Suspension FFT - which damper is better?

[Racer_D]

Hi there,

today in a lecture we got shown a graph that displayed a suspension FFT that looked like this



This was taken from spring travel data on one corner of the same car and same springs but with different damper settings.

Now in a discussion I had afterwards the question was: which one is better damped? And I got a little bit lost in the reasoning...
The bit up to about 2 Hz shows the same graph but after that the slope of both graphs is different. Am I correct in saying that the Red one has the better damping because it has less "energy" in the frequencies after 2Hz? I believe there was about a 10 dB difference between the lines after this 10 Hz bit all the way up to 50 Hz.

Is this better damping because the higher frequencies have less Magnitude or is the info too limited to say "better damping"?

[Jersey Tom]

Well for one I'd say that's a pretty hokey graphic for a lecture hall! Or is that just you sketching it out?

"Better" is a dangerous word. I suppose I'd say red is more damped.

[Tim.Wright]

Seems a bir strange that there are no peaks at the different wheel frequencies. Was it a real masurment or. just a sketch?

Ditto JT, the red one is more damped but more doesnt necessarily mean better. Otherwise you could replace the damper with a steel rod and youd get a perfect flat fft with zero movement at all frequencies.

[Racer_D]

Mine was a sketch from what I was told was a real measurement from one of the front springs of a small formula car.
But here's a picture taken with my mobile. Does it look unrealistic? Perhaps I should re-confirm this is not a simulated thing.


If you can't say which is "better" what can be concluded from an FFT?

[MadMatt]

Sprung and unsprung mass natural frequencies can be determined from this graph although unsprung mass is not really visible.

[netoperek]

courious, what kind of window has been put on the data

[Racer_D]

Hann

This was an example either left or right rear on two cars with same suspension settings to show they yield more or less the same results.
Here a peak is visibile at about 4 Hz it seems.

[Jersey Tom]

It's not that the curves look "unrealistic" it was just that we were trying to figure out where the sketch came from.

What can be concluded from a FFT? In this instance... I'm not really sure. Your question "Which damper is better?" - better is a subjective assessment. You tell me - what makes for "good" damping or damper performance on a car? A FFT is just a tool, one for evaluating the response of a system in the frequency domain rather than time domain. Without knowing specifically what end you are trying to achieve, to me a FFT by itself isn't very valuable. For that matter, there are times when damper performance is best evaluated in the time domain.

In any event, I feel like the only thing that can be said from the two curves in your original example is that one has more damping than the other. Can't make any sort of qualitative assessment on which one is better or worse, because there's been no criterion defined for best performance.

[speedsense]

Hann

This was an example either left or right rear on two cars with same suspension settings to show they yield more or less the same results.
Here a peak is visibile at about 4 Hz it seems.
http://i42.tinypic.com/116kinq.jpg

If the question is purely which setting has better damping, the answer is rather simple in the graph, that is, if the rest of components remained unchanged and the shock was the only setting changed and the car is being driven close to the same speeds.

The line that has less time spent in higher magnitudes has the better dampening control. The time spent at the "higher" velocities is reduced, thus there is more dampening occurring. And to define further, the shock beyond 2Hz and above is typically termed the High speed control and mostly dampening bumps, curbs etc.

You could apply a Derivative to the suspension signal in a signal plot to go along with this graph to see the bumpy areas of the track, would make it easier to define the shock control... and whether adding more dampening is making the car better or worse. Better dampening does not always mean "better" in terms of laptime..

[Tim.Wright]

Hann

This was an example either left or right rear on two cars with same suspension settings to show they yield more or less the same results.
Here a peak is visibile at about 4 Hz it seems.
http://i42.tinypic.com/116kinq.jpg

If the question is purely which setting has better damping, the answer is rather simple in the graph, that is, if the rest of components remained unchanged and the shock was the only setting changed and the car is being driven close to the same speeds.

The line that has less time spent in higher magnitudes has the better dampening control. The time spent at the "higher" velocities is reduced, thus there is more dampening occurring. And to define further, the shock beyond 2Hz and above is typically termed the High speed control and mostly dampening bumps, curbs etc.

You could apply a Derivative to the suspension signal in a signal plot to go along with this graph to see the bumpy areas of the track, would make it easier to define the shock control... and whether adding more dampening is making the car better or worse. Better dampening does not always mean "better" in terms of laptime..

I wouldn't agree with that. What we see is the the red shock has more high speed damping, but without knowing the track or any other info who's to say that its better?

You have equated less movement to better damping. My opinion is that less movement means more variations in contact patch force because the suspension is not doing its job. So if I had to choose one of the two curves, I'd go for the blue one because its allowing the wheel to move over bumps but its still not showing any uncontrolled resonant peaks.

[Racer_D]

Hann

This was an example either left or right rear on two cars with same suspension settings to show they yield more or less the same results.
Here a peak is visibile at about 4 Hz it seems.
http://i42.tinypic.com/116kinq.jpg

If the question is purely which setting has better damping, the answer is rather simple in the graph, that is, if the rest of components remained unchanged and the shock was the only setting changed and the car is being driven close to the same speeds.

The line that has less time spent in higher magnitudes has the better dampening control. The time spent at the "higher" velocities is reduced, thus there is more dampening occurring. And to define further, the shock beyond 2Hz and above is typically termed the High speed control and mostly dampening bumps, curbs etc.

You could apply a Derivative to the suspension signal in a signal plot to go along with this graph to see the bumpy areas of the track, would make it easier to define the shock control... and whether adding more dampening is making the car better or worse. Better dampening does not always mean "better" in terms of laptime..

I wouldn't agree with that. What we see is the the red shock has more high speed damping, but without knowing the track or any other info who's to say that its better?

You have equated less movement to better damping. My opinion is that less movement means more variations in contact patch force because the suspension is not doing its job. So if I had to choose one of the two curves, I'd go for the blue one because its allowing the wheel to move over bumps but its still not showing any uncontrolled resonant peaks.

Which is what you meant earlier by "Otherwise you could replace the damper with a steel rod and youd get a perfect flat fft with zero movement at all frequencies." which sounds reasonable I think.

[Jersey Tom]

Hann

This was an example either left or right rear on two cars with same suspension settings to show they yield more or less the same results.
Here a peak is visibile at about 4 Hz it seems.
http://i42.tinypic.com/116kinq.jpg

If the question is purely which setting has better damping, the answer is rather simple in the graph, that is, if the rest of components remained unchanged and the shock was the only setting changed and the car is being driven close to the same speeds.

The line that has less time spent in higher magnitudes has the better dampening control. The time spent at the "higher" velocities is reduced, thus there is more dampening occurring. And to define further, the shock beyond 2Hz and above is typically termed the High speed control and mostly dampening bumps, curbs etc

Have to agree with Tim here, and again... more damped does not mean better. To me.. it would be like looking at a spring / mass / damper system and saying the most overdamped case is "best damped."

[Greg Locock]

In addition to the above, from a signal analysis perspective I have a real suspicion of beautiful smooth curves like the low frequency curve in that data, and if it is right I'd be far more intereted in tiny differences in the 0-6 Hz range than the large differences in hihgh frequency performance.

[DaveW]

The bit up to about 2 Hz shows the same graph but after that the slope of both graphs is different. Am I correct in saying that the Red one has the better damping because it has less "energy" in the frequencies after 2Hz? I believe there was about a 10 dB difference between the lines after this 10 Hz bit all the way up to 50 Hz.

Is this better damping because the higher frequencies have less Magnitude or is the info too limited to say "better damping"?

Other than to observe that the response at frequencies < 2 Hz are probably driver-induced, I have nothing to add to the comments of JT & Tim. Madmat and netoperek were right to question the lack structure in the results.

The second screen shot you published added information. That showed, at least, that the rear dampers were doing something to control the heave mode (which you stated peaked at 4 Hz). That made me think that perhaps the subject was an "aero" car. If that is the case, then the front suspension is likely to have been buried in bump rubbers for most of the lap. It would explain the lack of structure in the front damper response, and would imply, perhaps, that the FFT of the front suspension was less meaningful than it might have been.

[speedsense]

Hann

This was an example either left or right rear on two cars with same suspension settings to show they yield more or less the same results.
Here a peak is visibile at about 4 Hz it seems.
http://i42.tinypic.com/116kinq.jpg

If the question is purely which setting has better damping, the answer is rather simple in the graph, that is, if the rest of components remained unchanged and the shock was the only setting changed and the car is being driven close to the same speeds.

The line that has less time spent in higher magnitudes has the better dampening control. The time spent at the "higher" velocities is reduced, thus there is more dampening occurring. And to define further, the shock beyond 2Hz and above is typically termed the High speed control and mostly dampening bumps, curbs etc.

You could apply a Derivative to the suspension signal in a signal plot to go along with this graph to see the bumpy areas of the track, would make it easier to define the shock control... and whether adding more dampening is making the car better or worse. Better dampening does not always mean "better" in terms of laptime..

I wouldn't agree with that. What we see is the the red shock has more high speed damping, but without knowing the track or any other info who's to say that its better?

You have equated less movement to better damping. My opinion is that less movement means more variations in contact patch force because the suspension is not doing its job. So if I had to choose one of the two curves, I'd go for the blue one because its allowing the wheel to move over bumps but its still not showing any uncontrolled resonant peaks.

Respectfully disagree with your comment...the graph is not a measure of movement, but a measure of HZ cycling of the spring, as the graph is in HZ, and the graph measurement of that in Frequency... lower hz numbers MEANS the dampening is lowering the Magnitude of the HZ...And I did not say one or the other is "better", just that one has more dampening, with "better" meaning more dampening, not that the setting itself is better,,... which I believe the "question" was about

From this graph, we cannot determine movement, not without a signal plot showing wheel movement differences..

Changes in dampening will not change the movement amount unless there was a "limiter or droop limiter" in the shock involved, it will change however the frequency that the springs cycle has, thus in turn lowering the hz operating number and the stated Magnitude..

To explain this further, an under damped spring will overshoot and cycle more times, thus increasing the frequency and the Magnitude of the frequency, over dampening creates the exact opposite effect, Less cycling, Less frequency in HZ.. IMHO