Sturmey Archer gear indicator chain failure?

[drossall]

It's decades since I've worked on an SA hub. However, after borking my Dahon, I'm on a borrowed Brompton for a couple of days while I sort out a replacement (probably a Brompton of my own). I'd like to give the borrowed bike back in at least as good a state as I got it. So far, I'm mending my second puncture, and now I've got a gear problem.

The gears have stopped working (stuck in third, usual thing). They were OK when I got the bike yesterday The indicator chain seems to be pulling out. I've screwed it back, but it never goes fully home; I'm used to them turning in until they bottom out, like most screws. This one just keeps turning. Then, attach it to the cable and it pulls out again.

The thread hasn't snapped. It's quite a short thread, like this, but looks intact. Do these threads strip, and is it likely to be the chain (hope so!*) or the socket? The threads on the chain look OK. Cheap to try a new one of course.

Suggestions welcome.

And what's the difference between chains for alloy and steel shells?

*I did once strip down and rebuild an SA 3-speed hub, but I was young and foolish then. I got it back together, but couldn't find the fault, so it still ended up at the LBS, where they diagnosed a worn clutch, if I recall correctly.

[Brucey]

the indicator rod is either not screwing into the toggle key, or it is screwing in and the thread is bad (stripped), so it is pulling out again.  I have seen both faults.

If you peer down the hollow axle you may see the hole in the toggle key. If you cannot, it may mean that the key is out of position and the only cure for this is to dismantle the hub (in part at least).

 If the indicator rod parts company with the toggle key when the hub is not in gear 3, then the toggle key is more likely to end up out of place.

With the current model of SA 3s hub you can (IIRC) replace the toggle key without removing the entire internal; just remove the driver and the clutch parts. You can attach the indicator rod to the toggle key before the clutch, clutch spring, driver etc are replaced, and this makes sure that the key doesn't come adrift before the hub is back together. Once the hub is together and is fully engaged in gear 3 then the indicator rod can be safely unscrewed.

Post 2000 Bromptons use SA hubs which use the same axle parts as the current SA AW (NIG) hub. The indicator rod varies with axle length; if a replacement is required get one with the same length in the solid part.

Axle parts are the same as in this pdf
http://www.sjscycles.com/Instructions/Sturmey_Archer/Sturmey_Archer_S_RF3_Hub_Instructions.pdf

cheers

[drossall]

Thanks. It does screw in, and indeed I can pull on the chain by hand and displace the toggle key when I have screwed the chain into place. However, in normal use, when attached to the cable, the threads are not holding.

It's pretty obvious really that the threads must be at fault, I suppose. I'll get a new chain tomorrow and try that, because it's an easy and cheap thing to do, so the obvious first step.

The other thing I noticed was that I was always taught that the shoulder on the chain lined up with the end of the axle in 2nd gear. At present, the shoulder lines up with the axle when the cable is relaxed, i.e. in third. Have things changed, or does that indicate something else?

[Brucey]

it probably means that the indicator rod is the wrong one, i.e. meant for a longer axle than is actually fitted. [The most common outcome is that the rod is about 8mm too long.]This in turn suggests that it has been replaced already, and a leading possibility as to why is that the threads are bad in the toggle key, such that a new indicator rod won't hold, or won't hold for long.

I measured up some new parts recently (to repair a hub with a similar issue) and it seemed that the tolerances on the threads were pretty slack, such that the difference between the ID of the toggle key thread and the OD of the indicator rod thread was as little as  ~0.2mm or so. A ~0.1mm reduction in both results in a slippage and fitting a new indicator rod in the worn toggle key is not a good long term solution; it is probably necessary to replace both parts.

BTW if the indicator rod used is too long, it may not be possible to select low gear easily or at all; whether this is related to the damaged thread I'm not sure, but it could be.

cheers

[drossall]

Thanks for the advice. I pass a branch of Cycle Surgery on the way into work, and had a good discussion with their knowledgeable mechanic. We looked at the rod, which did have quite severe wear to the thread, when we cleaned it up. It was indeed the wrong length; as you point out, this suggests that it has been replaced before, although the bike's owner wasn't aware of it.

I bought a replacement chain, and that does screw into place and lock as expected, so hopefully the internal thread isn't too badly worn. At lunchtime, I fitted it properly, and the gears now work