Sturmey-Archer S3X 3 speed fixed-gear hub

A few years ago when the folks at Sturmey-Archer announced that they were developing a 3 speed fixed gear hub, I was super-excited. This was tempered, however, with the worry that it would never see production due to it being something of a niche product.

Then, back in March of this year, suddenly they were actually available for purchase. Naturally, I wanted one. Bad. But, given that it’s a >$200 piece of kit, I wanted to get some first-hand opinions of it before I took the plunge and had a wheel built around one. I turned to the internet (specifically to the rec.bicycles.tech newsgroup) for info. Right away I got some feedback which further convinced me that I really “need” one. The only real issue anyone seemed to have with it was the amount of lash (15° at the hub seems like a lot, but when you factor in gearing, it’s not that bad at the pedals).

Then, just a few days ago, I got an email from “Rogerzilla” which was a far more detailed and technical review than I would have ever hoped to elicit. With his permission, I’m reprinting it below.

I’ve done about 200 miles on one and I currently have it stripped and cleaned (to get the contaminated grease out of it after running in). I will take some photos before I regrease and reassemble it, but I’ve done a little summary:

Mechanical details of the Sturmey-Archer S3X fixed-gear hub

It’s similar to SA’s four- and five-speed hubs with compound planets and two suns, which can be clutched to the axle or allowed to turn freely. There is a single gear ring fitting over the smaller ends of the planets.

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 that 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 fraught.

Means of operation: The driver ALWAYS drives the gear ring. The main clutch, on the other hand, never touches the gear ring and is solely used to drive the planet cage in top gear.

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 real part in this gear; it does turn the planets, but they are quite independent of the planet cage while the suns are free.

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 (25%).

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 (37.5%).

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 L/R spoke tension, even when cranking hard.

There is a neutral between medium and top when the clutch does not make contact with the gear ring or the planet cage. In theory simultaneous engagement of these two should not matter if the suns remain declutched from the axle until the main clutch has disengaged from the planet cage, but the neutral is there for all to feel when the hub is on a bike, so maybe SA didn’t want to take the risk. 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.

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 dangerous. 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 (10mm at the pedal) is mainly from the loose fit between the main clutch, driver, planet cage and gear ring (depending on the gear selected) and not from the gear teeth, as might be expected.

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.

Conclusions:

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 and the redundant freewheel ratchet in the RH ball ring, the hub betrays its origins as a “hack” of the current five-speed.

Amazingly, he followed this with another email, with this link to service instructions he posted and the following additional info:

I always intended to run mine in for 200 miles then strip and clean it to remove the contaminated grease. It is definitely quieter afterwards, probably because it has a bit more grease in it than it did from the factory.

The nice thing about the S3X is that the true drive side is always the left, so the RH ball ring, which is the bit you have to unscrew to pull the mechanism out of the shell, doesn’t get so tight that it needs extreme measures to remove it. With other SA hubs the ball ring drives the shell in at least the top two gears, and really gets screwed down hard.

There are actually only ten parts to the mechanism: the planet cage assembly, a circlip, the axle assembly, the clutch, its spring, the spring cap, the gear ring, the ball ring, the ball ring race and the driver. Much easier to service than an S-RF3, which has three pairs of sprung pawls.