Recently, I acquired a four speed Sturmey Archer hub. I was feeling a bit bored the other day, so I pulled it apart, and had a look at the bits.
Normally, taking apart these hubs requires a lot of effort, since the force of the pedals tightens the screw threads, but this time it was easy. Quite obviously someone else had pulled it apart before me, even the bearing lock nuts were only finger tight.
Here are some pictures of the internals:
I didn’t take any pictures of the bits of the internals, too busy focusing on what I was doing. Note the compound planet pinions: the way this system works is that there are two ranges depending on which sun gear is locked to the axle, a really wide three speed and a not so wide three speed. You can get 5 gears from this mechanism (middle gear of both ranges is direct drive, which is the same regardless of sun gear selection).
The four speed hub doesn’t use all of the available gears. Highest gear is the high gear of the not-so-wide range. Changing down from second to first (lowest) gear, the sun pinions are slid along the axle to engage the low gear of the really wide range.
In order to convert this hub to a five speed, we will modify the shift mechanism so that we can move the sun pinions independently of the main clutch. That way, we will be able to select the high gear in the really wide range; the only gear not available to the four speed hubs.