Regarding the front and the rear ride frequency

[Comrade Guevara]

I am confused to that of the selection of the suspension travel of the rear and the front. so that to calculate the frequency of the rear and the front as the frequency is related to that of the suspension displacement.

[Pippo69]

Roll the dices if you don't have enough time to learn such basics...

[MCoach]

Everything involving frequency is just wavelengths.


If you have a set length of a string, say 20cm, and you need to fit it to a curve, there will be a few things that affect it. The only requirement in my example will be a constant amplitude, no progression or digression. Lay out a graph with the string laying along the +x axis and makes wave amplitudes along the Y axis.

If you have a high amplitude, you will run out of the x axis travel shortly, leading to a low ride frequency.
Stretching that string out so that there are many, many little waves, but a long x axis distance, then the ride frequency will be high.
Imagine the peaks representing some arbitrary frequency value. High amplitude, less peaks, low freq.
Low amplitude, more peaks, high frequency.


Or you could reference a differential equations book that describes the motion of a mass-spring-damper system. The EQs check out.

[BillCobb]

Here's the easiest and best way. Just write a ...

Sorry, I have to go...

[Warpspeed]

I don't have enough time to read in the depth.
And we don't have the time to spare either.

[Claude Rouelle]

Anuj,

What don't you try to read any good vehicle dynamics book or simple Google your question before you ask this question on the forum?

People are making fun of your post and nobody is a real winner here.

[Comrade Guevara]

Boys it just very easy thing that we have
The out of phase motion between front and rear vertical motion, caused by the time delay
between when the front wheel and rear wheel hit the bump, is accentuated by the frequency
difference. A result of the phase difference is pitching of the body. To reduce the pitch
induced by hitting a bump, the rear needs to have a higher natural frequency to “catch up”
with the front. This notion is called producing a “flat ride”, meaning that the induced body pitch from road bumps is minimized.....

For a given wheelbase and speed, a frequency split front to rear can be calculated to minimize
pitching of the body due to road bumps. A common split is 10 – 20% front to rear.
The above theory was originally developed for passenger cars, where comfort takes priority
over performance, which leads to low damping ratios, and minimum pitching over bumps.
Race cars in general run higher damping ratios, and have a much smaller concern for comfort,
leading to some race cars using higher front ride frequencies. The higher damping ratios will
reduce the amount of oscillation resultant from road bumps, in return reducing the need for a
flat ride. Damping ratios will be explained in the next tech tip in detail. A higher front ride
frequency in a race car allows faster transient response at corner entry, less ride height
variation on the front (the aerodynamics are usually more pitch sensitive on the front of the
car) and allows for better rear wheel traction (for rear wheel drive cars) on corner exit. The
ride frequency split should be chosen based on which is more important on the car you are
racing, the track surface, the speed, pitch sensitivity, etc.......

[Owen Thomas]

Looks like someone found the Optimum G tech tips! They're very good, take the time to read them through.

Now that you have experienced the power of google, why don't you try and use it to answer the other question you asked? (pitch center)

[onemaniac]

Originally posted by Anuj Regmi:
Boys it just very easy thing that we have


Exactly. I'm glad you tried and found the answer your own.
Do this more often.

[MCoach]

*clap clap clap*

This is a day I never thought I would see.

Congrats on your research.

[Comrade Guevara]

i have read those things now what i mean to say is :
i need to know the stiffness in the rear and the front. let me assume a "X N/mm" front stiffness spring. This I assumed arbitrarily. According to the post i should fix more stiffness spring in the rear so that the frequency would match. So how to select the stiffness in the rear can i just take the "X N/mm" as the reference and take the rear one as just higher value. Is this so is there anything that i should know while selecting the spring..?

[jlangholzj]

Look at this (http://lmgtfy.com/?q=ride+frequency+%2B+spring+stiffness)


Would you look at that....even gives the equations and the whole deal! From the master himself...there's even a link a couple down thats good too!

Even a spark-E / engine guy like myself can make some sense out of that.

you owe me a beer now

[Z]

This thread, its links, and the other thread by the OP on "Pitch point", are typical examples of automotive cottage industry codswallop.

As usual, NO DEFINITIONS of terms used.
==================================
What is "front ride frequency"?
What is "ride"?
How does "ride" differ from "heave" or "pitch"?
Is there such a thing as a distinct "front-sprung-mass" that is somehow separable from the "whole-car-sprung-mass"?

Having made no effort with the first step, there is no point applying any rigour to the rest of it.
================================================== =====================
Is the discussion about a 2-D side-view of the car with the whole-car-sprung-mass conveniently (and unrealistically!) reduced to two point masses located above front and rear axle lines?
Why so, given that when Rowell and Guest did more realistic 2-D side-view analyses in the 1920s they found that, in general, the two fundamental modes of this 2 DoF system give nothing like the "front" or "rear" frequencies calculated in the links?

With regard to the "Pitch point" thread, is that whole discussion about kinematic anti-pitch properties of the suspension linkages (eg. anti-dive/squat/etc.), or about the side-view oscillation centres (as per R&G above) as determined by F + R spring stiffnesses, and CG position + pitch MoI?

Is the real reason that automotive engineers talk about "front and rear ride frequencies" because it sounds oh-so much more sophisticated than saying "its got stiff springs"?

Z

[HenningO]

Originally posted by Z:
Is the real reason that automotive engineers talk about "front and rear ride frequencies" because it sounds oh-so much more sophisticated than saying "its got stiff springs"?


You if anyone should know that spring stiffness is not only parameter affecting the natural frequency of a system... Using frequency is convenient as it combines two values into one and allows for easy comparison between designs. But you are probably right, automotive engineers don't do it for the convenience but to sound sophisticated!

[js10coastr]

Originally posted by HenningO:
But you are probably right, automotive engineers don't do it for the convenience but to sound sophisticated!

Well, I think we've at least defined "hypocrisy". http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[BillCobb]

It's ALWAYS entertaining when you read the comments that those outside of the Gates of Industry make up only to sound like they are inebriated with the exhuberance of their own verbosity.

Perhaps one day they can hitch a ride on a tour bus that takes them into the labs, corridors, conference rooms and cubicals to catch a fleeting glimpse (out of the corner of their eye) of how sophisticated, technical, creative and secretive the industry actually is. Once you have been there, you'll never have to pretend you know what engineering is.

Mechanical engineers design weapons, civil engineers design targets. Un-civil enineers wind up serving nourishment for others to dispute.

[Z]

The thing is, there is no such thing as "front or rear ride frequency". It is fiction.
~~~~~o0o~~~~~

If it was real, then surely you could measure it.

How would you do this? Push the nose of the car down then measure time for, say, ten oscillations?

Well, with dampers you barely get one oscillation, and they change the frequency. So remove all dampers and fit ultra-low friction bearings everywhere. Now push the nose down again.

Except this time you notice that pushing the nose down also lifts the rear of the car. Leverage, aaargh! So, push the car down directly above the front wheels.

So, the $64,000 question: Will the front of the car now oscillate at some fixed frequency?

Answer: Of course not! As explained (independently) by Rowell and Guest in the 1920s. And repeated in a lot of Vehicle Dynamics texts since then. (Hint: the "front and rear" motions of the car are coupled, because the car's mass is "joined up" in the middle! There are indeed two fundamental harmonics, but in general these are not "front and rear".)
~~~~~o0o~~~~~

But thanks to progress over the last ninety odd years this rather simple 2-D problem has been dumbed-down to 2 x 1-D problems (ie. two totally independent, and unrealistic, 1 DoF spring-mass systems).

And could you even start to use this "independent front and rear" gibberish to describe the side-view behaviour of an interconnected suspension, such as used on the Citroen 2CV? Well, the answer is also the reason why there are so few interesting cars these days. Or good suspensions...

Z

[js10coastr]

Originally posted by Z:
The thing is, there is no such thing as "front or rear ride frequency". It is fiction.
~~~~~o0o~~~~~

If it was real, then surely you could measure it.



Following that logic, "stress" is fiction too since we can't measure it... we measure strain and then calculate stress.

All things engineering are fiction... we just come up with equations to easily characterize object. Some represent reality more closely than others, but how much fidelity is "close enough"?

[Tim.Wright]

Originally posted by Z:
This thread, its links, and the other thread by the OP on "Pitch point", are typical examples of automotive cottage industry codswallop.

They are typical examples of terminology and methods used by people who actually build cars for a living mate...

[Tim.Wright]

So, the $64,000 question: Will the front of the car now oscillate at some fixed frequency?

It doesnt matter one iota. Nobody cares.

We use the ride frequency to get a baseline value for the spring rate and then we move on. The final spring value is then refined after looking at load tranfser calcs, full vehicle simulations and test data.

[Edward M. Kasprzak]

But thanks to progress over the last ninety odd years this rather simple 2-D problem has been dumbed-down to 2 x 1-D problems (ie. two totally independent, and unrealistic, 1 DoF spring-mass systems).

I think it's worth noting that a scenario exists where it becomes mathematically correct to treat the front and rear as oscillating independently. It depends on the value of the pitch inertia relative to vehicle mass and CG location.

[ben]

Originally posted by Tim.Wright:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">So, the $64,000 question: Will the front of the car now oscillate at some fixed frequency?

It doesnt matter one iota. Nobody cares.

We use the ride frequency to get a baseline value for the spring rate and then we move on. The final spring value is then refined after looking at load tranfser calcs, full vehicle simulations and test data. </div></BLOCKQUOTE>

You're right Tim. However, if I was going to be charitable towards Z I would say that the idea of using a simple calculation to get in the ballpark then testing and refining (i.e. development engineering) to get a final answer is an insight that is missed by a lot of students in the competition.

It's the "how do I calculate the right answer" syndrome. If more people were honest about what a given model can and more importantly can't do we'd avoid a lot of these debates.

Ben

[Will M]

"All models are wrong, but some are useful" - George E. P. Box

-William

[Moop]

While I don't agree with what Z says about automotive engineers or his tone, I do agree that it seems a bit silly to speak exclusively in terms of front/rear ride frequencies, given how simple the analysis is.

For a block with a heave(x) and pitch(y) degree of freedom(about its centre of gravity) and springs at the front and rear(no dampers), the spring compressions at the front and rear are -x + a*y at the front and -x -b*y at the rear. If heave is positive up and pitch is positive forwards, then your equations of motion are simply
m * x double dot = Kf*(-x+a*y) + Kr*(-x-y*b)
I * y double dot = b*Kr*(-x-y*b) - a*Kf*(-x+a*y)

with some rearrangement, you can write the matrix equation
[ x double dot] = [ -(Kf+Kr)/m________(a*Kf-b*Kr)/m ] [ x]
[ y double dot] [ (a*Kf-b*Kr)/I_____-(a^2*Kf + b^2*Kr)/I ] [ y]

The brackets are supposed to look like matrices/vectors. I had spaces but had to replace them with underscores because the forum wouldn't display the spaces. This forum could use something funky for writing equations!. I checked it and it looks right, but I'm tired so I might have messed something up.

This is obviously the equation of a simple harmonic oscillator. The positive square roots of the negative of the eigenvalues of this matrix will give you the two natural frequencies of the system and the two eigenvectors will give you the two natural modes. There's no reason why the eigenvectors have to yield pure front spring displacement with no rear displacement, so the eigenvectors will be some combination of the two degrees of freedom. I think this is the bouncy pitch and pitchy bounce modes mentioned in the damper chapter of RCVD. With these eigenvectors you could calculate the instant centres of these two motions.

I started trying to find an expression for the eigenvalues but algebra became a PITA - maybe I'll finish it later and see if I can find the fantabulous expression Ed is referring to. I haven't done so yet, but it's pretty easy to plug this into an Excel spreadsheet to calculate the eigenvalues/vectors numerically.

Of course, this was without any damping. I'd imagine the modes would change with damping maybe? I'm not 100% on that - my ODEs prof mostly skipped over matrix ODEs.

But I guess it really depends on what you're concerned with. If you're more concerned with talking about roll stiffness distribution, then this doesn't matter at all and you can speak in terms of ride frequencies. If you're more concerned with flat ride for a production car or perhaps the way your chassis movement affects your undertray, then it would be more important to look at it with this approach.

I also think that the suspension kinematics would have an impact on this. The way I see it, the sprung mass doesn't have a pure pitch degree of freedom like our block does, since it unfortunately has suspension arms/sliding pillars/trailing arms. Instead, it has a pitching while moving forward a bit and possibly up a bit degree of freedom.

[GSpeedR]

Moop, Ed is referring to the case where (a*Kf-b*Kr) = 0, which means that the K matrix have is diagonal and thus the 2 equations decouple. In this case, the two modes will be independent: a pure pitch input will result in no bounce response and vice versa.

Edit: to complete the picture that Moop started...

If we change our coordinates from bounce/pitch to zf/zr [zf=bounce-a*pitch; zr=bounce+b*pitch] then we have another special case:

m*a*b - I = 0

where the front and rear suspensions are fully decoupled (assuming we have still met our a*Kf=b*Kr case above) and an input at the front suspension has no effect at the rear and vice versa. Give it a try. Note that the above 2 clauses are pretty difficult to obtain in practice, plus you might find working with body modes to be much easier.

[Z]

js10coastr,

"Following that logic, "stress" is fiction too since we can't measure it... we measure strain and then calculate stress."

I would measure force and a cross sectional area to calculate stress. I might measure strain if I wanted to estimate modulus of the material. Frequencies are easy to measure. Just count the number of peaks, troughs, whatever, of some cyclic phenomena that repeats itself over a given time period.

But the important point is that the word "frequency" suggests the frequent (= "numerous") reoccurrence of the same single event, or cycle. To suggest that a car has a single "front ride frequency" is like saying that Beethoven's Fifth consists entirely of the single note "middle-C".
~~~~~o0o~~~~~

Tim,

"They are typical examples of terminology and methods used by people who actually build cars for a living mate...
It doesnt matter one iota. Nobody cares."

I take this as confirmation (possibly from someone in the auto industry?) that modern car designers are a bunch of gibbering idiots that use whatever buzz-phrases they think will make them sound clever, but as for actually building better cars, well "Nobody cares"!!!

"We use the ride frequency to get a baseline value for the spring rate and then we move on..."

So, for example, you choose a "ride frequency" from the range in the Optimum G Tech Tip;
1. Passenger cars (soft springs) = 0.5 - 1.5 Hz,
2. Sedan racecars (medium) = 1.5 - 2.0 Hz,
3. High DF racecars (rock hard) = 3.0 - 5.0+ Hz,
With Frequency = (1/2.Pi).Sqrt(K/M).

And what brilliant insight does that give you, other than that you can have soft, medium, or hard springs?

Why don't you use the simpler "static deflection" to get your baseline value? Eg. (using the same numbers as above, rounded);
1. Passenger cars = 100 - 11 cm,
2. Sedan racecars = 11 - 6 cm,
3. High DF racecars = 3 - 1(or less) cm
With Deflection = W/K.
(W=weight)

Oh, yes, it's not nearly as clever sounding, so it won't impress your bosses, will it? But you don't care anyway, which I am sure is very cool...
~~~~~o0o~~~~~

Edward,

"I think it's worth noting that a scenario exists where it becomes mathematically correct to treat the front and rear as oscillating independently. It depends on the value of the pitch inertia relative to vehicle mass and CG location."

Agreed! I note that there are an infinite number of possibilities of this happening, but these are only one infinitieth of all possibilities. Less cryptically, for each of the infinite possible CG positions (longitudinally in 2-D side-view) there is an infinite range of possible pitch inertias, but only one of these pitch MoIs (per CG position) will give independent F&R oscillation.

I find it disappointing that no one else has yet shown any interest in this real (ie. observable, measurable...) side-view behaviour of cars (Edit: See PS below). FWIW (to the students), and very briefly, there are two fundamental harmonics (because 2 DoF system). One has its oscillation centre outside the wheelbase, and the other inside the wheelbase. These are respectively, and loosely, called the "bounce" and "pitch" modes. When excited by any input at either axle both fundamentals combine to form a "beat", within which is a varying amplitude and wavelength. In the special circumstances of above paragraph, the oscillation centres coincide with the axle lines and constant frequencies can be observed.

All this very easy to understand with simple geometrical calculations. Students might try "Chassis Design... Olley" book by Millikens and Edward for one approach (I think the explanation can be simpler and more insightful).
~~~~~o0o~~~~~

Ben,
"If more people were honest about what a given model can and more importantly can't do we'd avoid a lot of these debates."

and William,
"All models are wrong, but some are useful"

My main concern here is that by using a model of separate "front and rear ride frequencies" the suspension engineers are giving themselves a metaphorical lobotomy. The car is cut into two halves in the very first design meeting, and henceforth there is never any communication between respective F&R design teams.

"What the... You want to CONNECT the front and rear springs!!!
But what sort of F&RRFs will that give???
Aaaaaaargh!!!... Go away, you're hurting my brain!!!"

Yep, "Idiocracy" here we come! http://fsae.com/groupee_common/emoticons/icon_frown.gif

Z

PS. Moop, you posted just as I was writing this. I haven't checked your equations, but I reckon you've got it. The geometrical analysis is very simple, just a couple of lines and circles. The gist of it is that you replace the real car with a "dynamically equivalent" (*) system of two springs under two masses (* ie. first 3 MoIs, for springs and mass, are equal to that of the car). These represent the fundamental harmonics, and their locations are the oscillation centres.

PPS. GSpeedR, as above... Good to see that some of you are interested in what actually happens. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Tim.Wright]

It was exactly my point, that the ride frequency is only used to tell you if your springs are soft medium or hard, nothing more.

Let me ask you some questions.
What extra insight do you reach by using a rigid body with pitch inertia in your calculations??
Why do you (incorrectly) assume the body is rigid?
Why arent you speaking of hysteretic friction in the suspension and tyres?
Why don't you account for the installation stiffness of the suspension?

To me the answer to these questions is exaclty the same as the answer to why we calculate the front and rear frequencies seperately.

The $64k questions is, how would YOU arrive at a set of spring rates for a car?


My main concern here is that by using a model of separate "front and rear ride frequencies" the suspension engineers are giving themselves a metaphorical lobotomy. The car is cut into two halves in the very first design meeting, and henceforth there is never any communication between respective F&R teams
The is complete BS. The car isnt developed seperately front and rear, I dont know where you got that idea from.

[GSpeedR]

Tim, maybe you understand the assumptions made when making (hopefully very quick) calculations using separated ride frequencies, versus using rigid body coordinates, versus adding extra DOFs and nonlinear elements, etc. However, I don't think everybody on this forum fully understand what these assumptions entail.

"Good enough and move on" is a good way to create bad engineers.

[Tim.Wright]

While I do agree that these simplifications are generally taught without the disclaimer that they are gross simplifications (and I think this is the only point where we are actually all in agreeance), I still don't see any reason to over complicate a problem which in the grand scheme of things is quite a small part of the overall picture.

I also disagree that "good enough and then move on" is such a terrible way to work. If you reach a solution that is "good enough", any extra time spent dicking about with it is a complete waste. The springs are a perfect example. What exactly do you gain by calculating the pitch and bounch frequencies using the pitch MOI (which nobody know accurately anyway, not even OEMs!!!) as opposed to a simple frequency calculation which requires only the sprung mass and its longitudinal distribution??

Then consider the fact that the spring value will change during the design phase as you trial different geometries for anti-dive, roll centres, roll gradients etc. Then once the car is built the springs will again change from track to track and from driver to driver. You will quicky see that all this masturbation over how you arrived at your initial spring values was a complete waste of time.

[GSpeedR]

Here's the issue: At some point, somebody is going to have to analyze and characterize this racecar, whether it is done at the early stages of development, or at some intermediate level, or 1 week before competition. It sounds like you jump directly from looking up ride freqs from OptimumG's website straight to looking at results from your 35 DOF ADAMS model after you've finished the vehicle(?).

How long does this analysis really take (how much time are you wasting)? If you are completely developing a vehicle on your lunch break, then OK there may not be enough time. Yes, various factors will change the parameters involved so you may have to do it again...write a program in Matlab. There is a significant amount of effort that goes into simplifying the results of hugely complex systems so that humans can understand it and make decisions with it.

[GSpeedR]

2nd problem: FSAE students need to ask themselves what the purpose of this activity is. Is it to build a competitive car, to learn skills and apply principles taught in the classroom to a very cool project, or both? I think that the GEandMO concept is applied too heavily by FSAE engineers to their own detriment when/if they want to make a career out of it.

Personally, I know I wasted a lot of time looking for easy ways to get fast answers.

[Tim.Wright]

For your first point, if we stay with the springs example, then OK its true there is not a massive amount of time wasted doing these extra calcs but the thing that counts it out for me is that the cost vs benefit is not there. I.e. you are adding complexity to your work which gives (in the end) zero added value for the reasons I've specified already (different different tracks and drivers = different springs). Like a mentioned before, if you want to add pitch MOI, why not add the chassis bending stiffness, suspension installation stiffness (which is often not negligible) etc etc?

Also:
1. A lot of other people in your design team might not be used to this method. 2Hz calculated with mass+MOI is not the same as 2Hz calculated with mass_f and mass_r. So you have nothing to use as a reference.
2. Nobody know the pitch inertia if their chassis. Even OEM's use values that were once measured on a previous car and then mathematically fudged to apply to the new car. For a new car there simply is no pitch inertia until you have finished the detail design. What do you do then? So again, whats the point of extending your working to include the pitch inertia when 95% of the time its wrong?

For your other point, I think I agree. if by GE and MO you are speaking of geometric methods of drawing lines to form instant centres etc... I too went to a vehicle dynamics seminar at the start of my studies and was told that the chassis rolls about the roll centre and that lateral movement of the roll centre is evil. I believed this for about a year because it was presented as an all encompassing fact rather than a simplification.

[GSpeedR]

I understand that springs will change due to many valid reasons, but vibration analysis certainly can give meaningful results at te design phase, and at the track. When a person looks up ride freqs on the Internet and calculates a natural frequncy they are performing a very dumbed down vibe analysis.

Inertias...we measured ours on a swing table and published the data, not to mention there are many ways to estimate.

The GEandMO was actually an acronym for good enough and move on. http://fsae.com/groupee_common/emoticons/icon_smile.gif However I agree with your example and I experienced the same thing.

[Z]

Some additional comments to GSpeedR's post further up (just after Moop's).

1. "Ed is referring to the case where (a*Kf-b*Kr) = 0, which means that the K matrix is diagonal and thus the 2 equations decouple. In this case, the two modes will be independent: a pure pitch input will result in no bounce response and vice versa."

The equation above (ie. a*Kf = b*Kr) says that the "spring centre" is at the same location as the "mass centre" or CG (ie. push down on the car at the CG and it stays horizontal). So the two fundamental harmonics are a "Heave" oscillation (car moves up-down, but always horizontal) and a "Pitch" oscillation (rotary motion about car's CG). Note that these are different to the loosely termed "bounce" (both ends of car moving in same direction, but by different amounts) and "pitch" (ends of car moving in opposite directions, but not necessarily rotation about CG) used by Rowell and Guest, and many others since then.

More importantly, all fundamental harmonics are always decoupled (in a sense) unless you forcibly couple them. In this case, any road bumps hitting front or rear axles will excite both Heave and Pitch modes (typically with different amplitudes), so the car body as a whole will develop a (complicated) "beat" motion.
~o0o~

2. "If we change our coordinates from bounce/pitch to zf/zr [zf=bounce-a*pitch; zr=bounce+b*pitch] then we have another special case:
m*a*b - I = 0"

Since I = m*k^2, the above equation is very often written as;
k^2 = a*b ("k-squared-equals-aye-bee", with k = Radius of Gyration (NOT spring stiffness http://fsae.com/groupee_common/emoticons/icon_smile.gif)).

This says that the total sprung mass of the car can be replaced with a "dynamically equivalent" system of two smaller masses sitting above the F&R axles (ie. Mf = Mtotal*b/(a+b) at front, and Mr = Mtotal*a/(a+b) at rear). (BTW, the method of "dynamically equivalent" systems is great for turning seemingly complicated problems into very easy to understand solutions. In this example the geometric calculation involves drawing a few straight lines and a semi-circle. It is at least as rigorous as the algebraic approach, perhaps more so, being based on Euclid's axiomatic-deductive approach. Highly recommended!http://fsae.com/groupee_common/emoticons/icon_smile.gif)

These two (1-DoF) single-mass-on-single-springs (eg. Mf/Kf) are now the two fundamentals and their frequencies are very easy to work out (and are, in fact, "the F&R ride frequencies"). This is the unique condition where excitation at one axle causes only that end of the car to oscillate at a fixed frequency.
~o0o~

3. There is a doubly special case where both above conditions are met. Ie. "spring centre" = "mass centre", and total mass can be distributed above F&R axles. Now Mf/Kf = Mr/Kr so both fundamentals have the same frequency.

The geometric analysis makes it clear that in this case the oscillation centres can be ANYWHERE! More correctly, you can specify any arbitrary position for one centre, and this then determines the position of the other. Or looking at it another way, both axle-lines of the car body oscillate at the same frequency, but can have different amplitudes and phases.
~o0o~

4. All of the above assumes NO longitudinal interconnection of the springing. As such, there is always one oscillation centre outside the wheelbase (for "bounce" mode), and the other inside the wheelbase (for "pitch" mode), with exception of special case 2 above.

However (!http://fsae.com/groupee_common/emoticons/icon_smile.gif), by adding, say, longitudinal Z or U-bars, the "equivalent spring system" can be changed to two springs that are not at the axle lines (eg. both springs within the wheelbase, or both outside, etc.). This means that both oscillation centres can be brought within the wheelbase, or taken outside of it.

Most importantly, some of the modes (hint; "Pitch") can thus be made soft enough that very little damping is needed to suppress oscillation. Excessive damping is almost always a very BAD thing. It gives a harsh ride over bumps, adversely affects handling, and lift the wheels off the road over bumps reducing grip.

Longitudinally interconnected springing allows less damping. Less damping = better.
~~~~~~~~~~o0o~~~~~~~~~~

Tim,

"It was exactly my point, that the ride frequency is only used to tell you if your springs are soft medium or hard, nothing more."
So why waste your time talking about "frequencies"? Oh, yes, I answered that before...

"Let me ask you some questions.
What extra insight do you reach by using a rigid body with pitch inertia in your calculations??"
See top half this post, and other posts......

"Why do you (incorrectly) assume the body is rigid?"
Because I have calculated the tolerances of assuming otherwise, and for most car bodies have found negligible difference.

"Why arent you speaking of hysteretic friction in the suspension and tyres?"
I did, several posts ago.

"Why don't you account for the installation stiffness of the suspension?"
I do, and also for Motion Ratio, Tyre rate, etc. All in the K's above.

"The $64k questions is, how would YOU arrive at a set of spring rates for a car?"
As I have discussed at length before, but briefly here, I would NOT have four corner springs + two ARBs. I would have:
1. A main Heave mode spring carrying all weight of the car, with smoothly rising rate.
2. A (much smaller) Pitch mode spring, with highly non-linear rate for level ride and good bump absorbancy.
3. A Roll mode spring similar to Pitch spring, but generally stiffer.
4. No Twist mode spring at all (= Warp), but linkage arranged to give mostly zero Twist rate with smoothly rising rate at ends of travel.

"The [metaphorical lobotomy] is complete BS. The car isnt developed seperately front and rear, I dont know where you got that idea from."
I got that idea from you, and from others in the auto industry, and from driving modern cars.

Let's face it, there are (almost?) no cars these days that have longitudinally interconnected springing (there used to be Citroen, Packard, even British Leyland!...). This is a direct result of starting the design process (as you said) with an assumption that there will be certain spring rate at the front, and another at the rear. This "axle-centric" thinking no doubt goes back to horse-and-cart days.

Most suspension engineers these days know NOTHING about fully interconnected springing. If you want better suspension, then you MUST start thinking about the whole car.

The real tragedy is that this crude approach to design is dressed up as being sophisticated by using BS buzz-phrases such as "We first optimise the ride frequencies, then use extensive numerical multi-body analyses...", when you really mean "Oh, bugger it, let's just make the springs stiffer, again! If the customer complains, err... well ... we'll just tell 'em to drive on smoother roads!".
~~~~~o0o~~~~~

Bottom line for anyone interested in good suspension is;
Forget about "front and rear ride frequencies", and start thinking about Heave, Pitch, and Roll motions of the car body, and (very importantly!) Twist in the road surface.

Z

[MCoach]

"The $64k questions is, how would YOU arrive at a set of spring rates for a car?"
As I have discussed at length before, but briefly here, I would NOT have four corner springs + two ARBs. I would have:
1. A main Heave mode spring carrying all weight of the car, with smoothly rising rate.
2. A (much smaller) Pitch mode spring, with highly non-linear rate for level ride and good bump absorbancy.
3. A Roll mode spring similar to Pitch spring, but generally stiffer.
4. No Twist mode spring at all (= Warp), but linkage arranged to give mostly zero Twist rate with smoothly rising rate at ends of travel.


I appreciate the banter between the simple and complex algorithms for the selection of spring rates. However, there is one defining feature between a mathematician and an engineer. An engineer knows when to say, "Close enough is good enough." He is not met with kind words with a partial project.
So, who cares if your calculations are off by 5lbs for your spring rates? I bet your driver won't notice.

Z, no one worries about static deflection because that doesn't really do a whole lot for calculations. It's more of a side effect than a value to use for dynamic simulation. The only thing I personally use it for is static ride height estimation. Beyond that, without rearrangement and application is essentially meaningless. Ride frequency becomes important because it is the first stepping stone to getting ball park numbers, rather than numbers that include compliance values after several iterations of ADAMS. It's a bit difficult to know those values when you are still in imaginary-car-land.


...Now, onto the quoted section. That's a beautifully theoretical layout, but scratching my head how that practically makes sense...

[Tim.Wright]

As I have discussed at length before, but briefly here, I would NOT have four corner springs + two ARBs. I would have:
1. A main Heave mode spring carrying all weight of the car, with smoothly rising rate.
2. A (much smaller) Pitch mode spring, with highly non-linear rate for level ride and good bump absorbancy.
3. A Roll mode spring similar to Pitch spring, but generally stiffer.
4. No Twist mode spring at all (= Warp), but linkage arranged to give mostly zero Twist rate with smoothly rising rate at ends of travel.

Well I suggest you build this and demonstrate some results, because until you do you're merely "preaching" in exactly the same way that any proponent of roll centres, force lines etc has done previously...

[Edward M. Kasprzak]

Originally posted by Tim.Wright:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">As I have discussed at length before, but briefly here, I would NOT have four corner springs + two ARBs. I would have:
1. A main Heave mode spring carrying all weight of the car, with smoothly rising rate.
2. A (much smaller) Pitch mode spring, with highly non-linear rate for level ride and good bump absorbancy.
3. A Roll mode spring similar to Pitch spring, but generally stiffer.
4. No Twist mode spring at all (= Warp), but linkage arranged to give mostly zero Twist rate with smoothly rising rate at ends of travel.

Well I suggest you build this and demonstrate some results, because until you do you're merely "preaching" in exactly the same way that any proponent of roll centres, force lines etc has done previously... </div></BLOCKQUOTE>

Something like this can be (and has been) done in active suspensions. Each mode can be isolated mathematically and controlled independently with not only its own stiffness, but its own damping. Unsprung modes, too.

Has active suspension been mentioned in the "Fantasy Car" thread? If not it should be. Having one mechanical spring & damper at each corner is a huge compromise--you never get the stiffness and damping you want in every mode.

[MCoach]

Originally posted by Edward M. Kasprzak:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Tim.Wright:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">As I have discussed at length before, but briefly here, I would NOT have four corner springs + two ARBs. I would have:
1. A main Heave mode spring carrying all weight of the car, with smoothly rising rate.
2. A (much smaller) Pitch mode spring, with highly non-linear rate for level ride and good bump absorbancy.
3. A Roll mode spring similar to Pitch spring, but generally stiffer.
4. No Twist mode spring at all (= Warp), but linkage arranged to give mostly zero Twist rate with smoothly rising rate at ends of travel.

Well I suggest you build this and demonstrate some results, because until you do you're merely "preaching" in exactly the same way that any proponent of roll centres, force lines etc has done previously... </div></BLOCKQUOTE>

Something like this can be (and has been) done in active suspensions. Each mode can be isolated mathematically and controlled independently with not only its own stiffness, but its own damping. Unsprung modes, too.

Has active suspension been mentioned in the "Fantasy Car" thread? If not it should be. Having one mechanical spring & damper at each corner is a huge compromise--you never get the stiffness and damping you want in every mode. </div></BLOCKQUOTE>


Yes, and in that reference almost all of those applications have still used a spring/damper package at each corner of the car whether it is mechanical,reactive,active. These would include the Bosch system, the Williams system, as well as the Lotus system. Mathematically, with full control over damping and spring rates of the these individual, great things can happen. Active suspension was the point that Formula 1 put its foot down, with good drivers and superior cars beating out characters like Senna.

But still, the idea of using a mechanical springs with the set-up that Z listed seems impractical...

[Z]

Originally posted by Tim.Wright:
Well I suggest you build this and demonstrate some results, because until you do you're merely "preaching"...

Originally posted by Z:
...[^^^]... gibbering idiots ...
For the record:

Citroen first started developing fully interconnected suspensions in the 1930s. The Citroen 2CV (developed pre-WWII, but only released immediately post-WWII, and then produced up until 1980s?) has side-pair interconnected suspension that couples Heave & Roll, and Pitch & Twist, but with very different spring rates for these paired modes. In 1955 Citroen released the DS line ("deesse" = "goddess") with active Heave & Pitch control via hydro-pneumatic springing. Almost all Roll stiffness is via a front lateral U-bar (very thin one at rear), which thus gives very soft Twist mode. This active H & P control allowed extremely soft rates, so very small damping forces can give "critical" damping.

By late 1950s there was huge development of many different types of fully modally separated suspensions, not surprisingly mostly in France. In USA, Packard jumped on the bandwagon and started using longitudinal Z-bars to separate H & R from P & T. Unfortunately, their stylists produced hideous cars and the management must have been incompetent, and the company went under.

Meanwhile, Britain had some first rate chief designers, such as Alec Issigonis who penned the "Mini" (the trendsetter for all modern small cars with East-West engines). Other similar Austin-Morris models started using side-pair springing like the 2CV, but with hydraulic interconnection to rubber springs ("Hydrolastic") or gas springs ("Hydragas"). This last one blending Citroen's 2CV and DS suspensions. Unfortunately, these efforts were let down by horrific detail design, totally incompetent management, and militant unions. There were multiple takeovers and name changes (A-M -> BMC -> BLMC ->???) as the various companies sank into oblivion.

For my part, as a tinkering "farm-boy" I converted some VW Beetle buggies to 2CV style suspension about 30 years ago. I took this a step further with Heave & Roll coupled by side-pair Z-bars (using coils like the 2CV), Pitch controlled by its own spring (lateral centre-pivot leaf), and no Twist spring at all. Compared with conventional spring-at-each-corner suspension the difference is night and day. But anyone who had driven a 2CV ("the world's cheapest car"!) already knew this. The mechanical implementation is also remarkably simple. Only three springs required, and it can all be done with only one!

But today all auto companies, including Citroen, are white-goods manufacturers. Except that the customers are seen in public with their "washing machines", so they want them to look "stylish". Must impress the neighbours! And, errr..., it must have lots of cupholders... And TVs in the seatbacks, to turn the kiddies' brains into mush... Furthermore, the OEMs have found that they can build-in a lot of electronic obsolescence by adding countless little boxes-of-grief, designed to do nothing more than make more and more lights flash on the dashboard, or maybe not flash at all, as the car ages.

The auto companies produce such rubbish, which a lapped up by a gullible public, by employing an army of witless drones who think that they have to calculate some sort of "frequency" before they can make a spring stiffer. Said drones are payed peanuts, but are kept happy with toys that have lots of flashing lights, and make pretty pictures, and let them pretend they are doing "highly intricate multi-body modal analysis". This makes the drones feel very clever, even though they have ABSOLUTELY NO IDEA AT ALL about the recent history of their particular field of endeavour.

Note that I can not bring myself to call current "Suspension Design" a field of "expertise", or a "profession". More a case of "a thousand baboons bashing away at keyboards". If any OEM supension designer out there disagrees, then please provide your detailed analysis of fully interconnected suspension, together with examples of research done, prototypes built, conclusions found, and so on. As noted earlier, I have never worked in the auto industry, have only the same passing interest in cars as most males who like tinkering with mechanical gadgets, but I know that interconnected suspensions are light years better than the horse-and-cart technology currently on cars.

As for the future, well I reckon that Moop, GSpeedR, the boys at UWA, and a small number of others on this Forum, can, or will soon be able to, tell the difference. They manage to do 2-D side-view H & P analyses, rather than having to dumb it down to 1-DoF "F & R ride frequencies". And at UWA they have (very simply!) separated H & P from Roll modes, with zero Twist stiffness. Separation of H from P is a simple next step, but only possible if you think along those lines.

However, I doubt that these students would pass the "intelligence test" to get into the auto industry. They are TOO CLEVER! Their bosses do not need that sort of thing. Geez, it might lead to CHANGE..., aaaaargh!!! And, heaven forbid, a sort of suspension "arms race" between companies. Nope, don't want any of that.

Better just a gentle slide into Idiocracy...

Z

[MCoach]

Originally posted by Z:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Tim.Wright:
Well I suggest you build this and demonstrate some results, because until you do you're merely "preaching"...

Originally posted by Z:
...[^^^]... gibbering idiots ...
For the record:

Citroen first started developing fully interconnected suspensions in the 1930s. The Citroen 2CV (developed pre-WWII, but only released immediately post-WWII, and then produced up until 1980s?) has side-pair interconnected suspension that couples Heave & Roll, and Pitch & Twist, but with very different spring rates for these paired modes. In 1955 Citroen released the DS line ("deesse" = "goddess") with active Heave & Pitch control via hydro-pneumatic springing. Almost all Roll stiffness is via a front lateral U-bar (very thin one at rear), which thus gives very soft Twist mode. This active H & P control allowed extremely soft rates, so very small damping forces can give "critical" damping.

By late 1950s there was huge development of many different types of fully modally separated suspensions, not surprisingly mostly in France. In USA, Packard jumped on the bandwagon and started using longitudinal Z-bars to separate H & R from P & T. Unfortunately, their stylists produced hideous cars and the management must have been incompetent, and the company went under.

Meanwhile, Britain had some first rate chief designers, such as Alec Issigonis who penned the "Mini" (the trendsetter for all modern small cars with East-West engines). Other similar Austin-Morris models started using side-pair springing like the 2CV, but with hydraulic interconnection to rubber springs ("Hydrolastic") or gas springs ("Hydragas"). This last one blending Citroen's 2CV and DS suspensions. Unfortunately, these efforts were let down by horrific detail design, totally incompetent management, and militant unions. There were multiple takeovers and name changes (A-M -> BMC -> BLMC ->???) as the various companies sank into oblivion.

For my part, as a tinkering "farm-boy" I converted some VW Beetle buggies to 2CV style suspension about 30 years ago. I took this a step further with Heave & Roll coupled by side-pair Z-bars (using coils like the 2CV), Pitch controlled by its own spring (lateral centre-pivot leaf), and no Twist spring at all. Compared with conventional spring-at-each-corner suspension the difference is night and day. But anyone who had driven a 2CV ("the world's cheapest car"!) already knew this. The mechanical implementation is also remarkably simple. Only three springs required, and it can all be done with only one!

But today all auto companies, including Citroen, are white-goods manufacturers. Except that the customers are seen in public with their "washing machines", so they want them to look "stylish". Must impress the neighbours! And, errr..., it must have lots of cupholders... And TVs in the seatbacks, to turn the kiddies' brains into mush... Furthermore, the OEMs have found that they can build-in a lot of electronic obsolescence by adding countless little boxes-of-grief, designed to do nothing more than make more and more lights flash on the dashboard, or maybe not flash at all, as the car ages.

The auto companies produce such rubbish, which a lapped up by a gullible public, by employing an army of witless drones who think that they have to calculate some sort of "frequency" before they can make a spring stiffer. Said drones are payed peanuts, but are kept happy with toys that have lots of flashing lights, and make pretty pictures, and let them pretend they are doing "highly intricate multi-body modal analysis". This makes the drones feel very clever, even though they have ABSOLUTELY NO IDEA AT ALL about the recent history of their particular field of endeavour.

Note that I can not bring myself to call current "Suspension Design" a field of "expertise", or a "profession". More a case of "a thousand baboons bashing away at keyboards". If any OEM supension designer out there disagrees, then please provide your detailed analysis of fully interconnected suspension, together with examples of research done, prototypes built, conclusions found, and so on. As noted earlier, I have never worked in the auto industry, have only the same passing interest in cars as most males who like tinkering with mechanical gadgets, but I know that interconnected suspensions are light years better than the horse-and-cart technology currently on cars.

As for the future, well I reckon that Moop, GSpeedR, the boys at UWA, and a small number of others on this Forum, can, or will soon be able to, tell the difference. They manage to do 2-D side-view H & P analyses, rather than having to dumb it down to 1-DoF "F & R ride frequencies". And at UWA they have (very simply!) separated H & P from Roll modes, with zero Twist stiffness. Separation of H from P is a simple next step, but only possible if you think along those lines.

However, I doubt that these students would pass the "intelligence test" to get into the auto industry. They are TOO CLEVER! Their bosses do not need that sort of thing. Geez, it might lead to CHANGE..., aaaaargh!!! And, heaven forbid, a sort of suspension "arms race" between companies. Nope, don't want any of that.

Better just a gentle slide into Idiocracy...

Z </div></BLOCKQUOTE>

What happened to you promoting 4 shocks at each corner, directly actuating, and the Z rule #1:

"Rule 1. Football is a simple game, keep it simple!
The most shouted rule! Even when playing games, kids want to complicate things!!!"

We're just trying to keep it simple here...and your spouting all these ideas of interconnected springs, not being clever enough and not making a brown go-kart that doesn't also meet the tolerances of the gods, with the mathematical modelling of a genius. At least keep it consistent.

To go a few other things, because this is honestly bothering, to be attacked every time I read a post about how we choose to design something...

"If it ain't broke, don't fix it. -Z"

"Rule 6. Never, ever, ever, let the opposition pull your pants down.
Stay awake. Corollary: If the opposition are asleep (and in FSAE, most are Wink ), then pants them!" -Z



Also, Z...The 2CV uses bell cranks in it's suspension which you vehemently fought against in previous suspension threads:

Push/Pullrod-and-Rocker actuated Spring-Dampers.
======================================
Of all the features on an FSAE car, this is the one I consider the least justifiable.

I could repeat the criticisms I have given elsewhere for these "fashion accessories", but this post is already too long. However, if anyone wants to provide supporting arguments (eg. "they help lower the unsprung mass"), then I will happily discuss...
~~~o0o~~~




Don't let the kids catch you with your pants down. http://fsae.com/groupee_common/emoticons/icon_razz.gif

[Z]

MCoach,

1. FSAE cars don't need any suspension at all to do well (only to pass scrutineering). The above posts are about cars with suspensions that have to work, and how they can be improved.

2. Three springs (H, P, & R) is simpler than the six on most current cars. UWA's 2012 car is very simple, has low parts count, and uses just three springs. Using only one spring is even simpler, and it can work very well indeed. (BTW, my suggestions for a "brown go-kart with a spring-at-each-corner" are for the students who have to take baby steps first.)

3. The 2CV has Leading-Arm and Trailing-Arm suspension, with the spring connections and dampers acting DIRECTLY on these arms. They are not "rockers" or "bell-cranks".

4. If you think the "ride frequency" equation is somehow more instructive than the "static deflection" equation, then please tell us why. I see very little difference (the conversion equation is trivial).

5. If change scares you, then by all means "... keep doing what you've always done..." (ie. polish a turd). But try to be rational about it, for example by counting the number of springs, rockers, ball-joints, chassis nodes, etc. This is helpful advice, not an "attack".

6. Did any east coast Australians see the Benji Marshall try last night (Tigers vs Eels)? Now that was a pantsing! (~70m untouched because all the opposition had their backs' turned.) The same is on offer in FSAE. http://fsae.com/groupee_common/emoticons/icon_smile.gif

But note again, in (current) FSAE there are negligible bumps, so little need for really good suspension. However, ... a brown go-kart with aero-undertray...

Z

[Tim.Wright]

Originally posted by Z:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Tim.Wright:
Well I suggest you build this and demonstrate some results, because until you do you're merely "preaching"...

Originally posted by Z:
...[^^^]... gibbering idiots ... </div></BLOCKQUOTE>

If you're going to bring your argument down to this level then its pretty clear the discussion isn't going to progress anywhere. I'm out.