Spectrographic analysis of chassis shimmy
#1
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.
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.
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.
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.
Last edited by JasonC SBB; 08-21-2011 at 03:30 PM.
#2
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).
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).
Last edited by JasonC SBB; 08-15-2011 at 03:34 PM.
#6
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!
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!
#13
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.
I'd be curious to see if the results are affected with different suspension setups, or even just different damper settings.
#16
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.
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.
#17
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.
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.
Last edited by JasonC SBB; 08-16-2011 at 01:38 AM.