Spectrographic analysis of chassis shimmy
 

I bought this $20 3-axis acceleromter:
http://www.adafruit.com/products/163

and this $29 4-channel USB data acquisition box:
http://www.dataq.com/products/startkit/di145.html

I attached the accelerometer to middle of the top hoop of my rollbar. I started a datalog, then I drove over 6 speed bumps in a parking lot and made a U turn to hit them again. I then stopped, got out, loosened one end of the STB, and repeated the drive.

I then converted the datalog from the left-right axis (the main mode of the miata, torsional vibration), into a WAV file and loaded it into spectrographic analysis software. See below.

https://www.miataturbo.net/attachmen...1&d=1313425425

Left to right is time, (total is about 240 sec), bottom to top is 0 to 30 Hz. The bright columns are each speed bump. The bright red spots are at about 16.2 Hz, which is the chassis torsional resonance. The left half (first 2 groups of columns) is with a tight STB, the right half, is with the STB bolts loosened.

(Due to a quirk of the spectrographic software I had to tell the conversion software that the log is 600 Hz and not 60 Hz sampling, so the frequencies are multiplied by 10 and time is shortened by 10. )

I expected loosening the STB to show a decrease in torsional resonant frequency signifying a decrease in torsional stiffness, but instead what I found was an increase in the strength of the "ringing" (red spots are brighter), and an increase in 'Q' (duration of the ringing), evident as larger islands of red.

The spectrogram software I used is this one:
http://web.archive.org/web/200011180...ams/gram50.zip

It's no longer availalbe on the original website and I found it on the wayback machine (hooray).

what STB are you using in this comparison?

mazda 01+

interesting so it didn't change the stiffness but it changed the damping?

My theory for now is that the engine bay has a resonance that is close to that of the body torsion, when the STB isn't there. Maybe the engine bay vibrates at said resonant frequency and then the body vibrates sympathetically. With the STB, engine bay resonance is moved up, and so it doesn't excite the main torsion resonance.

Mazda wouldn't add a large hunk of metal if it didn't improve NVH or otherwise *do* something.

That the resonant freq isn't changed, but is less prominent, is what my butt says. In contrast, the door bars, and FM butterfly, raised the resonant frequency. But no, I'm too lazy to go try disconnecting the door bars!

BTW I compared the plots also from just driving to and from work with and without the STB connected, with similar results.

BTW played back at 10x speed the chassis resonance sounds like a drum!

Love it!

Go Jason, Go Jason!

I use Spectra for some analysis along with audio design related software. It would be nice to see your data portrayed or displayed in different ways.

Try loading the zipped wav file into it.

This is on the top loop of the roll bar? I'd say they're impressive results for being so far away from the change you're making.

Yes because I figured that would be a good point to measure torsional vibration, and it's easier to tiewrap the accelerometer to it than to the windshield header!

Awesome experiment! Interesting results and analysis too, which I think is spot-on (I'm a mechanical engineer with a *little* bit of vibration experiment experience).

I'd be curious to see if the results are affected with different suspension setups, or even just different damper settings.

The stiffer the dampers' high speed damping, the more the chassis resonances will be excited
And also, the closer you are to the stiff part of your bumpstops (the more you hit them).

take one for the team! :laugh:

How did you determine that the peak around 16Hz is a primary torsion mode? I'm interested in reading any documentation that you have found on the subject. Do you know if anybody has performed any sort of modal testing on a Miata (simulation or real-life impact)?

Assuming the 16Hz peak is a torsional mode, I would expect it to be the torsional mode along the major axis of the frame. If so, the STB shouldn't affect that mode at all since it is tying 2 points together perpendicular to the frame axis which adds zero stiffness and pretty much zero mass.

16 Hz is the strongest vibration frequency of the left-right axis. A long time ago testing with a borrowed accelerometer, this axis had the strongest vibrations, at the top of the rollbar. I suppose if I had a 2nd accelerometer, I could look to see if the left-right movement of the rollbar and the windshield header are in opposite directions.

Next time you are going over a road where the car shimmies, place a hand on the top of the windshield header, or the top of the rollbar. The left-right motion is the most pronounced.

P.S. I just examined the other axes' datalogs and they don't have anywhere near the amplitude of the left-right axis.

And see my theory above about sympathetic vibrations of the chassis, from the engine bay.

16hz, give or take a little is the frequency of wheel/tire rotation at 65 or so mph. I think you just found the 65mph shimmy and conclusively demonstrated that it is related to chassis resonance.

So would adding FM rails and stiffer shocks/springs move the resonance point up or down? Seams like my 65mph shimmy it at about 70-75 now. Keep swearing I have a tire out of balance even tho the tire shop swears I don't. Just curious, with I could get data like that.