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Creating a Front Mech/Derailleur Shim (31.8 to 34.9)… From Beer Cans!

Like many things with push bikes, there’s a part that you’re missing and there are a huge number of ways to fix it. They are usually:

  1. Buy the part made by the original manufacturer (a set of shims is £4.99) for more than you paid for the main item (I paid £4 for the derailleur)
  2. Bodge something else to make the part (I tried an inner tube first)
  3. Have a quick google for suggestions and make something someone else already has.

Option three is normally the best and cheapest method of doing this.

Here’s how I made mine:

Firstly, I took measurements of the circumference of the seat tube (diameter of 31.8mm) and subtracted 10mm for the compression created by the band: This is about 95mm.

The mech band is about 15mm in height, so the shim should be about 20mm tall so you don’t have to be very careful about position, but only protruding 2.5mm from top and bottom.

If we half the difference between the two diameters we want, we get 1.55mm for the thickness of the shim.

So, here’s the list of supplies I used (and you’ll probably need):

  • 2x 440ml beer cans
  • Rule
  • Electrical tape
  • Scissors
  • Sharp knife
  • If you’ve got one, and can use it: a micrometer

Steps to create the shim:

  1. Cut the top and bottom from your cans. Use the sharp knife to create the initial cut, and then the scissors to create an edge with less burrs.
  2. Cut the resulting large cylinder length ways and you should end up with a sheet of metal about 200mm x 120mm.
  3. At this point I measured the thickness with a micrometer to work out roughly how many layers I would need, and it came out to 0.11mm thick. Assuming the electrical tape that we will wrap it in is 0.05mm thick on both sides, we need about 14 layers to create a shim 1.53mm thick.
  4. Along the long edge, measure 95mm intervals and cut down them, this should make 2 sheets of 95mm x 120mm with a little excess.
  5. Then on each of these mark 20mm and create 6 strips that are 95mm x 20mm x 0.11mm. Keep doing this until you have 14, and discard the rest of the can. Measure the thickness of this stack (carefully, it’s sharp!) and it should come out about 1.5mm (because they’re not perfectly flat)
  6. Then cut two small strips of electrical tape to cover the ends, and stick them on. This should hold the layers together.
  7. Then start wrapping the whole shim with electrical tape and overlap the layers a little so nothing is exposed when it’s bent/stretched.
  8. You should have something that looks like this!

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