OK, I'm back to report on progress
engineering a solution for this darn drone. It's been very tricky. I haven't figured it all the way out yet, but I'm making progress. The first thing I worked to do was to fix the resonating glass. If you recall, the glass resonated strongly:
View attachment 7869510
This resonance at ~60Hz was close enough to the door resonance at ~48Hz that the window was amplifying the issue. I tried putting EPDM gaskets around the glass touching the plastic trim, but it didn't do much. On to something more dramatic: I adhered a 1" aluminum section vertically to the glass. This both dampens the glass, but, more importantly, increases the resonance frequency to well above the door itself. Here's what that looks like:
View attachment 7869513
And here is the resonance of the glass now:
View attachment 7869514
Quite a difference. Note that the absolute scale (brightness) is not calibrated here, but the strength is way lower and the (minimal) resonance is now up at 80 hz. So, did it do anything the overall drone? Well, I don't have the best testing lab, but with alternating tests my guess is that it helped the overall drone about 2dB. Is that a lot? Not really. It's like turning the volume down one or two clicks. More work to go...
[Insert hours of trying stuff that doesn't do much]. Then, after two days of testing and very little progress I had a
miracle run, where the resonance was almost completely gone (down 10dB!) I of course attributed it to the last thing I had changed (lowering the pressure of the spare tire) and wondered how that had helped so much. My joy was short lived when I did another test and the drone came right back. What?!
Racking my brain, I remembered that the door might have shut a little funny for one of the tests. I decided to investigate. It turns out the the lower latch on the larger door has three positions it can "click" into and still report to the truck that it is shut. After more experimentation,
I was able to reproduce the miracle. If I am very gentle (push the big tailgate shut, hear the click on the top latch, hear the first click on the bottom latch, but no more, then shut the small tailgate hard) I can get my Grenadier into it's magical state. In this state there is only a "weak shut" on the bottom of the tailgate, but it is actually latched... Also in this state, the vehicle's drone is essentially completely gone! (Presumably due to the looser coupling of the tailgate to the latch). Here are the graphics:
Baseline, with the door shut normally:
View attachment 7869516
And "magical" with the door shut to one click on the large bottom latch:
View attachment 7869519
You can visually see how much less intense this is. Detailed spectrum analysis is even more dramatic, and shows
a 8+ dB reduction in the resonance peak (like 5 clicks of volume)! This a massive improvement and essentially completely fixes the issue.
So, I now have a way to reliably shut the tailgate that fixes
my drone issue. Will this work for you? Maybe, but this also
might partially explain why some people seem to have this worse than others--it's dependent of the exact fit of the latches and how hard you slam your doors! Speculating, the exact amount that the U bars protrude from the vehicle might affect this a lot. If they are aligned perfectly top to bottom the tailgate might be in a nice state of floating (which the latches are designed to allow). If the top and bottom are misaligned, it might preload the latches, undermining their floating design and tuning it into a braced drum.
I'm going to keep the work going. I have a tailgate table coming, and I'm going to experiment with that, and also use it as a mechanism of apply a considerable amount of constrained later damping (CLD) to the tailgate area. I don't expect much, but it's worth the testing. (
Quick update: GP Factor tailgate table arrived tonight and I bolted it on. It might have helped a tiny bit--within my margin of error. It stiffened the door a bit and shifted the resonance up 3Hz--not a bit deal.)
I'll be out for a week on a trip so no more progress imminent, but I figured you folks would want the update.