This drone continues to drive me a bit crazy, as I just feel like I'm driving around inside a drum in my new Grenadier. I've owned a good number of cars, but never one with this issue.
So, I've done a bunch of investigating for us. I have a background in acoustics and engineering, so bear with me, this will get a bit technical. TDLR: I have good news and I have bad news: The good news is that
I think I know what the problem is (not what anyone thought!), the bad news is I don't have a good way to fix it yet. OK, let's get into it...
First, for the doubters, this is a spectrogram of my Grenadier
driving down a road at 65 mph and showing a very strong drone. The Y axis is frequency in Hz and the X axis is time (time is not super important, we're usually looking at like 30 seconds of driving):
Yes, that bright yellow line is
a pronounced resonance at ~48Hz. A low bass drone.
Also, this is not only a 65mph thing (though it is worst there for reasons I will explain later). Here is a Grenadier
driving down a road at ~25 mph:
As you can see, the same fundamental resonance at ~48 Hz is there
even at low speed.
Well, the roads don't have lots of 48Hz noise by themselves--so something in the Grenadier is amplifying this frequency. (I verified this by analyzing interior frequency spectrums produced by banging the frame and suspension in various places with a "impulse hammer", i.e. rubber mallet, and seeing the same effect -- yes this is how they do things in industry.)
So, I set out to figure out what could be causing this 48hz resonance. This is tricky business because sounds this low are basically impossible to localize by ear. My first order of business was testing
air resonances.
This shows me doing a bunch of clapping (generating an impulse) and seeing what the resonant frequencies of the cabin are. There might be a bit of a resonance at ~85Hz, but nothing pronounced. So, cabin acoustics are not the main culprit. Next up was investigating structural resonances. I won't bore you with the graphs for the dozens of tests I did, but I analyzed the spectrum of tapping every panel in the inside of the vehicle, searching for the resonance source. Roofs seemed suspicious, but were too high frequency, doors were fine, heat shields were a non-issue, etc. Most tests looked like this one (of the
driver's left roof panel), or less interesting. (The vertical lines are individual taps, if you were wondering.)
Looking at all the data I collected, I found only one culprit:
This is the tailgate. It is large, flat, and vibrates strongly at ~45-50 hz. It's basically a giant speaker, and rings at
the same exact frequency of the drone we see in the vehicle.
The tailgate is causing the resonance. You can experience this yourself by locking yourself in the trunk area and banging the middle of the door. You will hear a pronounced drum effect ringing in your ears. You can also have someone else do it as you sit in the drivers seat--same thing.
Which leaves one last mystery: Why 65 mph? The explanation must involve something on the car that tends to generate 48Hz vibration at that speed (which then gets amplified). Again, long story short, I calculated all of the natural frequencies for the drivetrain at 65. Tires, engine rotation, exhaust, etc., none were even close to the right frequency. Except the driveshafts. Using the axle ratio and Bridgestone's data on KO2 revs/mile, the driveshafts (and associated gears) work out to generate a
48.6 Hz rotation frequency at 65 mph. Yup the same exact frequency.
And that's not the only piece of evidence... If you look closely you can see a "pulsating" effect every few seconds at 65mph that you don't see at lower speeds... (And, just for fun, yes, this graph was taken with the car in 7th gear at 65mph, showing that doesn't change much of anything.
Without going into math, this pulsating is highly indicative of
two vibration sources that are vibrating at very close to the same frequency. And to further implicate the already obviously guilty driveshafts, there are indeed two of them... Front and rear.
So, to sum up:
The Ineos has a highly resonant rear tailgate that rings like a drum. It resonates all the time, but is most noticeable when going at ~65mph when the driveshaft vibrations stimulate the resonance.
So, the next step is figuring out how to reduce the problem. I worked on that a bit, but it seems like it will be tricky. Ideally, Ineos would have built a tuned mass damper into the tailgate to mitigate this. (Porsche did this with early Caymans which suffered a similar issue.) Anyway, at least we have reasonably high confidence about what is going on. So, no, bending your heat shields will not do anything about this, and no, sadly 7th gear at 65 won't either. Driving around with the tailgate open will
(It's possible that applying a bunch of constrained layer damping material on the tailgate could help, but it's hard to say as it's a low frequency we're dealing with.)
I'll let you know if I figure anything else out. - Dave
!
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