A glance into a Sturmey Archer FW hub

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.

5 thoughts on “A glance into a Sturmey Archer FW hub”

  1. Highest gear is the high gear of the wide range, not the not-so-wide-range. You can’t convert this hub to a 5 speed unfortunately because the dogs on the sun are ramped on one side. This is the side that the 5th gear would need to press on, therefore it would slide out of gear.

    1. I should say the highest gear available in the original 4 speed hub was the high gear of the not so wide range.

      Although some FW hubs may have had the dogs on the sun gear ramped on one side, that actually isn’t the case on the version that I have. As a matter of fact, I’ve been using it as a 5 speed hub for four years now.

      The hub that I have dates from 1959, which I believe was approximately 10 years before Sturmey Archer produced a 5 speed hub on the same shift/control arrangement. I’d be interested to know when your hub dates from.

  2. Hi I have a FW that has the ramped dogs ok K408. I believe its a 1965. I’m working on converting it into a super wide ratio three speed that uses the commonly available AW indicator rod and shifter.

    1. Well, that’s interesting. According to Denis Watkins, in later versions of the FW hub the dogs on sun gear K408 were ramped, which hinders conversion of such hubs to 5 speed. The ramped dogs would freewheel forward in the highest gear of the 5 speed hub.
      (source: https://hadland.wordpress.com/2020/04/19/converting-the-sturmey-archer-fw-4-speed-hub-into-a-5-speed/)

      There are a few other options you may consider:
      1 – Acquire a non-ramped sun gear (although they may be rare), which is the by-the-book method.
      2 – Use as a 3 speed with AW axle key and indicator chain. Mine would be a close ratio 3 speed, as it’s the close ratio sun gear engaged by default.
      3 – Use as a 4 speed (AW indicator chain on the sprocket side, and push rod & bell crank on the other); you would simply need to avoid using the no-drive high gear of the wide range
      4 – It may be possible to remove the ramps from K408 using a milling machine or similar, then you can use it as a 5 speed (same control arrangement as option 4).

      1. I’ll send you pics WHEN I get chance to work on it. The conversion involves machining a collar to lock a ramped K408 to the axle dogs so it cannot ramp away under the reaction torque when the planet cage is driving the gear ring. I thought lots of FW hubs must have been scrapped due to not being able to convert the ramped K408 to 5 speed and the scarcity of 4 speed triggers. Currently in the UK we’re in home school hell due to kids being constantly at home from school lockdowns so I haven’t had much chance to work on it and I have been obsessing about this idea for three months.

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