I have bar-end mirrors on my Buell 1125CR. They're fairly heavy anodized aluminum pieces that pivot on a ball & socket joint for adjusting the angle of view.
One of them fits snugly on the joint, so I can adjust it and it stays in place as I'm riding. The other is looser, so it won't stay in place - if I goose the throttle, it vibrates out of position and just dangles until I readjust it. Obviously that's not the safest thing to do while I'm riding.
Is there some way I can tighten up the ball & socket joint? I'm thinking of some sort of viscous grease that I can inject in there to reduce the amount of play. I considered threadlock or even silicone sealant, but I want to retain the adjustability of the mirror if possible.
Great solutions, everyone! I really appreciate the help.
In the end, I tried a quick fix with Loctite Blue since I had some lying around. I squeezed out a couple of drops onto the back of the ball so it would drip into the socket, and left it to cure overnight.
This worked reasonably well - the mirror no longer droops under brisk acceleration, although "flooring" it still creates enough vibration to shake it out of place, albeit not nearly as much as before. I feared that the Loctite would really sieze up the joint, but it hasn't - it still remains pliable enough to allow me to adjust the mirror.
I suspect that Loctite Green might have done a better job of wicking into the tight void between the ball & socket, so I might try that next, although the set screw idea also sounds viable and should be a permanent solution.
There's probably no "correct" solution to this problem, so I've accepted the answer with the most votes.