AM 3 Speed Medium Ratio

The AM 3 speed medium ratio hub was introduced by Sturmey-Archer in 1936. The thirties had been a productive decade for the company and the AM was introduced with the ubiqutous AW and the ultra close ratio AR hub gears. Recall the 1950s ladies Raleigh touring bike found on the streets of Newcastle on the Sturmey-Archer hub gear page? This alloy AM hub was spotted by my brother Geoff's eagle eye and he stoically carried the abandoned bike back home. Looking at that old hub, you might be forgiven for thinking that medium ratio sports hubs were a long gone technology. No chance of ever finding one in as-new condition. NOT SO! A new-old-stock Sturmey-Archer AM with all the parts came up on ebaY recently and it was too strong a temptation for my brother Mike to resist. In terms of discoveries, for hub fans it really doesn't get any better than this.

Thanks to Mike Perkins for the photos of the NOS AM hub and fittings. The AM is an incredibly tough design, being based on the AW but with the AW's four single planets replaced by three compound planets rather like the FWs. Later AMs had a two piece axle key with an indicator rod flush with the end of the axle in 2nd gear. But you can replace the axle key for a threaded version from an AW and use the freely available AW toggle chain.

The most important thing to remember if you are rebuilding an AM is that, like the FW, the compound planets must be put in in the same relative positions. This is called "timing the planets". They have a little tick mark on one gear tooth and should be fitted as shown right.

Direct drive in the AM is 2nd gear with a step down of 13.5% and a step up of 15.5%. When I tried an AM on my Moulton TSR, it felt to me like it had hardly changed, but it was too wide for some clubmen from the mid 1930s to the mid 1950s. To satisfy this demand Sturmey-Archer made the AC close ratio and also the ultra close ratio AR hubs.

The slideshow below shows the stripdown of an AM. Click on the first photo to start. The hub shown is actually one that I have aready rebuilt for the Racing Midi. It was a 40 hole and I only had a 36 hole rim. "Two hub holes missed on each side; what difference can it make?" thought I. The answer was, quite a lot. For one thing, all of the spokes seemed to be either very long or very short. For another, the rim just wouldn't stay true. I had to true it at least five times and it still went slightly buckled. There is a white plastic stopper on the axle that I used to try to prevent grease coming into the hub from the left side wheelbearing. That was just an experiment and normally wouldn't be present.

This is a typical 1950s alloy AM from a top spec. sports bicycle. Them were the days.

The illustrious names of jounalist Sturmey and name-signer Archer.

The hub has a newer driver for a better choice of small sprockets.

Note missed holes. This is a 40 hole hub in a 36 hole rim. We don't need to remove left cap.

15mm spanner fits nuts and usually the cone locknuts too...

However I fitted newer locknuts which require a 17mm spanner. Start on the left side.

These cone spanners are available on ebay and help to fit in the small gap under a washer.

With the locknut off, lift off the anti-turn washer. Sometimes this is just a plain washer.

Next unscrew the left hand cone. Note the U channel ring which is a dust trap.

Cone off. Not the shiniest bearing surface but it felt fine in use.

Nice clean grease in the bearing. I made an attempt to stop the grease being washed into the hub by the oil.

I'm jumping the gun a little. It would be best to take the entire mechanism out of the shell next.

Nevertheless, let's just go with it. Take off the cone locknut.

Here it comes. Sorry for the tatty skin. Note the square locking tab below.

Remove the square locking tab. Make sure this is refitted to the right hand side.

Now I realise that the mechanism must be removed. Knock the ring with a hammer and old screwdriver to unscrew it.

The ball ring has a twin-start thread. You can mark it and the hub before removing to ensure it goes back in the same place.

Withdraw the mechanism from the hub. It is surprisingly not very oily. The old steel caps do leak oil more than plastic ones.

Here is the plastic guard I added to keep the grease in the left wheelbearing. It worked well.

The hub has been well used but the planet pinions look good. Plenty of life left.

Hub shell has almost no oil left in and I was generous when I built the hub. Note that grease has stayed in the left bearing.

Use a Sturmey Archer cone spanner (if you have one) to take off the right hand cone.

Now lift off the driver. This would have been from an AW, but it is a identical to the AM's apart from the wire ring instead of a thread.

Next remove the ball ring. I won't bother with taking the bearing balls out. See the AW stripdown for this.

Here is the view of the mech. Grease has come from the wheel bearing onto the spring. The spring cap goes on the outside.

Lift off the gear ring- often called the annulus. Note the interesting alternating projecting dogs inside.

Take off the clutch spring. May as well leave the collar in place on the spring.

Lift off the thin washer that protects the end of the spring from wear.

Remove the axle key collar....

... followed by the axle key. This is a replacement from an AW in order to use an AW toggle chain instead of the earlier two piece rods.

Remove the clutch. This component is very hard and usually remains undamaged no matter what age the hub is. William Brown's innovation.

The planet timing information from Sturmey Archer. This is necessary because the smaller cogs have a particular orientation to the larger.

Note the tick markings on the planets. There is some wear on the planet cage dogs from pressured changes. Not too bad.

View from above. The high gear dogs are a little chewed but no ill effects were felt when using the hub. Three planets like the FW.

Lift off the axle key sleeve.

Invert the axle and rotate the planet cage on it. The panet axle pins will slide out.

All planets removed. We could remove the low gear pawls next if wanted but they are in good order so I will leave them be.

View of planets, axle and cage. None of the mechanism is very well lubricated which shows the importance of topping up regularly.

Slide the planet cage rightwards off the axle. That is as far as we need to go. See the AW page for any more stripdown information you need.

Here are all the parts laid out in order along the bench. You were doing this as you went along weren't you?

And reorganised to give a closer view. Well done if you got this far. Reverse the process to reassemble the gear.

Once you have completed an AW assembly, you will easily be able to put an AM back together. Or just step backwards through the slideshow above.

Hub Gears

AM 3 Speed Medium Ratio