moment diagram with weight transfer

[Silente]

@jpusb,

yes, i correct the corner radius for each output point of the diagram. Actually, since it is a constant velocity simulation, calculating the Fy gives automatically an Ay, so the radius is defined automatically as an output. From there i also then derive the slip angles.

By the way, here below the results i got for the three different TLLTD scenarios. I will add another message with the diagrams corresponding to the same scenario.

[Silente]

second part, diagrams.

[Z]

REGARDING "YAW-MOMENT vs LATERAL-ACCELERATION" DIAGRAMS.
================================================== =========
(Or "YMDs", or "MMMDs", or "M3Ds"?)

Asked by Silente (p11);

I would love to have your feedback about the results I am showing...
... some hints at what you would actually look, to identify car behaviors and tendencies (max lat acceleration, max lat acceleration in trimmed conditions, stability, control, ...?).
... advice on "realistic numbers for these metrics"?

and,

Again, I would be interested to share about the metrics to look at and their values...
... interesting to discuss about how to "read" and use dN/dDelta or dN/dBeta (as suggested by Claude to measure stability and control) both close to the origin and to the maximum lateral acceleration.

Asked by Jpusb (p11).

I would love to read the input of the rest of the known or unknown authorities on this subject, including Z's.

I, too, would be interested in hearing more about above issues!
~~~o0o~~~

However, and this is probably because of my current NON-EXISTENT experience in using such diagrams, I am becoming skeptical of their use in gaining a deeper understanding of the relevant VD. Especially so, to the challenge of winning FS/FSAE competitions.

Here are some reasons why.

1. Both Silente and JP have produced these diagrams themselves, yet are having difficulty extracting "meaning" from them. What I find especially puzzling here, is that S and J have at their fingertips ALL THE INFORMATION necessary to interpret their diagrams, namely all the input stuff (eg. the tyre curves, etc.), yet the process of manipulating that data into the form of the diagrams seems to have resulted in a LOSS OF UNDERSTANDING.

Why? (I have my suspicions, but would like to hear other peoples' opinions.)

2. The presentation of the diagrams, especially JP's close-up of his right-corner "candy wrapper", suggests a very UNREALISTIC degree of precision. I would prefer to see the lines painted with a 12 inch wide paint-roller, to better represent the vagaries of real tyre-road grip. That way, as long as the roller-painted corner is somewhere near the horizontal-axis, the reader can interpret it as "close to balanced ... depending on conditions...". (Ie., "Any measurement is MEANINGLESS, without knowledge of its uncertainty...".)

3. To win FS/FSAE, I would NOT bother chasing "perfect-balance/neutral-handling". Instead, as I said way back on page 2, IMO much more important is to make your diagram AS BIG AS POSSIBLE!

Yep, "Small but perfectly formed..." can work well on the dating scene, but in FS/FSAE I reckon you really need SIZE. If you can make your car's diagram twice as big as the opposition's (ie. 2 x as high and 2 x as wide), then you blow said opposition away even while driving comfortably INSIDE the diagram, and well away from any "unstable limit edges".

Q. And how do you increase the size of your diagram?
A. Stickier tyres, and/or MORE AERO!!!
~~~o0o~~~

Despite above criticisms, I would still like to hear how the experts "read" such diagrams. What features are better or worse, etc...? Especially for FS/FSAE conditions?

Z

(PS. Jpusb, back when I worked in the packaging industry we called your corner feature a "butterfly wrap". I recall renovating some beautiful machines, built about 100 years ago, that would butterfly-wrap "candy" (= "lollies" here) using an almost entirely "mechanical" mechanism. Just a single 3-phase electric motor in the base that drove a big camshaft, then multiple push/pullrods, levers, linkages, etc. They were almost completely silent, and would run all day and night ... forever. The more modern versions had lots of VERY NOISY (!) pneumatics and electric-stepper-motors, and kept breaking down!)

[theTTshark]

Despite above criticisms, I would still like to hear how the experts "read" such diagrams. What features are better or worse, etc...? Especially for FS/FSAE conditions?

Basically if you want to be able to put a handling metric on a vehicle setup you need to at least calculate certain portions of the MMM Diagram. Knowing targets of where the balance of the handling, driver control of the vehicle, or stability of the vehicle should be saves you time, money, and gains you respect by not wasting other people's time.

Essentially at the beginning the only thing you can really tell is if you are in the ballpark of having a balanced car, and roughly how many Gs you're going to be able to pull at a certain speed. This is because you have no testing data to correlate them with. Now if you have data from an older car and you did your job in diligent testing preparation and note taking you can model that vehicle and essentially see where your driver likes to live. But let's say the car is completely different and you can't. You make a couple of diagrams at several speeds and for a couple of easy setup changes that you plan on performing to get a snapshot of how the balance, control, and stability changes through the various corners you might encounter and you store that information in your testing folder. Then you go testing. You ask the driver okay how did the car feel in terms of balance, driver control, and vehicle stability. You make one of those pre-planned setup changes, do more laps, and repeat the feedback process. After enough setup changes you will eventually learn the delta that your driver works in. This delta takes into account the simplifications in your model as well as driver preference, but it begins to allow you to make incredibly smart setup decisions and saves you time while testing. At the University of Kansas we used these, a Total Lateral Load Transfer distribution spreadsheet, a quarter car model, and damper travel estimations in a variety of conditions to estimate how we should initially setup vehicles and how we should make changes. In 2014 JT and myself never even changed the springs through our whole competition season (Michigan and Lincoln). We were able to dedicate our testing to more important concepts and because of that we gained more respect with our team, peers, and ultimately got us both jobs. Eventually you don't even worry looking at the whole diagram or even any of the diagram. You focus on your balance metric and your control and stability derivative at the limit. From there you create MMMs for entry then for exit, and you start piecing the puzzle together of what the hell the driver wants.

Is it always right? No. Is anything always right? Absolutely not. Does it narrow your setup options down? Yes and thank god it does. I can't speak about my professional life on here very much, but Claude and Optimum G have successfully used this process to win races in a wide variety of series. Do all race teams use it? No. Do successful race teams use it? In some way, shape, or form they are or are working towards using it. There are simply too many benefits to not use it.

[MCoach]

Yep, "Small but perfectly formed..." can work well on the dating scene, but in FS/FSAE I reckon you really need SIZE. If you can make your car's diagram twice as big as the opposition's (ie. 2 x as high and 2 x as wide), then you blow said opposition away even while driving comfortably INSIDE the diagram, and well away from any "unstable limit edges".

Q. And how do you increase the size of your diagram?
A. Stickier tyres, and/or MORE AERO!!!

There are options that are stickier than the current "standard" Hoosier offerings, however ever they have all demonstrated to show their ugly side after lap 1. Mostly because these tires are typically Avon (or some competitor) hillclimb tires and not meant to be heat cycled for more than a minute or so.
The simple answer would be to just get a larger tire, but then what is found is that the tire has too much mass to heat up in an FSAE envirmonment. Dirt track tires? Similar problem, soft, capable, but too much mass with too little contact area. Not a simple solution here.

2x high and wide? I'd settle for 1.1 times bigger. However, this only nets you potential of the vehicle. With increasing the boundary that the vehicle operates, it inherently means the driver needs to be capable of using that area predictably. If the car has "little quarks" here and there, then who would want a car that is 5% faster but ready to bite their head off and can't even use it. Of course, the answer to this was answered many years ago by pilots who now zoom around in statically unstable aircraft that will do circles around their stable brethren from yesteryear. But, without the computers that aid them, the planes handle like monsters and several have crashed in the development stages. Some, like the B-2 have crashed due to simple things like moisture on sensors, showing their nasty side.

Trent gave a pretty good summary of it all, and Chris Patton's paper from a few years ago explores such modelling a little more deeply for quasi-static simulation.

[jpusb]

Don't get me wrong, I never said I did not understand the diagram or how to read it. Neither did I say that I did not understand the realities/limitations/uncertainties of modeling tires (especially at the limit), complianceless car, etc vs. the real situation. I work doing much more complicated experiments all day which are later correlated to very complex CFD models other people do, so I have a very good idea of this. This does not mean, however, that I don't get curious for how and why my idealized model is behaving like that. The zoomed in candy feature I just posted is just one particular doubt that came up, purely out of curiosity. I just wanted to know if other models (don't care about reality at the time of this doubt) show this. I haven't had time to work on it since I posted them, and when I posted them I just prepared the figures but I haven't worked on the model for 6 months or so.
Sadly, I am not trying to win FSAE anymore, I am just doing this just because I am curious. So I don't have a car to spend my time testing instead of spending it in this model. However, being an experimentalist myself, I see many, many uses for this model and the diagram, most of them already discussed in different ways here, during design events, and in RCVD. Is it the secret of life? Of course not, but it is just another tool in your toolbox. The more tools you have (both from modeling and from testing) the more educated your guess will be when the time comes. This is a tool I would have loved to have in my FSAE suspension guy/driver days.
However, the fact that I understand how and why my diagram is built this way does not mean that I am an expert in the matter (so I am not expected to know what is "usual" and what not), that I can solve the effects of all variables instantly in my head, and that I have seen 200 of these for very different cars. At the same time, with so many variables in the middle, so many angles and coordinates, and especially so little time to go deeply through this, it is normal that I am not entirely sure of if this A-feature is produced by that B-variable, etc. Therefore, I just asked for expert's opinion on how the diagram looks (in a general sense, besides the cool colors I put ) and if the candy feature was found usually in other cars' diagrams.

[BillCobb]

I'm disappointed that the multiple requests for information on the direct use of MMM analysis and diagrams from experts has resulted in the threadbare patient dying on the operating table. As the only result, we are left with impressionistic works of art and without any consistent or meaningful engineering values or metrics. Is that the requiem for it ?

What I wish to see are diagrams produced from an elementary introductory and exploratory vehicle architecture, followed by a kitchen sink diagram for the same vehicle from the advanced hardware stage of the design, with test data obtained from a track, overlayed in a best of the best achievable correlation scenario. If you as a manufacturer's representative were hauled into court by plaintiff's legal team demanding these engineering documents, and all you had was candy wrappers, I believe a judge would park you in a jail cell for a day or so to contemplate your contempt. And, I have seen this done in a Federal Court in Washington, D.C..

Now what say you 'experts'. There is no jury, only jurists.

Otherwise, this thread belongs in an art museum to be contemplated for its passion, and personal subjective interpretations of the vigor of the lines and colors in dramatic repose.

In the applicable words or Claude Monet: (Do I need to remind you that he was an artist?)
"For me, a landscape does not exist in its own right, since its appearance changes at every moment; but the surrounding atmosphere brings it to life - the light and the air which vary continually. For me, it is only the surrounding atmosphere which gives subjects their true value."

His work is a Moment diagram, too, eh? and even with aero effects... !!

[rwstevens59]

Attached (I hope) are a few photos of a very crude example of the kinematics I believe Z was trying to describe on page 10 of this thread.

After reading his post I had a look in the attic and in a very short time produced this 'model' to play with to more fully understand what Z was trying to describe.

Worked for me.
Train Kinematics 01 Web.jpgTrain Kinematics 02 Web.jpgTrain Kinematics 04 Web.jpgTrain Kinematics 05 Web.jpg

The object of the last photo using two different radii is that in the world of oval track skewing of the solid rear axle wrt the car centerline is a common practice and I just wanted to have a look at what 'crabbing' down the track might look like.

Again, Sorry for the toys.

Ralph

[Z]

Ralph,

I cannot make out the writing on the arrows ... so I am not sure what they represent? Are they "the forces, from-car-to-ground, at centre of F&R-axles"?
~o0o~

Anyway, I have drawn five sketches of the Transient Cornering Kinematics/Dynamics of "The School Bus", and am just finishing writing up the words. Should have these up by next week.

Interestingly, having gone through this "planar" example in detail now, I am wondering why I bothered. It all seems so obvious! And so simple! Nevertheless, many long posts coming soon...

Z

[rwstevens59]

Z,

The arrows simply represent the individual coordinate systems of Earth (shown in all pictures on the tile floor grid), body front truck pivot, body rear truck pivot, and an assumed body CG in the middle of the flat car body. There are also small arrows on the front an rear couplers indicating front and rear truck 'steer' relative to the body pivot coordinate systems or Earth system (track).

Being a kinematic problem I was not showing any forces, just directions and displacements to get a feel for how front truck steer on a body with a long wheelbase moves on corner entry in very small steps. Far removed due to the rear truck pivot point, but somewhat similar to your school bus.

Kinematic-ally as I studied this more I realized with a fixed path radii (the track) and pivots front and rear, that if you use equivalent links, as the front truck enters the corner this reduces to nothing more than a crank and slider 2D kinematics problem.

Far removed from a car with pneumatic tires I know, but I think it illustrates your point on how a corner is initiated.

Ralph