..Do they actually hold the light steady enough for singletrack riding?
I don't want to be messing with re-aiming my light at the same time as trying to make my way over rocks / fallen trees / etc.
I'm used to a nice niterider clamp assembly, and seeing these o-ring mounts on even the higher end stuff puzzles me a little bit.
Man, I don't know what kind of single track you are on - but I was surprised by how easy it is to use several strips of old inner tubes to lash up any light I want to use.
I've clamped on several styles of lights using both side by side as well as front to rear lashing - either works well.
The key is knowing how tight they need to be and providing a "non-slip" tubing surface, not bare metal.....
O-ring mounts like the Lupine and MagicShine work perfvectly. The part that rests on the bars should have a piece of rubber on it and the rubber on the O-ring will help keep it steady. Unless you build a DIY boat anchor, they will not move with normal singletrack bumps, jumps, and drops. If you use some sort of O-ring mount with a plain old flashlight...then you're asking for trouble b/c the light is weighted off center and there is no rubber thing to contour the bars.
Should be fine. (I've never had any trouble widdit ballin my Komodo/Pike rydah over logs and such.