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Thread: moment diagram with weight transfer

  1. #11
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    Quote Originally Posted by Viv View Post
    ...I am wondering if it is truly possible to attain zero (or close to zero) yaw moment at max lat g ...
    Short answer is yes, this is possible, I've seen it many times. But perhaps it is not possible with your combination of CG location and choice of tires (as modeled) along with any other effects included in your model.

    How far are you off? What is the max lateral g available at yaw moment=0?

  2. #12
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    Quote Originally Posted by Viv View Post
    ... I am wondering if it is truly possible to attain zero (or close to zero) yaw moment at max lat g
    Viv,

    I think you are asking the wrong question. How about:

    Q1. In order to win, does an FSAE car really need to be "balanced" at the limit (ie. have zero Yaw-couple at Max-Lat-G)?
    Q2. Why?
    Q3. If not, then how much unbalance is acceptable, and which way is better (US or OS)?
    Q4. Again, if balance does NOT have to be "optimised", then what performance factors should you be focusing on?

    And ... a whole lot of other important questions... But, for now, I would answer:

    A1. No.
    A2. Because bigger fish to fry (see A4).
    A3. Production cars usually aim for considerable limit US, but in FSAE conditions quite a lot of OS can work well, although either US or OS are OK.
    A4. Aim for MAXIMUM Max-Lat-G, and high enough Yaw-couple (with low enough car Yaw-inertia) to make the driver dizzy.

    Note that "balancing" the handling of a car is often done by REDUCING the lateral grip capability of the "strong" end of the car, to bring it into balance with the weak end. This can make sense on 200+ mph Indy oval type racing. Makes little sense in FSAE conditions with virtually NO "steady-state" corners.

    Rather than cripple the strong end of the car, keep trying to strengthen the weak end. Meanwhile, drive to suit...

    Z
    Last edited by Z; 03-19-2015 at 11:48 PM.

  3. #13
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    Answer to question 1, 2, 3 & 4: What's the point of having a large Max Ay if the driver can never use it?

    In my experience, the driver starts complaining of instability and general driving difficulties BEFORE you reach the point of "mathematical" instabilities. I.e. a change of sign of static margin or stability factor. In other words, you NEED a stable balance at the limit otherwise the driver (particularly amateur drivers) will never find it.

    FSAE needs high maneuverability but not instability. There are ways to achieve this...
    Last edited by Tim.Wright; 03-20-2015 at 03:15 AM.

  4. #14
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    Quote Originally Posted by Viv View Post
    Hi everyone. My name is Vivek and I am the suspension lead from the University of Toronto team. I attended Mr. Claude's seminar in Indianapolis where I learnt further details about the yaw moment diagram and have also read chapter 8 from Mr. Milliken's textbook. I have since spent hours programming excel to create yaw moment diagrams for constant radius turns. It uses pacejka models fitted to TTC tire data, ride/roll camber and toe, ackermann steering, and weight transfer. I am however having problems getting the zero yaw moment at max lateral g even after adjusting various vehicle parameters. Currently the vehicle understeers at max lat g.

    Pardon my ignorance if this question is trivial but I am wondering if it is truly possible to attain zero (or close to zero) yaw moment at max lat g outside the theoretical world with all these effects accounted for or is there something I am doing wrong with my program. One reason I believe I might be getting these results are because of slight asymmetry in tire data lateral forces (left turn vs right turn). Also the yaw moment at max lat g currently sits at around -500 Nm at lat g of approx 2. I also have little intuition as to how big an effect such a yaw moment would I have (which I suppose really depends on the yaw inertia of the vehicle) and if it is worth spending further hours on the issue considering this is not even the full picture considering compliance, tire aligning moments, etc.
    Have you done a bit of a sanity check on the model to see if its working correct? I.e. replaced the pacejka model with a linear one and checked that the results match up with the basic bicycle model equations of motion?

    If thats ok the next step I'd try would be to see if the US/OS moment changes in the correct way and direction when you change the CG longitudinal position and the lateral load transfer distribution?

    I have seen that when you include non-linear tyre models into a static vehicle model that is close to neutral steer at the limit you can run into some apparently strange numerical behaviours as both axles saturate at the same time. It has even happened to me that the model may not even have a single unique solution in some conditions.

    One question, what is the yaw moment digram that you speak of? Is this the MMM?

  5. #15
    Thanks for your replies everyone. I will definitely heed your advice especially do take a few steps back and do sanity checks like the one Mr.Claude suggested and Mr. Tim Wright. My person preference from having driven the car last year is to have a bit of oversteer actually because for drivers like me (not experienced) oversteer maintains steering control (as stated in Milliken). If the car were perfectly balanced and if driver did not realize they are about to breakaway, there would be little opportunity to correct as all wheels breakaway at same time. (At least this is what I understand).

    Hi Mr.Milliken,
    I am not too far off. Max Lat G at 0 yaw moment is about 1.9 g and the max available is about 2.1.

    Hi Claude,

    Currently neither of them passes through the origin (a quick sanity check I should have done before). This is odd... The car is generating lateral force at 0 body slip and steering. The isoline at Beta = 0 crosses the yaw moment axis at about -500 Nm and the isoline for gamma = 0 at about -650 Nm.

    I shall posted further progress.
    Last edited by Viv; 03-20-2015 at 03:31 PM.

  6. #16
    Viv,

    1. "Currently neither of them passes through the origin (a quick sanity check I should have done before). This is odd... "

    That is the #1 mistake made by 99 % of the students who create a Yaw moment Vs lateral acceleration for the first time.

    The issue is that you forget to switch from the tire coordinate system to the chassis coordinate system.

    A beginning of explanation: Imagine a tire that you put on the tire testing machine. Let's say that you test that tire at 0 slip angle and 0 camber. You notice that that tire has a slight Fy. That is very possible because no tire is absolutely symmetrical. Let's say that that tire has an Fy pulling towards the left. Put that tire on the LF and on the RF of the car and you have 2 Fy pulling to the left, that means some lateral acceleration and some yaw moment without even a steering or a CG slip angle. You see were we are going....

    2. If you yaw moment show you a positive number at maximum lateral G that means that the car over-steers ....on the simulation. There are tons of things that you assume in your simulation that are not necessarily real.
    - How real is your tire model?
    - Are the tire forces and moments that you measured on the tire testing machine for a given input of Fz, slip angle, camber, pressure the same that you will have on the track? And I am not even speaking about tire temperature....
    - Does you model take into account compliance? Because your chassis, your suspension elements your uprights etc... are bending, twisting etc... maybe the camber and the slip angle you thought you have is not the one you effectively have.

    Can you car effectively take 1.9 G in the skip pad? If the recorded data do not match the simulation. If not then AT LEAST one of them is wrong. on

    Does it mean then that this kind of simulations not useful? No. but you need to remember that YOU WORK IN DELTA. THERE ARE TOO MANY PARAMETERS TO BE STOP ON IN ABSOLUTE VALUE. You yaw moment could be wrong by what you are interested in is the DELTA of yaw moment per % of weight distribution or per degree of front camber or rear toe. Then your simulation will be useful.

    3. Tim Wright is right.... Before you get to maximum lateral G at the apex you need to enter the corner. Is the car drive-able?

    Imagine a tire A that has +/- 10 % of his peak grip between 4 and 6 degree of slip angle. Imagine now a tire B that had twice the grip but only between 4.8 and 5.2 degree. That tire could be beating the lap record on a simulation software but in reality no driver would be able to drive it. The Yaw Moan Vs Lateral Acceleration has several useful indicators.
    - The maximum G
    - The maximum G at yaw moment = 0
    - The control at the corner entry = variation of the yaw moment per degree of steering angle when the CG slip angle beta = 0
    - The control at Apex = variation of the yaw moment per degree of steering angle when the CG slip angle beta is the one you have at maximum lateral G. Food for thought: Remember that if all 4 tires are at the ideal slip angle you have no reserve of front grip: if you steer more or less you lose front grip. Mario Andretti said " if everything is under control that means you are not going fast enough"
    - The stability at the corner entry = variation of the yaw moment per degree of CG slip angle Beta when the steering angle delta is = 0
    - The stability control at Apex = variation of the yaw moment per degree of CG slip angle Beta when the steering angle delta is the one you have at maximum lateral G. In other words does your car still has some stability has maximum lateral G? you car

    4. Most of the drivers are not very sensitive to the grip but are very sensitive to the balance (yaw moment) the control and the stability. We have worked with professional drivers who can feel the 100 NM or even 50 NM of yaw moment difference but do not feel a difference between 3.0 and 3.1 of apex maximum lateral G.

    5. A good advice. Start a yaw moment Vs. Lateral G exercise with a 2 wheel model and constant cornering stiffness, then with "real' non linear tires then and then only with a 4 wheel model and weight transfer. Simple then complicated not he other way around

    Good luck,

    Keep us posted.
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  7. #17
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    Quote Originally Posted by Claude Rouelle View Post
    ... Start a yaw moment Vs. Lateral G exercise with a 2 wheel model and constant cornering stiffness,...
    This model is available as part of the RCVD Program Suite, on the CD that is packaged with "RCVD: Problems, Answers and Experiments". Of course you will learn more if you write your own, but comparing with ours may be useful as you debug yours.
    http://www.millikenresearch.com/rcvdps.html
    At the bottom of the webpage are instructions to "register" this older software for recent versions of Windows.

  8. #18
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    Quote Originally Posted by Tim.Wright View Post
    ... What's the point of having a large Max Ay if the driver can never use it?
    ...
    ... you NEED a stable balance at the limit otherwise the driver (particularly amateur drivers) will never find it.
    Tim,

    I sense too much time spent in the book world (or Matlab++?), rather than the real world.

    Consider two cars:
    1. Tim-car - PERFECTLY BALANCED at the limit, which is at Max-Ay = 1.0 G (*).
    2. Z-car - AWFULLY UN-BALANCED at the limit, which is at Max-Ay = 2.0 G. Car is only "balanced" (ie. Tz = 0) at Ay = 1.5 G.

    Many Tim-cars are out there on the Enduro track, and thanks to their fantastically driver-friendly "balance", they are all cornering right on their Ay = 1.0 G limit,

    Now a Z-car goes out on track. Because of its awful balance it cannot get anywhere near its Steady-State limit (although it does have mind-snappingly fast turn-in, which is working a treat through the slaloms). Nevertheless, the Z-driver plods along well below the car's balanced SS limit, and ONLY manages about Ay = 1.2 G through the corners (ie. only driving at "8 tenths"). Said Z-car proceeds to lap the Tim-cars lap after lap after lap... (Do the sim of 1.0 G vs 1.2 G!)

    Moral - To win FSAE you DO NOT have to drive your car at its absolute limit. You ONLY need to drive faster than the other (student designed and built! ) cars.

    Z

    (* Tim-team achieved its perfect balance by crippling the grippy end of the car, to bring it into balance with the weak end. Z-team just kept chasing more grip at whichever end was the weakest.)

  9. #19
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    If you think you are able to pull an extra 0.5g of steady trim by reducing stability at the limit its quite clear which one of us has never engineered a car in his life...

  10. #20
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    He was just proving a point...
    ex-UWA Motorsport

    General team member 2013-15, Vehicle Dynamics Team Lead 2012
    Project Manager 2011, Powertrain minion 2009/10

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