[smilinsteve]

I might buy a new stem, and most of the ones in the shops these days seem to be for 31.8 bars.
My next bar might not be for a while, but will probably a 31.8, so I'm thinking maybe I should buy a stem for a 31.8 bar and use a shim for my current 25.4 bar until I get a new one. Is there any problem with that?

 

[norcom]

I'm using a shim for my On One Mary cromo bar with a 31.8 stem. It's a little bit of a pain to adjust the bar as the two shim pieces move around; I have a two piece shim. Otherwise I found no problems.

I wouldn't use a shim with a carbon bar. You might not get the exact same force around the bar with a shim.

 

[kiwi_matt]

I talked to a mechanic at my lbs about this not too long ago. He didn't recommend using a shim because they didn't grip as well. But if you're not doing jumps and drops etc it shouldn't be too much of an issue.

 

[smilinsteve]

I wouldn't use a shim with a carbon bar. You might not get the exact same force around the bar with a shim.

I do have a carbon bar. I have been thinking about stress risers at the radial and axial shim edges, but a stem clamp also has those edges, so I'm not really sure it would be any different.

 

[norcom]

I do have a carbon bar. I have been thinking about stress risers at the radial and axial shim edges, but a stem clamp also has those edges, so I'm not really sure it would be any different.

If you use the two piece shim LIKE THIS ONE it's hard to align it perfectly; that's the shim I use. It will sit a little crooked no matter what. I tried to put the spacers so the slits align with the stem but they move once you start tightening the bolts. And then you have to adjust the bar, so it's even more of a pain when everything starts moving.

If you can get everything to align perfectly I'm guessing it would be fine. But with carbon you better be sure. This is my first shim and from my first epxerience I'm just saying I wouldn't use it for carbon.

edit:
Contact your bar manufacturer and see what they say about shims. That could put your mind at ease.

 

[smilinsteve]

Could you put a rubber band or tape around each end of the shim to keep it from moving while you are adjusting and tightening the clamp?

 

[smilinsteve]

It looks like this shim has a groove and O ring to help keep things from moving around: