jezmellors

Hello all,

is it permissable to use a "coke-can" shim between a carbon steerer and a carbon stem, they both say they are the same size but it's a bit loose and i don't want to over-tighten the carbon stem.

THANKS

Airburst

Anything that's clamping on a carbon tube has to be quite precisely made to ensure that it puts the tube in uniform compression rather than risking crushing it. I doubt a shim made from a can would be that precise. If you're having trouble with carbon stuff slipping, carbon assembly paste is a much better option.

FBinNY 

The issue with carbon bars isn't clamping force, it's distortion. Quality stems for carbon bars are dimensionally very accurate, and quality carbon bars very round at the clamp, so the curvatures match exactly, and clamping force doesn't ovalize the bars which leads to cracking.

If you need a shim (not if you just think you do) something is already very wrong, and the risk of the bar cracking from clamping pressure is very real. Adding a shim won't address the match in curvature, and if it was right all along would mismatch it.

So whether you need it or not, using any kind of shim on carbon bars is asking for trouble, and I recommend it against it. You should be able to achieve a solid grip with reasonable clamping pressure, and the match of the curvatures will prevent distortion and cracking.

If you feel it's as tight as you're comfortable and still not secure, the better solution is to use traction compound between the bar and stem. My favorite is lapping compound (grit in grease) which when clamped between two surfaces bites into both mechaincally locking them together at low clamping load. Note, if you do use lapping compound, position the bar exactly before clamping, and loosen completely to move it. Tweaking a lightly clamped bar a few degrees will score the bar, and the grit cuts into it.

fietsbob

Feel? Get measurements to the nearest say .001mm so you can say How much difference exists.

mconlonx

No.

jezmellors

Thanks very much for the replies guys, i'd never heard of assembly paste, that sounds like exactly what i need.

I just ordered some made by Tacx in a sachet, is there anything i should know about using it or is it just a case of smear it on both surfaces and tighten it up?

jimc101

So what brand kit are you using? if it doesn't fit, send it back, rather than trying a potentially life threatening bodge.

For the paste, this is a friction paste, not a filler, other brands like Finish Line are also available.

Kimmo

Guys, OP said between stem and steerer, not stem and bars.

I'd be aghast at a bar shim, but a steerer shim seems much more doable IMO...

If there's room for it. 0.2mm out would be pretty awful manufacturing tolerance.

Airburst

It's still a carbon tube, and no less vulnerable to failure if it's in a less-than-excellent clamp. Not saying it can't be done, more saying it'd not just a case of cutting up a can and wrapping it round the steerer.

Kimmo

A carbon tube within a carbon tube, at that.

I'd guess that if the gap is close enough to the thickness, a nice job is only just possible...

It'd have to be cut to quite exact dimensions, and it'd likely be a good move to sand the sharp corners off the cuts. Extreme care would be required to correctly position it.

FBinNY 

You're right, but the same rules apply, maybe more so. Steerer size has been standard for years, so I wouldn't trust anything that couldn't give a good fit here, especially if it were off by enough to allow room for that fat a shim (coke cans are pretty thin, but way beyond acceptable error for this job). Either the OP is worried about nothing, or the stem or steerer are way off the mark. Either way a shim is a no go.

jezmellors

All sorted, thanks guys. i used the Tacx assembly paste and it's rock solid with a lot less tightening.