Sturmey Archer RX-RD3 ..good idea...??..

[Brucey]

....or not...?

For the past 10+ years the SA 3s hub eg X-RD3
http://www.sturmey-archer.com/files/catalog/files/307/PART%20LIST%20-%20X-RD3.pdf

has been fitted with a thing called an 'actuator plate'. This exists for one purpose only; without it, when you are in top gear and you wheel the bike backwards, the clutch is being driven backwards by both the gear ring and the planet cage, but at different speeds. If the clutch doesn't ride up against its spring (which it arguably should do, there are ramps on the clutch), something in the hub will break. The actuator plate is meant to defeat the driver pawls when the clutch is driven backwards and this prevents any risk of the hub breaking when the bike is wheeled backwards. However more usually it falls off the driver or breaks up (through repeated backpedalling) and then bits of it fall off inside the hub and trash it. I think it is the single least reliable part in these hubs. However, you can usually hear the clutch riding up against its spring even if the actuator plate isn't working, so I have wondered if you really need it or not.

I have just examined the parts diagram for the CS-RK3
http://www.sturmey-archer.com/files/catalog/files/299/PART%20LIST%20-%20CS-RF3%20CS-RK3.pdf

and it appears not to have an actuator plate! Instead it has a clutch that has an easier set of ramps on it, so it should work just fine without it, even if it might make a funny noise if you wheel the bike backwards. All well and good then.

However I have also just looked at the diagram for the rotary shift RX-RD3
http://www.sturmey-archer.com/files/catalog/files/268/PART%20LIST%20-%20RX-RD3.pdf
and I am not so impressed with that, sadly. They have bowed to pressure from somewhere and they have changed the world's simplest shift control system (a toggle chain) for something that looks neater but is more complicated and expensive to make, more trouble when you remove the rear wheel etc, even if it mightn't get damaged so easily should the bike fall over.

The RX hub has an actuator plate, just like the X-RD3 does. However if the X-RD3 actuator plate fails, the clutch ought to lift easily; it only has to fight a spring, so the hub still shouldn't break. But in the RX-RD3, if the actuator plate fails, the clutch will still try to ride up its ramps, only now, instead of simply compressing a spring, it is trying to compress the entire shift control cam system (parts 30 and 31). Now it might be that this simply forces the gear shift backwards.... but then again it might just mangle something instead.

I've not had the chance to inspect or test these parts in person, so maybe my interpretation of this isn't quite right. Anyone else have any experience of the RX hubs as yet?

cheers

[[XAP]Bob]

Break a toggle chain by falling over? Onto what, and angle grinder?

[Brucey]

yeah, it can happen......it is easy enough to avoid breakage or replace it should the worst happen, but I guess when the other IGH makers have externally neater gear controls (no matter how flawed in practice they might be), a toggle chain might look at bit, er, '19th century' or something.

cheers

[TrekMad]

A good reason for getting shot of the toggle chain:

http://m.youtube.com/watch?v=LJeeNGh3P-M

[iow]

been running a £5.00 ebay X-RD3 (minus the drum brake and converted to oil lubrication) without an actuator plate for 2 years (c.600 miles) on my 'pub' bike with no issues - see HERE

[Brucey]

Re the video; I don't think that is anything to do with the toggle chain per se; what is happening in that video is that when freewheeling in top gear, instead of the ring gear pawls being overrun in the normal way, the ring gear is being dragged around with the hubshell (probably because there is something in the works eg a failed actuator plate, or perhaps because the hub is badly adjusted/lubricated) and the clutch is riding over the dogs on the planet cage.

Once the clutch is riding up like that, it makes that noise and pushes the toggle chain out every time it moves. In and of itself it doesn't do any real harm but it does indicate that the hub needs fettling.

If an X-RD3 (or other with similar internals) does that when you are wheeling the bike backwards in top gear, it means the actuator plate has failed. If a SC-RK3 or CS-RF3 does that under the same conditions, that would be 'normal', because that is what it is designed to do.

If a RX-RD3 is wheeled backwards in top gear and (unbeknownst to the owner) the actuator plate has failed, I think it'll mangle the shift control cams or perhaps exert a mighty force on the external shift controls.

BTW if you want to test the actuator plate function, I recommend that you get the rear wheel off the ground, select top gear and turn the wheel backwards gently and slowly. If you encounter any appreciable resistance, stop before you break something; it means the actuator plate has failed. If the toggle chain starts doing StVitus's dance as per the video, it also means that the actuator plate isn't working and should be replaced immediately.

Note also that if you load the transmission forwards (eg by having your foot on the pedal or if the crank gets stuck behind a propstand) as the bike is wheeled backwards in top gear, the actuator plate will see far higher loads than normal; the pawls that it is required to move will effectively be locked in position as long as there is chain tension. I think that in extremis (if the clutch doesn't lift for some reason) this could cause the actuator plate to fail instantly. Certainly the actuator plate will (alone) see the full load on the clutch until the pawls are defeated or the clutch overrides the dogs on the planet cage.

The same actuator plate design is used in all NIG three speeds, as well as all 'small shell' five speed hubs with a single toggle control i.e. from about 1995 onwards, including the 'sprinter', the X-RD5, the X-RD5(W) etc.

If you want to buy a new 5s hub of this sort you should get on with it, because SA are now shipping the new 5s hub, the 'C50', which is a much larger, heavier hub, ostensibly meant for e-bikes and the like. This might turn out to be a really good hub, we just don't know yet; all we do know is that it is built big and heavy (think bigger and heavier than a Nexus 8 ).

cheers

[Brucey]

been running a £5.00 ebay X-RD3 (minus the drum brake and converted to oil lubrication) without an actuator plate for 2 years (c.600 miles) on my 'pub' bike with no issues - see HERE

so what happens if you turn the wheel backwards in top gear? (please be careful as per my suggestion above)

cheers

[iow]

so what happens if you turn the wheel backwards in top gear? (please be careful as per my suggestion above)

cheers

same as when back pedalling in top - the clutch rides up over the drive/pawl pins, accompanied by a loud (relatively) click and the toggle going monetarily loose.

[Brucey]

jolly good!

Your clutch must have ramps on it then! I have a feeling that at one time they may not have...

cheers

[barrym]

Dunno if this is relevant or not, but if these actuators are fragile, could you use one of the fixed gear 3 speed hubs with a freewheel cog? Do they have actuators? I have often wondered what they were for, perhaps this is it.

[Brucey]

I think you are correct, you can fit a threaded freewheel to the driver of an S3X hub, and inside, there is no actuator plate.

However the S3X is actually not really a three-speed hub.

Eh?

Well, if you examine the way the hub works, it is more like a kind of hamstrung five speed (in which you get to use the first three gears only) than it is other three speeds. This means that the high gear is direct drive, and the ring gear or the planet cage are engaged with the driver, never both at the same time. This in turn means that there is no need for an actuator plate in this hub.

cheers

[barrym]

And does that mean it is a stronger hub (if that were even necessary) than the standard modern 3 speed?

[Brucey]

And does that mean it is a stronger hub (if that were even necessary) than the standard modern 3 speed?

Probably not; both the dogs locking the sun pinions and the planet pins on the (W) hubs (and by extension the S3X, I would suppose) are IMHO less good designs than in most 3s hubs.

My favourite 5s hub is the first SA version (or converted FW/FG). Nothing is perfect of course but the sun pinion locking is several times stronger in that hub than in any single-toggle SA 5s hub, or the S3X come to that.

I think that the sun locking is strong enough for most solo uses but not for the heaviest use and not under fault conditions. The planet pinion bushings (if they are like the (W) ones) are considerably inferior to any other 3s bushings; basically the planet pinions are solid, and have (rather narrow) end projections that rotate in the planet cage.

In other 3s and 5s hubs there is a loose pin through the planet gear and whilst the pin could rotate in the planet cage (a la (W)), it doesn't ever do this unless there is a fault of some kind; the planet gear always rotates around the pin instead, because there is less friction this way.

cheers

[SA_SA_SA]

my 1997 red band toggle sprinter had a neutral unlike its NIG replacement ball locked sprinter (blue toggle mark).

[Brucey]

I think that is because of the sun locking arrangements, and the neutral should appear between 1 and 2 as well as 4 and 5.

Both hubs should, I think, have an actuator plate on the driver, and exhibit similar NIG behaviour in 2-3-4, just like a 3s gear with an actuator plate.

cheers