Riding a fixed gear is fun and good exercise, but for longer and more challenging rides being able to coast down hills is a big plus. I could have set up a second rear wheel with a single speed freewheel, or just one "flip-flop" double-sided wheel. But this hub gives the advantage of a couple of additional gears while retaining much of the elegant simplicity of a single speed setup and quick and easy interchangability with the fixed gear wheel.
Gear ratio changes are half those of the standard AW wide range hub. The actual ratios are 45/52, 1/1, and 52/45, giving changes of 15.5%. With a 46 tooth chainring, 18 tooth cog and 700x25 tires, the gear inches are about 60, 69 and 80.
The AM hubs were made from 1936 until sometime in the 1950's, described as a "club gear" and "ideally suitable for sports machines". This one was made in 1951 and has a lightweight aluminum alloy shell with 40 spoke holes - but is built into a 32 spoke wheel (every 5th hole in the hub flange is empty). Click here for a close-up showing the spoke arrangement in the hub flange. This spoking pattern is quite symmetrical - there are only two different spoke lengths.
The internal parts are similar to the more familiar AW, except for the three (rather than four) one-piece compound planet pinions, the larger diameter of which engages with the sun pinion and the smaller with the ring gear, which is smaller than the AW's ring gear. The sun gear is also smaller, and machined on the axle rather than being a separate part. All parts except for the axle, planet pinions and pins, planet cage and ring gear are interchangable with the AW.
This hub came with the relatively rare 12-spline driver that uses a left-hand threaded lock ring, but because the 12-spline sprockets are so hard to find I substituted a standard threaded driver. The more modern 3-spline driver with circlip would have worked as well, but seemed less elegant to me.
During the heyday of Sturmey Archer hub gears from the 1930's to the '50's there were several other hubs with relatively close-ratio gears made for "club" or sports riding, or racing, as opposed to the "roadster" gears made for utilitarian bikes. In addition to the ubiquitous AW 3-speed the latter included the FW 4-speed. The former included the AC close ratio 3-speed, the FC and FM 4-speed models, and the ASC 3-speed fixed-gear hub which was based on the bottom 3 gears of the FC. But the AW and the AM are mechanically the simplest, having only a single sun gear and a single stage epicyclic gear train. For more information about these and other Sturmey Archer hub gears check out Sheldon Brown's Sturmey Archer web page.
Of the above mentioned models, only the AW is still being made. Sturmey Archer now also offers 5 and 7 speed hubs, and there are a few other hub gears now available, with up to 14 speeds. But unlike the old Sturmey Archer classics, none of them are really intended for high performance road riding.
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Modified December 27, 2002