any of the traditional Sturmey 'pratfalls', such as insufficiently supported planet pinions, pawl tips running on alloy etc.?
The only reason I ask is that I was 'gifted' one a while back; it is presently enjoying a life of leisure in my attic and I had the curious notion that it should be put back to work. It has some sort of brake attached so is likely doubly a "friction box".
20k is as reported by a contributor here who had just used the hub (in a 26" wheel I think) with the factory lube in it, for a few years of commuting. The pinions are made with integral stub bushings instead of being mounted on a pinion pin. This means that they have 'half-bushings' that are only about 2mm wide; not wide enough. [FWIW the (W) series of 5s hubs have a similar issue and the pinion stub bushings are a little wider, ~2.5 or 3mm. Cynically, I could say it doesn't seem to matter in this hub, because it usually breaks in some other way before these wear so badly it is a problem. I don't think I've seen one go past 10k yet, mainly because the shifters cause the hub to break.] In any case improved lubrication would certainly help but not cure the problem.
There are many potential design weaknesses in the 8s hub; I looked into repairing a faulty one only to find that individual spare parts were not available and in addition the hub in question had about five separate damaged areas, so it wouldn't have been economic anyway. I think that there have been several revisions to the design. It is very clever, just flawed in conception (two stages in use in 'cruising gears' is arguably never going to be super-efficient) and flawed in execution. For example each of the three planetary gear stages is locked solid when it isn't 'gearing up' and the method used is to have pawls that are overrun when 'gearing up' (much as low gear pawls are overrun in gear 2 and gear 3 of a three speed hub) and are locked in the 'solid' mode. Only the pawls don't engage with a separate dog ring, they engage with the teeth of the mating ring gear if the neighbouring stage. This is a really elegant solution in some respects but
a) the parts have to be exactly concentric, slop free, in order for all three pawls to engage reliably (echoes of SW...?) and they are not. The ring gears have about 80 teeth and it doesn't take much slop before 2/3 pawl drive or 1/3 pawl drive results.
b) any slippage results in shock loads that can break the teeth off both the pawls themselves and the mating ring gear. The pawls (whether it is the early design or the later one which is different) can't be changed (they are riveted in place.... :
) and the net result is often that a small fault of this type causes two out of three gear stages to need replacement.There are other issues with gear selection too.
Regarding the brake; SA drum brakes are easily set to be drag-free. SA drum brakes have probably held more HPV records and race wins than any other single type of brake. No fear or problems there.
If I had an 8s RF (rim brake) hub knocking about I'd probably build it into a 349 (Brompton size) wheel and use it (in a Brompton or an F series Moulton). The hub builds down to a relatively slim OLN and this makes for an easy fit and a nice ride in a Brompton; it is unlikely to suffer damage (small wheels = less torque) and folk don't tend to do intergalactic mileages on those bikes, or would overly begrudge a new hub internal once every five or ten years. You even need a smaller chainring than normal on a Brompton to get the gearing right. It is not quite a match made in heaven (the hub is not super- lightweight) but that is arguably still the best place for one.
Unfortunately the RD hub version is a bit wider and a bit heavier yet than the RF version, so isn't so well suited to a Brompton. However it shouldn't be difficult to get a RF shell (used) and convert if you wanted to. The RD hub would go well in a 20" wheeled folder perhaps?
hth
cheers
