Suspension FFT - which damper is better?

[DaveW]

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

I think it is time to reveal more, Racer_D. For example:

Were your results obtained from track data?
What type of vehicle (e.g. F3)?
What were the spring rates (front & rear)?
Can you publish the load/velocity damper trajectories (both front & rear)?
Front & rear motion ratios?
What type of circuit?
What was the sample rate?
What was the FFT frame length (or, alternatively, what was the frequency interval)?
Did the "FFT" procedure use overlapping?
What was the total sample length (or, how many FFT's were used to compile the plots)?
Do you have amplitude (or, better, velocity) probability densities for all corners?

[Greg Locock]

Yes.

But if I refocus on the original question, and the lousy photo, then there is a simple answer. One of the two curves has a lower response at its primary resonance and is therefore more highly damped. All the HF stuff is just irrelevant to that question.

The difference is not small, or even believable. But that's the only conclusion i'd draw.

[DaveW]

But if I refocus on the original question, and the lousy photo, then there is a simple answer. One of the two curves has a lower response at its primary resonance and is therefore more highly damped.

Yes, but the modes are not necessary better damped - as first suggested by Tim, I believe.

Edit: They probably would be on a road car, but not on a race car....

[speedsense]

Wow. How do you come up with this stuff? "Magnitude of the HZ"? Really? This is a measure of movement given it's an fft of a displacement signal. Do you want to tell me how much this car is moving by the signal present, seeing how you state it's a measurement of movement....how much movement does this car have at 3 HZ?
....Magnitude as you rightfully said later in this post, is the Y axis...Magnitude in dB as is written... not the way I would set up an FFT, but the algorithm is what it is.

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..

Damping (not dampening) will affect displacement. HOW? In a perfectly normal racing shock...and not jacking the car down by over dampening rebound.


And technically the spring doesn't have a frequency, the system does. The graph is a suspension sensor measuring suspension movement, IE... spring movement. the ride change,Pitch, Heave, warp, tire influence, road influence, ETC. are not evident and cannot be measured in this graph. And in the ONLY thing that is not subjective is frequency of the movement that the suspension sensor is measuring...meaning spring movement. Lest you want to tell how much the body is moving in this graph? Or how much the tire, track is influencing the graph...
And to add why is it, I change the springs, I can change the frequency of the system, if the spring doesn't have a frequency...technically speaking of course.


What? Again, not the spring that's being damped, it's the system. You do understand that Hertz is just a unit of frequency, right? Frequency in Hertz is on the x-axis. The y-axis is not frequency nor Hertz. Yep, as I stated, the author posed Magnitude db

I'm afraid you're a bit messed up and think you may need some fundamentals review. The number of cycles is not what's being measured, you are looking at magnitudes at a range of frequencies. In this case the number of cycles is driven by the input and has little to do with the damping, frequencies, or the plot being shown.

We have been presented with an FFT with Y Magnitude DB and X in HZ...and the question which shock is better. Which I take as the question as being which one has more damping...There's a reduction in HZ @ 3,4,5 and alas 6 Hz...I would throw out anything over this, as noise, considering that this a small open wheel car... and the fact that 2 Hz seems to be the major HZ that the car is operating at and we don't know anything else... and neither do you. The HZ number is lower AFTER 2 HZ, with a shock change... the shock has more damping..is it better? What we do know from this graph is it has more damping properties, as I stated previously, other than that we know nothing else.....

[RideRate]

Do you want to tell me how much this car is moving by the signal present, seeing how you state it's a measurement of movement....how much movement does this car have at 3 HZ?
....Magnitude as you rightfully said later in this post, is the Y axis...Magnitude in dB as is written... not the way I would set up an FFT, but the algorithm is what it is.

Knowing how the signal was transformed, yes, I could tell you the displacement. The displacement of the suspension spring only, being as that is what was measured. The magnitude is some normalized displacement. The method and FFT used to make that plot can simply be un-transformed and you would be left with the original signal giving you the displacements again in time as was measured. It's just a transform so the same information lives in both.

HOW? In a perfectly normal racing shock...and not jacking the car down by over dampening rebound.

Suspension displacement is the response of the system to inputs. You change the system (daming, mass, spring, otherwise) you will change the response which will include displacement. Small under typical shock adjustment scenarios but real and possibly important nonetheless. I really don't feel this needs an explanation.

The graph is a suspension sensor measuring suspension movement, IE... spring movement. the ride change,Pitch, Heave, warp, tire influence, road influence, ETC. are not evident and cannot be measured in this graph. And in the ONLY thing that is not subjective is frequency of the movement that the suspension sensor is measuring...meaning spring movement. Lest you want to tell how much the body is moving in this graph? Or how much the tire, track is influencing the graph...
And to add why is it, I change the springs, I can change the frequency of the system, if the spring doesn't have a frequency...technically speaking of course.

The spring (suspension) movement is evident, I've already explained it. There is no "measurement" of frequency here. There is a measurement (relative/normalized) of displacement in the frequency domain (a transform of time). That is what is not subjective. You can force the system to any frequency you desire, what you are changing is the system's displacement response when excited at that frequency.

...and the fact that 2 Hz seems to be the major HZ that the car is operating at and we don't know anything else... and neither do you. The HZ number is lower AFTER 2 HZ, with a shock change... the shock has more damping..is it better? What we do know from this graph is it has more damping properties, as I stated previously, other than that we know nothing else.....

Haha, you said "HZ number" again. The car is not "operating" at 2 Hz, it may just have resonance there. It's going to "operate" at the frequency at which it is forced, excluding non-linearities and possibly free vibration.

So let me pose this as a thinking question. What would the plot look like if you only excited the car in heave with a 3 Hertz sine wave? Just a 4.5 Hertz? A 5 and 3 Hertz sine wave? A sine sweep? Track data?

[Tim.Wright]

Damping (not dampening) will affect displacement.

HOW? In a perfectly normal racing shock...and not jacking the car down by over dampening rebound.

Simple, more damping = less displacement

Yes, but the modes are not necessary better damped - as first suggested by Tim, I believe.

But Tim doesn't really know sh!t about analysis in the frequency domain. He prefers to work in the time domain and today t=friday.

[DaveW]

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.

Pull the other one. This is what I was referring to.....