For a clamp on front derailleur, it is important to get one that matches your seat tube size. Next best to matching the size is to get a clamp that is larger than the seat tube. I can't help you if you have a clamp that is smaller than the tube.

I've given this shim a good run up and down hills and through the full range of gears on a nice long ride. If done correctly, this is a dependable shim.

Step 1: Recipe

First, you'll need some basic pieces. At a minimum, 1aluminum can, some tape and scissors. Any type of tape can be used including adhesive tapes. I ended up using 2 larger beer cans (Beamish 14.9 oz. cans).

Step 2: Cut Off the Ends

First, cut the top and bottom off of the can. This will make it much easier to the body into strips. I used some heavier scissors to make the initial cuts but I have performed the same with a razor knife. Use scissors to cut the body of the can in half cutting lengthwise of the can. Try to cut the can into 2 even pieces.

Step 3: Cut Into Strips

With the body cut into 2 halves, begin cutting each half into equal sized strips. The width of the strip will need to be at least as wide as the clamp of your derailleur. The length of the strip can be determined by wrapping a strip around the seat tube. There should be about a 1/4" (~6 mm) space between the ends of the strip when wrapped around the tube. This will save some issues once the compression clamp of the derailleur is cranked down.

Step 4: Stack and Wrap

The difference between your seat tube and derailleur clamp should drive how much of the can you use. Stack as many strips of the can as is needed to make up the difference between your seat tube and clamp. I had to use 1.5 cans but I wanted to make sure I took up some extra space to get things nice and tight.

To make clamping the derailleur to the tube easier, and for some aesthetic changes should you not want to highlight your shim, wrap the stack of strips in some form of tape. I used teflon tape (normally used for plumbing) because my dogs ate my roll of electrical tape. The white teflon also matches my frame color. Only use as much tape as is needed to hold the stack together. This will keep the shim from being spongy.

Step 5: Clamp It On

After the stack of strips is wrapped sufficiently*, go ahead with using the new shim to clamp your derailleur to your seat post. Be sure to tighten the clamp very well. A front derailleur sliding on the tube is asking for broken or damaged components.

sufficiently for me was about 2-3 layers of telfon tape. Don't use so much as to need to make a new shim. The tape is really just to keep things together.