I had the same problem with an old Sears unicycle.
**** warning **** ideas below may violate your unicycle’s ability to
function at all. Also, suggestions involve using tools that are dangerous
even if used properly.
My first solution was to use a metal file to file down the flat on the axle that
the cotter pin is supposed to fit against. If the damage isn’t too bad, this can
be pretty straightforward.
(see vise clamping idea below)
Two concerns I can think of are:
- The flat must end up parallel to the flat at the other end of the axle (so
the cranks are pointing 180degrees from each other)
- The metal file must stay flush with the flat as you file. Unless you’re
conciously fighting the urge, you will end up rounding the flat by rocking
the file as you drag it to and fro.
It also helps to get new cotters. They are cheap. And a slightly damaged cotter
will work a little loose, then voila! more axle damage.
***** The ultimate goal is to have the cotter and the axle fit securely -
surface roughness on either will cause things to work loose, which will avalance
into more axle and cotter damage.
Now this solution only worked for one of my cranks, as the other crank had had
this done so many times that the axle was now too narrow. (The deeper the flat
in the axle, the farther in the cotter pin will go. If the flat is too deep, the
cotter pin can’t reach and just falls out.)
To fix the too-deep groove, I had a friend weld some metal into the groove.
Since the axle was just steel, he just melted welding rod into the groove till
it was full. (overfull). Then I re-cut the flat with a milling machine, which
was pretty quick, and allowed me to force the 180 deg alignment with the groove
at the other end of the axle.
Had I no access to a milling machine, I was going to use a metal file.
Clamp the axle in a vise with the other flat (groove) facing one face of the
vise and to it. (trap a metal bar with a square cross section between the flat
and the vise face, so that the round part of the axle doesn’t touch that face of
the vise. This will force the axle to be aligned with the flat parallel to the
vise face.) Then run the file to and fro, re-cutting the groove, paying
excessive attention to keeping the file parallel to the vise faces so the flat
ends up facing the right way. (This is the set up I used on the milling machine,
only I used a cutter rather than manually filing).
To help visualize the above suggestions, the axle ends up pointing
vertically and the wheel (which, unless you disassemble it) is level with
the ground or table.
*** Before starting any welding, make sure to practice filing metal on a scrap
piece of steel to get an idea how much effort it is and how long it will take.
You may decide it will take way too long for your patience. Also it’s not
trivial to generate a clean, flat surface, and practicing on scrap will help you
decide if you’re going to end up ruining your axle completely.
Another technique I tried involved putting flat pieces of shim steel between the
axle and the cotter to make up the extra distance. I didn’t have too much
success, as the steel shims got squashed to funny shapes. And funny shapes won’t
hold well against flats.
If you have no access to a machine shop, the best solution might be to get a new
axle and rebuild the wheel. Or just get a uni with cotterless cranks.
If anyone is actually going to try to follow any of my suggestions, feel free to
email me. I’d be happy to try to explain how to make this work in an
Martin Jaspan email@example.com