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Sturmey-Archer X-RK8 (W)

Let me begin with a warning: this is not a product I'd recommend to every rider for every bike. It has worked reliably on my folder for years, but I don't put more than 69 Nm into it (rider weight 70 kg, crank length 170 mm, chain transmission 42/25). My colleague Jura, who cranks something over 100 Nm (chain 28/25, the rest assumed to be the same), has had two tooth failures already. I've found several reports of similar failures on the net. Helios (Czech bike parts dealer) decided to stop selling these hubs because they return quite often. That means: the hub is suitable for smaller loads only (small wheel and overdrive chain transmission), not for anything else (standard-sized wheel and chain about 1/1).

Tactical data: eight gears, total range of 325 % (a bit more than 8-speeds by Shimano or SRAM). Gear 1 is direct drive, all others are overdrives. Weight of almost 2 kg. There is also a drum brake version X-RD8 and brakeless X-RF8.

The shifting is quick and troublefree as long as you don't push on the pedals during the shift. Six middle gears are evenly and quite tightly spaced (I often shift by two at a time), the first and last are further apart. Pulling the cable shifts to slower gears, releasing it to faster, as usual. The hub is quite noisy, especially gear 4 whirrs like an electric motor. Until the control cable beds in, you have to check the adjustment periodically; the tolerance is quite narrow and hitting an intermediate position is asking for trouble. Cable tension is adjusted by a screw on the shifter and checked by a yellow square under a slot in the cable drum (the yellow is not very visible, so I repainted it red - see photo above). It's good to start with the adjusting barrel screwed all the way in, so it can take up the slack developing in the cable.

The only occasion where the hub can't be relied upon is bumpy terrain. Shifting pawls are not statically balanced and their return springs are weak, so if the wheel bounces hard, they can engage randomly. You can always reset the gears by a quick backpedal, but when you are trundling uphill slowly at gear 1, hit a rock and suddenly appear in gear 2 (30% difference), you are probably going to get off and push. Cobblestones are OK though.

Installation

Shifting grip or thumb lever, cable and all nuts and fittings are included in the package. It is possible to lace the big hub into a 20" wheel with 36 spokes and standard triple crossing, but the spokes slope a bit too much, double crossing would be better. I don't recommend any bigger wheels.

The rotary gear selector is completely hidden behind the dropouts, so it's shielded from damage. Cable stop can be attached in two possible positions for horizontal or sloped dropouts, so the cable hides neatly behind the chainstay. It is not necessary for the function though, so if you for example have small rear fork with quickly converging chainstays, you can let the cable stick out up or down to clear them. Possible problems are the cable stop rubbing on outward-dished sprocket, and anti-rotation washer tabs being thicker than the dropouts. Both are easily fixed with a file.

You only have to adjust cable position once. When unmounting the wheel, you just unhook it and then put it back without further adjusting.

Maintenance

One thing that may need maintenance or periodic replacement is the control cable. The sprocket will also wear off one day, but considering the tooth count, its life span should be in the order of tens of thousands kilometres (spares are available and inexpensive). The internal gearing is well adjusted and greased by the factory, so I guess it can run for years without maintenance. The manufacturer recommends major overhauls be done by "authorized dealers who are equipped to carry this out" and gives us amateur tinkerers just one drawing and a part list. Fortunately, the mechanism is not very complex, so we can do our tinkering even without a manual - see below.

Design

Hub shell is made of polished aluminium, cable drum on the right side is plastic, everything else is galvanized steel (I can't see if the cogs run on any bushings). Weather protection is done by labyrinth dustcaps, so riding axle-deep in water is not a good idea. But normal rain, dust and mud are kept out very well, as proved by perfectly clean grease I found in the bearings after every yearly checkup. And the bearings are of good quality, they still look like new after four years of daily use.

Hub with a (W) in its name is a second generation. The first one had a separate gear and ratchet ring on each stage (see example here). Now there is just the gear ring which doubles as a ratchet. Saved space and material was probably used to reinforce everything that was left, but it's not enough yet. This happened to Jura:

First failure (leftmost picture) was two broken teeth on the gear ring of last stage (the part which also contains main freewheel). It happened in normal use during a stronger push on the pedals, after about 500 km of use. There's a planet-cog-shaped hole in one of the teeth, so I guess it was a local material flaw - the tooth was softer than it should have been. The other tooth might have been broken off by a fragment of the first tooth, but it's just a guess.

Second failure (middle picture) happened a year later: one of the three ratchet teeth on first stage broke off. First breakdown was covered by warranty, dealer delivered a new ring and complete previous stage. The second one wasn't, but the dealer (already mentioned Helios) sent a new hub anyway.

My hub still has all teeth intact, but a lockwasher on the right end of the axle has cracked apart after about 6000 km of use. It is cast of some rather brittle metal (cast iron?) and has little bumps along the perimeter to bite into aluminium dropouts and help keep the axle in place. But my dropouts are made of steel, so stress peaks and more shear loads developed, eventually destroying the washer. The event was quick: after the washer cracked, right axle end moved forward and chain fell off. So I stopped on a sidewalk and found what happened and how lucky I was - had it happened half a mile sooner in a steep downhill before a crossroad, maybe I wouldn't be writing this now. Quick solution: fabricate a new washer of a good quality steel (see last two pictures). Lesson learnt: cast iron washers NEVERMORE.

How does it work

The gearing consists of three epicyclic stages connected in series, each with a different gear ratio. Driving torque is input on the planet cage and output on the outer gear ring. Sun gear is either locked with the axle and the stage works in overdrive mode, or it spins freely, the gears lock by the integrated ratchet and work in direct drive. Each stage is controlled independently by a rotary cam and a pawl sticking out from the axle. Three stages by two positions give the resulting eight gears. The best efficiency (the least cogs engaged) occurs in the low gears - good for quick starts and hill climbing, not so good for high speed cruising.

gearCBA
1000
2010
3001
4100
5011
6110
7101
8111

The cams are marked A, B and C. Each of them controls one epicyclic stage according to the table on the left. 1 means engaged pawl, locked sun gear and overdrive mode; 0 means retracted pawl and the sun gear free in direct drive. The pawls engage via springs and get pushed back by cams. Engagement is theoretically possible under full load (a pawl pops up and waits for the sun gear splines to meet it), retracting works only if no torque is present. That means shifting from 1 to 2 and from 7 to 8 works under load, all other transitions do not. Not really worth remembering, better to always ease off before shifting.

How to get inside

Note: if it works, don't mess with it. If it doesn't, something has probably cracked apart and you can't fix it anyway. If you just need to pour water out, replenish grease or remove the cracked bits and limp to the nearest bike shop, keep reading. There are no traps (except the cam mechanism which usually doesn't have to be disassembled), all threads are right handed and all you need is a flat 16 mm spanner, two 17s, C-clip pliers and a vise.

First remove the sprocket and dustcap on the right side, then the cone on the left side and then the big notched gray ring on the right (it requires either a special wrench, a vise, or tangential tapping into the notches). After that, the internals fall out in one solid bulk.

If you remove a C-clip on the left side, you can pull the output ratchet and all three gear stages off the axle. If the control pawls don't let them go, shift to gear 1 to retract them.

The hub comes from the factory well greased: yellow grease for the bearings, black graphite goo for the cogs. Be very careful to keep dirt away from the gears - the stages are riveted and can't be taken apart for cleaning; all you can do is to add grease.

I really don't recommend to disassemble the cam mechanism, there's nothing interesting in there and it's very difficult to put back together correctly (most parts fit in multiple positions and only one of them is correct).

OK, you were warned. All you need to do is to unscrew the nut on the right end of the axle and to pull out everything you find under it. Look out for pre-tensioned springs and other traps. It's a good idea to take photos of the process so as to have some reference for the reassembly. By the way: the cams go back in A, B, C order; if you mix them up, they won't shift.

As you can see, I've already been through the complete stripdown. The main reason was curiosity, the excuse was a new noise that sounded like something loose lightly rubbing on something spinning. The hub started to emit it in gears 4..8 after 2100 km of use. It was probably caused by the planet cages - they slide on each other and if the grease creeps out from between them, you start to hear it. I've found no mechanical failures, all moving parts were in good condition, ball races shiny like new and the grease perfectly clean.

Conclusion

X-Rsomething8 is a good choice for folders and similar small-wheeled machines which would otherwise require huge chainrings or tiny sprockets. Good efficiency of low gears works well for hills and stoplights; the high end is only used downhill and there are speed limits in towns anyway.

The hub is not suitable for standard-sized wheels where it can't be trusted to survive all the torques. Unreliable low gear also disqualifies it from bumpy terrain.

Durability? Over 10000 km so far on my folder, still alive and well. If you have any more experience (mileage, failures etc.), feel free to share.

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