I got a deal on a CF drop bar, and replaced the aluminum ones that came with my bike. I kept my old stem, though, which has a 31.8 mm clamp, while the bars have a 26 mm clamping surface. I'm using shims, partly because I like the stem, and partly to spread the tension over a larger surface, which seems like a good idea for carbon fiber.

The "corners" of the bars have a textured surface, to attach the brake hoods. But the center of the bar is smooth. So, if I'm riding on the hoods, sometimes I'll hit a small bump, and the bars will rotate as much as a few inches. It's a bit scary, and a lot more annoying. Riding the tops or the drops seems to solve the issue, but I like having all three hand positions available.

What should I do to get more friction between the bars and the shim? I was thinking of taking it all apart, and wrapping the center of the bars with electrical tape.

are you sure you're tightening the faceplate bolts down to specified torque?
If you are and it's still slipping, you can scuff up the shims with some 400grit sand paper.

Please don't try the electrical tape. You'll only make the slipping problem worse. First, get some carbon assembly paste: https://www.amazon.com/Finish-Line-Bi.../dp/B0012RIEM6 Use it between the shims and the bars and the shims and the stem. Ideally, you'd ditch the shims though. They are only complicating the slipping problem by adding another set of tolerances.

Second are you using a torque wrench to install the stem face plate? You may need to be using more torque.

That's a LOT of shimming required. It's likely that you're just not able to achieve the even pressure all around on the bars due to the shim stock itself or the fact that you're not correctly able to measure the true LOADED diameter of the shim when clamped. If that's the case then you're only pinching the bars in a couple of spots and that would be the reason why they are not grabbing well.

And I just about gagged when I read your question about using electrical tape. That would produce a soft enough joint that first the bars would still move as the tape would allow them to ooze around and second you'd lose all the ridigity that you want in order to effectively lever power to the bike through the bars. It would be like they were mounted in rubber. It just will not work even as a make do for now.

Frankly this is a case of "in for a penny, in for a pound". I strongly suspect that your attempt to shim the stem to use these bars will just plain not work. And certainly in the end it will look kludegy as hell. Bite the bullet and treat the bars to a correctly sized companion stem.

Thanks, both of you! I'm not sure if I'm applying the right amount of torque ... actually I'm kind of mechanically inept. I'm a little concerned about over-torquing and cracking the bars, so I might have done the opposite.

I didn't know they made some kind of gel for this. I'll try to stop by the local bike shop tonight and see if they have any, or get it from Amazon if they don't.

Make sure you get actual carbon assembly paste which is different than just plain grease or assembly lube (though the latter would only be found in auto shops). Carbon assembly paste has some sort of grit mixed into it that adds friction, a good thing for slippery carbon fiber components.

You SHOULD be concerned. You can't get away with stuff on carbon parts that you can with alloy.

A shim CAN work but the key is that you want it to be a precise sort of thing and not just a bunch of wraps of pop can metal. The shim has to pinch the bars just right so that when tightened in the stem there is even pressure all around the circumference of the center section. You just can't assure that sort of accuracy with a generic shim or wrapping it with multiple turns of metal shim stock without some careful measuring and allowing for the inter layer slack generated in a multiple turn wrapping.

I'm assuming Seattle is using a shim specifically made for adapting 26mm handlebars to 31.8mm stems. If using anything else, please ditch it immediately and purchase a proper solution, as in a 26mm stem or proper shims like these: https://www.universalcycles.com/shopp...ls.php?id=7414

It's the type of shim JoeJack951 linked to. I got it from the fitter at a bike shop in my neighborhood this past weekend. He suggested that it would help distribute the pressure evenly across the entire width, instead of only where the stem connects to the bars. I had the same problem before I put the shim in place, when I was using a 26 mm stem, but it seemed less pronounced. I'm not really sure.

The 31.8 mm stem is very adjustable, and I haven't been able to find the same thing in 26 mm yet, which is why I'm using the setup I've got right now. I was getting sore shoulders after long ( three hour ) rides, and moving the bars around is one of the things we did to fix that.

Adjustable like this?: https://www.universalcycles.com/shopp...5&category=173

Exactly! Or very close to it - I think mine is 90 mm, but if they make them in 80, 100, and 120, I'm sure I can find one in the right size - or that I'm confused. And it's good to see they do make them in 26 mm. I'm not sure why the shop I go to doesn't have any...

Thanks!

Ah, I see. In that case it would be a matter of evaluating how well the shim fits in the stem saddle and how well it fits the bars. If it's still turning then you either have a slightly loose fit or you're just not tightening it enough because you're afraid of bruising or busting the carbon.

The shim should need to be pressed onto the bars. It should seat correctly but only with a firm push from your thumbs. If it doesn't then it's too loose. A shim of pop can metal or something thinner may be required to achieve that sort of fitting. Similarly with the shim on the bars it should take an equally firm two thumb push to seat the bars and shim into the stem. Again, if it is loose then you're only going to get solid pressure at the most fore and aft points.

I had the slipping bar problem with an alloy bar, even when tightened as much as I dared. The stem clamp inner surfaces were smooth, which I think was causing the problem. I fixed it by using my Dremel tool and a thin cutting wheel to cross-hatch the clamp surface. Now they do not slip when reasonably tightened.

Don't know if this procedure will help you, but it worked for me--without doing anything to the bars.