HID Headlights Fluttering, Flickering, Vibrating, Shaking
I'll preface this by saying I've searched the forum before posting this and have found no definitive explanation posted anywhere, so I thought I'd share a few things I've found out about the HID bulbs fluttering over harsh bumps. All the following information comes from 6 months of studying the issue with the assistance of a Master Mazda Technician who is a friend of mine, and the help of some horrible Maryland rural highways.
It comes down to several reasons, the first and most obvious is that our projectors are NOT designed for HID headlights. I have a friend with an '08 Miata with HID & a friend with a 1st gen Mazda3 with HID. The projector housing and bulb are completely different. HID bulb shake does still happen with some factory HID's as I found out from an Audi coming towards me one night. Kind of surprised that a $100k Audi would actually shake, but it happened. And because our projectors are not made for them, the metal piece that the bulb is held by is actually suspended with rubber. It doesn't move a lot, but it is enough to have the bulb physically move.
It is also true that not all bulb bases are made the same. My assumption is the cheaper the HID kit, the looser the fit. I paid $60 for my cheap HID's (look identical to DDM's) and the bulb fits tight. There is no movement between the bulb and the projector, except that tiny bit allowed by the rubber surrounding the mount.
The other question people have had is why don't you see it with stock bulbs, and also why does it seem to always be the passenger side? Stock bulbs you will not see the vibration because the light is softer and more scattered. HID bulbs, the light is stronger and more focused, so the edge of the beam is more obvious and therefore more noticeable. That small amount of rubber in the projector allows just enough movement to be noticeable. As far as seeing more on the passenger side, Mazda directs the left headlight more straightforward than to the side as to not blind opposing traffic. Many people have noticed this and questioned how they can fix it, but simple answer, you can't. Because the light is more directed to the front and right, it overlaps the beam from the right headlight and enhances the shaking towards the right side of the car. There is also that whole thing about driving on the right side of the road, so the trees and objects off the road are closer and reflect the light closer to your view. From what we found, the shaking is not noticeable to opposing traffic, or the people driving in front of you, unless you are tailing some big ass suv, and the light cutoff is at his back window.
I hope this helps those of you out there that can't seem to get a definitive answer on bulb shake. And after 6 months of driving with HID's, you kind of get used to it.