Bars slipping in stem... help!

[Hard-Drive]

I do all my own spannering on my bikes but I have an issue that's stumped me.

On my gravel bike, when I do an off road descent on the hoods, my bars are "rotating" in the stem and the whole hood/bars/garmin assembly is lower after a rough descent. It's fine on road, it's just the jarring element that's causing the issue.

I don't get it. The bars and the stem are matched Deda items, both aluminium. The stem bolts are stage torqued properly, in an X pattern order of tightening. The visible thread is the same on all bolts. I've tried using assembly paste to increase friction, no joy. I've tried slightly over-torqueing the bolts. I've checked my torque wrench against another. I am not unduly heavy (around 80kg), and it's happening on rural bridleways and tracks, and I'm late 40s (i.e. I'm not some twenty something smashing it down a black run at a trail centre!). The bars are just normal gravel drop bars, not some crazy bend pattern, there's no aero extensions, bar bags or anything.

Short of carefully getting some 80 grit paper on the inside of the stem clamping area, I'm totally out of ideas. Does anyone have any ideas? Thanks!

[bobbo89]

You 100% sure the torquing pattern is correct?

All my (admittedly mtb) stems you tighten the top two bolts first to zero gap then torque the bottom two to the correct torque...

Edited by bobbo89 on Thursday 2nd November 14:01

[outnumbered]

I would ignore whatever the torque spec is, and just tighten them up until they don't slip...

[lufbramatt]

Have you checked the faces of the clamp and bolt holes to make sure there are no burrs or something that would be holding the clamp apart?

[Lotobear]

...you could try some loctite bearing fit on the clamping area of the bars

[BOR]

Do the bolt spin in easily by hand?

I would check that there is no old Loctite on the threads, or any misalignment that would increase the friction on the bolt as you do it up, which could mean your clamping force is too low.

Is the contact surfaces between the head of the bolt and the mating face on the cap smooth?

Are the caps themselves the right way round? It shouldn't really matter, but if they are designed to fit a certain way, then you need to fit them the same way round as they were machined.

Finally, if clutching at straws, clean threads and apply some fresh threadlock.

[lickatysplit]

get some Carbon grip paste and add that. that sould help

[Liamjrhodes]

Clean the mating surfaces before assembly and tighten the bolts up until it doesn't move, its just a case of not enough clamping force

[snotrag]

bobbo89 said:

You 100% sure the torquing pattern is correct?

All my (admittedly mtb) stems you tighten the top two bolts first to zero gap then torque the bottom two to the correct torque...

Edited by bobbo89 on Thursday 2nd November 14:01

SOME stems operate the zero gap policy but its very rare - absolutely do not do this unless clearly marked and asked for on the stem/manual itself.

For the vast majority of stems this is wrong and will be putting a bending load into the bolts.

[Hard-Drive]

Thanks guys. I'll try winding it up a bit harder...perhaps an email to Deda might be worthwhile in case there was a bad batch of stems, or if if torque settings have been revised.

Thanks again!

[profpointy]

Could put a bit of shim round the bars; aluminium beer cans being pretty much perfect. That said, they shouldn't really slip if sized correctly and done up tight enough, but better a bit of shim than stripping a thread