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. Alloy AMRecall 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 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.

new am box side thumb
new am and box thumb
new am and box thumb
new am whole hub thumb
new am cablestools thumb
new am cablestools thumb

Thanks to Mike Perkins for the photos of the NOS AM hub and fittings.planet pinion timing thumb 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.

am in tsr rear thumb 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.

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.