I've just started to use suspension spectrum analysis to tune my spring rates.
I tried different suspension settings with the same car and the same track.
All the graphs show a natural suspension frequency peak. Sometimes it is tall but it's often small an surrounded by two or three tall peaks. What do they mean?
In short I did not managed to find myself how to interprete correctly these graph, each one has an unique shape. I didn't find the track frequency either..
I quess, it just does mean, how spring loads thus frequencies change. As you can't be absolutely consistent, you just get unique curve for each test run. The peaks can represent, again, weight transfer... Sounds credible to me, but I'd anyways rather wait for colcob or further Bob's analysis
if you did an fft over the whole track, the graph is (as it seems to me) meaningless, because you are outside of "stationariety" of the signal...well i am assuming that the suspension response it is not over a whole track, i think is better you consider many fft over small time windows, enough small for the suspension response to be considered "stationary"...also i think the update time of the phisic model should be important for the aliasing issue, but as long we assume (as i think) the lfs phisic model is good, the i believe it is fast enough to avoid aliasing.
so my hint is to try to fft only a small time...maybe the crossing of the chicane or much better the test wich mercedes class A failed: a sudden steer and countersteer on a plain straight. hope it helps
Well as you are only analysing suspension frequencies under g, then as I understand it there should be a peak either side of the "natural frequency". For cornering, this will be due to the left/right weight transfer, changing the mass rest on the spring, and therefore the freqency will change. Being that cornering is usually done at the limit of the tyres, there weight transfer will often be the same amount, and depending on it being a left or right corner, those peaks make sense.
I still have a few questions though:
What is the y scale?
What data is it you've actually plotted.
If you changed the scale to be track time (x) vs. frequency (y) you should see the left frequency going high and the right low during cornering (or vice versa), and going high together under braking (since you have been analysing the rear wheels).
Edit: you posted while I was browsing, to answer your question:
The relationship is that the spring stiffness is fixed (the value you see in the garage), while the frequency depends on the mass on the spring. Weight transfer is taking off load from one spring, and putting more on another. So while the spring stiffness never changes, the frequency should drop on the additionally-loaded spring, and increase on the less-loaded spring (during cornering, this is the inside wheel).
no i only meant short period of time, if u go straight it helps finding bump frequency of the track, i only intended that if suspensions are not solicitated, the spectrum of the output will be (teoretically) zero, because it has zero input...only this
the "merceds test" should help finding compression and rebound frequencies in a single step.
I disagree. Following the discussion about suspension frequency change with downforce loading I went right back to the differential equations for a spring-mass-damper system and confirmed that spring load does not affect the suspension frequency. Only the mass supported by the spring and the spring rate (and, if underdamped, the damping constant) change the suspension frequency. During cornering the weight distribution changes but the mass distribution doesn't. This is ignoring change of CoG with chassis roll obviously, but that should be fairly small.
Yah, he's right, this is the problem I ran into back when I was making my suspension spreadsheet. I initially thought that downforce would have an effect on spring frequencies, but eventually we determined downforce had no effect at all.
But then that begs the question, why do the graphs have multiple peaks, above and below the natural frequency?
Further fuel on the fire--don't forget that the tires, being flexible, also have a frequency, usually much higher than the suspension itself (anywhere from 5-13 Hz, given a quick look at Bob's GRC). With critical damping (generally not actually used), the flex of the tires over bumps will be pretty significant.
Just a few quick contributions from me, its past my bedtime.
Firstly, Spring frequency doesnt change with load transfer as I understand it, its certainly not the case that weight transfer changes the MASS resting on a spring, it just changes its current load, therefore should not influence frequency.
Also, I presume your input data was suspension travel. Now what we call 'suspension frequency' is actually the frequency of the sprung mass resting on the suspension under gravity.
But a measurement of suspension travel doesnt just contain data about the movement of the sprung mass, it also contains data for the movement of the unsprung masses, and without incorporating data about the absolute z-position of the two masses, its impossible to separate the two in the suspension travel data alone.
So your graphs are a bastard child of spring mass frequency and unsprung mass frequency, and probably tell us as much about the bumps on the track as they do about the suspension. Although even that data is distorted by the fact that all the bumps will have been run over at different speeds.
as i said before i believe that the suspension output, as a signal, cannot be considered stationary (statistically speaking), so it's "meaningless".
what should be done IMO is calculating fft over short periods of time...short enough that the suspension signal can be considered stationary within it.
then plotting all the fft results in a graph of the spectrum against time, this helps finding the track-bumps frequency...
using math is cool, but it should be applied under the theorem's hypotesis to be meaningful, which is not this case IMO.
Yeah OK then, cornering is more like changing the value of g for different sides of the car than actually moving the mass. But that makes it much harder to understand the data.
So, thinking about the sprung/unsprung masses bit that Colcob brought up, wouldn't we then technically have two frequencies, which could be seen with a spectral view? Actually we'd have two moving bodies, both of which could have more than one frequency. I'm just thinking about sound, since when a speaker cone moves it's moving one path but can be moving at many different frequencies.
More thinking: if it is suspension travel is what was used to create the graphs, then even though weight transfer won't affect frequencies, it will still cause the suspension to compress on one side and extend on another (i.e. roll). So this would still affect the graph even without mass transfer occuring. Yes/No?
Oh and finally as for the tyre frequencies that are present in GRC, do not take them to be overly accurate as I've very little data to base my tyre deformation formula on (that and it's also very simplistic).
Open RAF with F1perfview, export it to a csv file, import it into excel, calculate suspension velocity, export it into a FFT program, and (in my case) import FFT data in excel.
OK I think now that working FFT with LFS is useless, I can't see what I thought I could see, and as I was afraid, suspension are mainly setup with feelings and many tests, not with calculation. I'm a bit disappointed since I spend very long time for it but I'm cool now because I don't need to break my head for things I can simply do.
Um, I'm not sure what you mean, but I'll try and explain it a bit better, I was in a rush last night.
Firstly, I dont think its a waste of time what you've done, I'm sure that this kind of analysis can be useful, but filtering and interpreting the data is probably something that only gets taught in third or fourth year of a auto engineering degree course, so its going to take us a while to work it out from scratch.
To try and explain a bit better what I meant, imagine the following:
A car is driving a course on a perfectly flat level surface, so the unsprung mass, ie the wheels and some suspension components, do not move vertically at all.
The Sprung mass, ie the chassis, moves around on the suspension and induces changes in the suspension travel.
In this situation, an FFT analysis of the suspension velocity will show you something about the natural frequency of the suspension and any harmonics.
But in the real world, the road surface has small high frequency bumps, large low frequency changes in elevation and everything in between, which all induce changes in the suspension travel, even if the car is driving in a straight line at constant speed. So all of those oscillations feed into the suspension travel data, and add frequencies to the resulting FFT plot.
But thats not necessarily a bad thing, because we only really care about how the car drives in the real world, and handling those bumps and elevation changes is one of the main problems behind suspension setup.
It would be interesting to see plots from a short section of straight from 3 different tracks, in the same car with the same setup, to see if this picks up noticeable differences in the dominant frequency of bumps on the different tracks.
This could actually be useful data for car setup, as say you know that South City has predominant bumps of about 5hz in a braking zone, it might be a bad idea to have a front suspension frequency of 2.5hz for example.
Thats all getting pretty hypothetical, particularly as the bump frequency depends entirely on how fast you drive over them, but you get the idea.
did you scale those plots ? normally they should go up to 100 Hz
EDIT:
oh and this step is just plain wrong you have to do the fft over the deflection of the suspension and not its time-derivative
and another thing from whar youve said ive gathered that you did cut out blocks from the data sets put those together and did the fft over that
is that right ? cause if you did youll get false fft results
EDIT2:
oh and another thing ... the derivation to get suspension velocity explains the spectrum you get as what you did is a prediction so the overall signal should become more or less white which is exactly what those plots show
Sorry to be not clear, I understood that you wrote and it proves that my FFT graph are misused and not accurate. Anyway I'll keep my tool for later if needed.
hum, I don't understand what you mean, but I can say that the scale was automatic and linear, I only display a range from 0Hz to 5Hz.
Yep, you must be in the thruth. However, I'm not sure about FFT the deflection. I tried once and the results were weird.
What do you mean by "white"?
raf files sample data at 100 Hz therefore the fft should produce points between 0 and 100 Hz
then there must be something wrong with the way you calculated it
you want the spectrum of the spring oscillations so youll have to analyze the amplitude of the springs deflection over time
white as in same magnitude for all frequencies
which is exactly what you want
for a valuable spectral analysis the ideal road would be one that has bumps at all frequencies (or at least at all frequencies up to half the sample rate at which you measure spring deflection) in essence a white noise road surface
Since the 'suspension travel' is measuring the extension of the spring, you'd have one frequency for the vertical motion of the unsprung mass and one frequency for the vertical motion of the sprung mass. You'd also probably get frequencies corresponding to the roll and pitch frequencies as both of these will affect the ride height on an individual wheel.
I dont necessarily agree with that, as a real road almost certainly doesnt have uniformly distributed bump frequencies up to 50hz, but probably has certain characteristic bump frequencies that might be worth taking account of. Only guessing though.
i think the problem here is a fundamental misunderstanding of the relation between a fft analysis and what happens on a real road on your side
let us assume for the moment that the wheel always stays in contact with the road surface and that the tyres are solid (or at leat that their effect is negligable) so all the bumps in the road directly affect the spring with the cars mass attached to it
think of the road (or rather the bumps it has ie the changes is surface height and the frequencies at which they occur) your using for the analysis as an input signal (at this point the solid tyres are important so the bumps in the road directly excite the suspension without prior filtering through the tyres ... generally this assumption isnt neccesary but the relation between bumps and the suspensions reaction wouldnt be as tighly as it is under said assumption)
by driving across that road your feeding this signal into your suspension which is the system you want to analyze (here the assumption of a non lifting wheel comes into play as we dont want to analize two interacting inseparable systems)
the output from the system ie the reaction of your suspension to those bumps is the spring deflection
this is basically a system identification problem
for system identification (especially if all you care about is magnitudes which is the case for suspension analysis as phase rarely matters) a white input signal is a good choice since it excites the system at all frequencies so youll get the systems frequency response for all frequencies
the main idea here is that the spectrum of the excitation relates to the nodes where you are able to gather data about the fequency response
the spectum you get at the output ie the spectrum of the springs deflection is actually the multiplication of the excitations and the systems sprectra (and to be exhaustive also the spectrum of the window you use for the fft)
therefore unless your excitation is truly white (ie same magnitude for all frequencies) the outputs spectrum will be distorted by the inputs spectrum
after you have the response to all those frequencies you can evaluate any road surface by multipliing the spectrum of the road surface with the spectrum of the suspension ... the result of this will be the spectrum of the suspensions response so you can tell at which frequencies the suspension will swing and who lage the amplutide of those oscillations will be (unfortunately a fourier analysis isnt exactly useful for transient behaviour)
another way to do this would be sweping through the fequencies ... unfortuatelly the only way i can think of to do this is by wiggling the steeringwheel left to right at increasing frequency while driving on a perfectly flat road surface ... unfortuantely that way its practically impossible to ensure that the amplitudes at which you excite the suspension are the same for all frequencies
