Monday, 5 July 2010

What's inside the Sturmey-Archer S3X hub

The S3X is a three-speed fixed-gear (or fixed-wheel, if you prefer) hub. This makes it a rather unique item. You can change gear but you can't stop pedalling. Imagine driving a car fitted with a manual gearbox but no clutch pedal; when you changed gear, the engine would have to instantaneously speed up or slow down. Well, that's what your legs have to do with the S3X.

This hub fulfils no real need, addresses no real gap in the marketplace. It's a curiosity, and it's there for fun. Consider it in those terms and you won't be disappointed.

On the outside it looks much like any other current Sturmey-Archer hub. Nice alloy shell and a slightly crass name, presumably because it's supposed to be S3XY. Hmmm.

It's the spiritual successor to the 1940s/1950s ASC hub, which gave ratios of direct, -10% and -25%. This one, for reasons of re-use and economy, has considerably wider ratios of direct, -25% and -37.5%. So top gear is direct drive (and therefore almost lossless) while the gear you're going to spend most of your time using isn't. This isn't as bad as it sounds. The bottom gear is a bit noisy and you can feel that your effort is going through a bunch of cogs, but the middle gear is very good once the hub has been run in - 100 miles gets you most of the way there, and it won't really improve further after 200.

Looking inside, you might wonder why you paid nearly two hundred quid for it. It's reasonably well known that it's a hack of the current five-speed hub, using the bottom three ratios. Some parts are carried over from that hub such as the right-hand ball ring, with redundant freewheel ratchet, and the planet cage assembly.

The way the hub works is a bit complicated, but not as bad as some of the esoterica that Sturmey-Archer and its then chief designer, William Brown, were turning out in the golden years of hub gears in the 1940s.

I wrote this piece (slightly edited and improved here) for the newsgroup.

First impressions: the hub is extremely simple and easy to strip down,
with a total absence of pawls and "R" springs. The simplicity is
largely because SA have riveted the suns and the planets into the planet
cage assembly so it can't be further disassembled. Given that these
parts don't normally wear out and compound planets require careful
"timing" when being refitted, I think that's a reasonable decision. I'm
not so enamoured of the way the axle key is fixed into the axle, of
which more later. The mechanism does not get as tightly screwed into
the shell as on other SA hubs, which makes regular servicing less

Means of operation:

The S3X is a single-stage epicyclic with selectable suns and compound planets. It has a lot in common with the old FW and the five-speed hubs.

The driver (to which the sprocket is fixed) ALWAYS drives the gear ring.

In top gear neither of the suns are clutched to the axle. The main
clutch drives the planet cage via six arms and trapezoid dogs on the
face of the cage - no AW-style extended planet pinion pins here.
Because the suns are out of action the entire planet cage turns at the
speed of the clutch (and hence the sprocket) and transmits the drive
straight to the hub shell via a three-armed dog. The gear ring plays no
part at all, since the planets and suns it drives all rotate freely and have no effect on the movement of the planet cage.

In middle gear the main clutch is disengaged from the planet cage. The small sun of the pair is clutched to
the axle by means of a sliding axle key. The gear ring drives the small
ends of the compound planets, which rotate around the small sun via
their large ends. The planet cage is forced to rotate slightly slower
than the gear ring, giving a medium gear reduction (1/4).

In low gear the large sun is clutched to the axle instead. The gear ring
drives the small ends of the compound planets which rotate around the
large sun. The planet cage is forced to rotate considerably slower than
the gear ring, giving a large gear reduction (3/8).

Note that in this hub "drive side" is always the left, since the torque
is transmitted to the shell more or less where the LH flange is.
However, a large-diameter SA shell is so torsionally stiff that this
makes very little difference to left/right spoke tension, even when cranking

There is a neutral position between medium and top when the clutch does not make
contact with the planet cage and the small sun is not yet clutched to the axle. The hub is highly unlikely to
slip out of top gear when the cable is correctly adjusted because of the
proper dog arrangement on the planet cage, so this is not as serious a
fault as on the AW, with its kludgey extended planet pinion pins.

There is also a potential slip between low and normal gear when the axle
key is out of contact with either sun. Cable adjustment is critical
with this hub, and the shifter allows selection of intermediate
positions which can be dengerous. An old-fashioned flick trigger has a
lot going for it but isn't offered for the S3X cable pull. When
changing gear, always ensure that the shifter has clicked into position
before re-applying power to the pedals.

Quality and specification:

Most parts are very heavily built, especially the gear ring which is
probably overspecified. On the other hand, the sliding axle key, which
is required to move the main clutch and to lock the suns, is very small.
The dog which does the sun-locking part was very shiny on one side after
only 200 miles' use. I don't know what proportion of the total torque
has to be resisted by the sun/axle attachment (the AW gets away with a
small cotter pin) but, coupled with the loose fit and frequent reversals
of drive, it doesn't quite seem up to the task. The slot in which the
axle key slides also shows significant wear marks on its edges.

The key
is not easily removable from the axle as it is held tightly between two
halves of a long compression spring (what was called a "compensating
spring" on the old FW), and it would be best if SA made the axle, spring
and key assembly available as a spare part considering that all of these
will wear. Nothing else in the hub showed any signs of wear beyond the
normal light polishing from running-in.

The lash in the hub is mainly from the loose fit between the main clutch
and the planet cage or gear ring (depending on the gear selected) and
not from the gear teeth, as might be expected. There is a small
clearance to allow the clutch to engage and slide freely.

Personally I dislike grease lubrication as it means periodic stripdowns,
but the SA grease is an unusually brown and oily one and does flow a
little. SRAM grease is more like toothpaste and will not migrate to
coat any uncovered internal parts.


The hub is unique and therefore it's not worth comparing it to anything
else on the market. The design is neat and the ratios are well-chosen
for general riding, although low gear is quite noisy and top (direct
drive) will rarely be used except for diving downhill. Fortunately,
middle gear is almost as efficient as top, with only fractionally more
drag when the wheel is spun off the ground; I would expect the
additional losses to be about 3%. The low gear noise and inefficiency
is likely to be due to tooth shaping, which has always been sub-optimal
on SA hubs with multiple suns.

The design has two main weaknesses: the presence of "neutrals" and the
small size of the axle key, which will wear rapidly. The "neutrals" are
compounded by the choice of a shifter which does not flick positively
between gears. The lash, I believe, is unavoidable in a 3-speed fixed
hub (the simpler 1930s two-speed TF is said to have virtually none). In
some areas, such as the choice of ratios, the hub betrays its origins as
a "hack" of the current five-speed.

Next post: how to service it and put it back together so it works. Much, much easier than most SA hubs.

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