Geared Unicycles

[M.G. Rushton]

I wrote a while ago about my plan to make a hub gear which I could use on a
unicycle. Well I have made a hub now out of two old Sturmey Archer Hubs the only
problem is that the hub has 40 spokes and I am having trouble finding any 40
spoke 20" rims. Does anyone know where I could get one preferably an alloy one.
If these are not available is it possible to get undrilled rims and where from.

Mark Rushton

[M.G. Rushton]

James, Converting the Sturmey Archer hub gear was a lot harder than I thought it
was going to be but here is how to do it anyway.

You need to start with an old four speed hub, anyone will do but
preferably not the dynamohub as this requires an old three speed hub as
well. In principle all you need to do is to lock the planet cage to the
outer shell of the hub and remove the pawls from the annulus, this will
give you three fixed gears. This is not good enough though as the
shifting is very dodgy and there is 60 degrees of backlash in the bottom
two gears, also if you started with a FG or FW hub you will only be able
to load the gear in one direction in bottom gear.

To cure the bottom gear propble all you have to do is to deposite some
tool-steel on to the dogs of the large sun to convert the ramped
sloping side of the dog so it resembles the vertical face, then grind
to final shape.

To cure the shifting problem you need to add a quarter of an inch long
tube to the none chain end of the indicator rod on the narrow none
threaded bit this should put your gears in the right place. Then
measure the lengths of nesesscary to be in each gear and modify a
trigger to do this.

To cure the backlash you need some serious engineering equipment but
here is how to do it. Grind off the old pawl holding ring on the annulus
Then grind out the old dog teeth inside, having done that use a tipped
boring tool to lathe out the inside a few millimeters. Now lathe
yourself a new blank to replace what you have just removed. Now all you
have to do is to get someone with a CNC wire-cutting machine to cut out
a new dog tooth profile this time with eight dogs instead of the
original four this peice then needs to be welded to the annulus and then
ground so it fits comfortably inside the ratchet ring. Now heat treat
the annulus to harden it.

Now you can reassemble it and you should have a three speed backlash
free hub with two different sized reductions and a direct drive top
gear. My annnulus is at this moment being heat treated so it should all
be finished next week and when I find a rim it will be on my giraffe
very soon afterwards. This is a 6' giraffe so not very practical on the
roads so in the summer I plan to build a low rise carbon fiber giraffe
hopefully in time for the unicycle convention in August.

Mark Rushton.