Suspension FFT - which damper is better?

[Jersey Tom]

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... which the "question" was about

Thread title - "Which damper is better"

And within the first post...

Now in a discussion I had afterwards the question was: which one is better damped?

Your reply...

If the question is purely which setting has better damping, the answer is rather simple

Surely you can see how it could come across that your first assertion was that one was in fact better than the other.

[Tim.Wright]

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

I'm not following you here... A frequency plot such as the one shown is simply displacement vs frequency. The displacement can be represented in dB or mm but its still movement. I think you've got a pretty fundamental misunderstanding of what a frequency reponse is actually representing...

[Greg Locock]

Which does lead me to a question. The y axis is dB. dB Force? dB mm? dB m/s? dB m/s/s?

As it happens I have lots of real world data for suspension forces and travel, so if the OP will nominate a y axis I'll post REAL data showing why I think that curve is produced by bad signal analysis. The other alternative is that it is an FFT of a single bump event.

[mep]

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.

Wait....
The bode plot is showing the response of a body relative to its input, -yes?
In other words it shows how well the body follows the road inputs you give on the tire.
When you fit a solid link instead of a damper, then you still have the tires as spring. The sprung mass is basically added to the unsprung mass by the solid link, which creates a single degree of freedom model.
But due to its high inertia the tire will struggle to follow the road inputs at increasing frequencies hence it will not generate a perfectly flat FFT with zero movement at all frequencies!
Comments...

[Tim.Wright]

You are right in that a solid suspension will follow the road badly due to the tyre, but the FFT in discussion is taken from damper data not laser body movements.

So a solid suspension would give a zero input for the damper FFT but not the overall ride height FFT

[Greg Locock]

we still don't know what that is a graph of.

[Jersey Tom]

First post indicated it was "taken from spring travel data."

[netoperek]

Well, can't decide which post to reply, so I'll try to make some general comments.
First, "flat" FFT can by observed only on a perfect white noise signal. When there is a completely static signal probed, there will be one spike at 0Hz.
About measure of FFT. It is (or at least should be) normalised tool! It does not have an unit. Its not in mm or degrees.
What it does tell us, is where the energy lies. Specifically, if there is a single harmonic in the signal, there will be one peak on the FFT, assuming the transformation has been made including the exact frequency. If the probed signal is the superposition of 2 harmonic functions, carrying the same amount of energy, there will be two equal peaks on the plot, with the same assumption as made earlier, AND assuming the sampling meets the Nyquist-Shannon cryteria, AND if there is no windowing performed on the signal (hence my previous question). When the window is put on a signal, the energy from each bin, leaks into the nearby bins (frequencies). What we really want to see - where exactly the resonanse is and what is its energy is blurred.
FFT is just a tool, the most important questions are not about the deflection of the tires and so on, but what exactly the data represents. How the test was made, what was the object excitation, was the data processed, and how, etc. Without that knowledge we know absolutely nothing from the plot.
If its just a FFT from bare measured signal, I'd guess that what we see on these plots its some unknown input put through 2 different filters - thats how IMO we should treat the whole package - car, its suspension, mass, tires, etc. Without the knowledge of the input we cannot even estimate the characteristics of these filters, let alone trying to compare them.

[DaveW]

Forgive me, netoperek, but there is rather more to understanding the "FFT" process. It is actually a Digital Fourier Series estimator and achieves its remarkable efficiency by assuming the source data is exactly periodic. "Windowing" in that context is a very useful tool. This might help, see the section labelled Leakage.

[Greg Locock]

OK, sorry I missed that. Now I'm interested in knowing what transducer fitted to a car could give 0-80 Hz displacement data.

Anyway, I'm guessing that was taken off a shock dyno, with a single step input.

here's some real road data from a 25 second event.

http://files.engineering.com/getfile.as ... velfft.png

Frankly i don't believe anything on that above 10 Hz, but then my models are only good for 6 Hz so that's all the data i need.

[Jersey Tom]

OK, sorry I missed that. Now I'm interested in knowing what transducer fitted to a car could give 0-80 Hz displacement data.

Not uncommon to log linear pots say on coilovers at several hundred Hz, no?

[Greg Locock]

I don't know, I haven't used them. We use stringpots, their upper frequency is limited by the string mass/recoil spring rate. For high frequency work we use accelerometers. I haven't tried overplotting accel and string pots, might do that today.

[RideRate]

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...

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.

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. And technically the spring doesn't have a frequency, the system does.

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

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.

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.

[RideRate]

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.

Wait....
The bode plot is showing the response of a body relative to its input, -yes?
In other words it shows how well the body follows the road inputs you give on the tire.
When you fit a solid link instead of a damper, then you still have the tires as spring. The sprung mass is basically added to the unsprung mass by the solid link, which creates a single degree of freedom model.
But due to its high inertia the tire will struggle to follow the road inputs at increasing frequencies hence it will not generate a perfectly flat FFT with zero movement at all frequencies!
Comments...

The plot is not showing the response of the body relative to the input. From what he said it is just strictly displacement of the spring and we don't know the input. So relative to road we're missing loads of info. Knowing nothing of the input to the system causes all sorts of unknowns and speculation. However, we can determine some stuff relatively if we assume between the two shown plots the input was the same and the only change made was to the damper. We can't learn a whole lot, but could have a small bit of useful information.

So with a perfectly solid link for a spring/damper the time history of the plot he used would just be all 0's since you're just measuring the displacement (or lack thereof) of that link.

[mep]

We corrected that already, and I actually realized it soon after I wrote it but thanks for repeating it.