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Thread: Simulate what you can test and test what you can simulate

  1. #11
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    You might take a look through SAE 205A "Motions of Skidding Automobiles", Radt & Milliken. 1960. If you can't get the paper from your library or SAE, let me know.

  2. #12
    If you don't have any data acquisition at all....do not dispair! You still have some tricks at hand that you can use.

    If you run skid pad at several radii increments you can use the vehicle time to complete the each cycle, which allows you to calculate estimated lateral acceleration.
    While running these tests, if you place an O-ring on your damper shaft and push it up to the face of your body, you can that to find your max damper travel during each event.
    (as pictured in attachment, Doug may remember me suggesting this during the MIS design review last year )Combining this with the shop/lab tests suggested by Bill, you can tell a lot about your car.
    You can do the same with your steering rack. This will allow you to find max steer travel during the event. Again, combining this with lab kinematic measures can yield useful data.

    You can go on to do this for transient events such as slaloms as well. But, I suggest you start small, go fast, and turn left.

    I suggest you take those lab measurements, and turn them into suspension travel fit equations, then with a laptop and a set of calipers to measure the O-ring distance from the shock body you can figure out almost exactly how much steer angle you are using, your camber used during these events, the ride height of the car all around, pitch, roll, etc.
    If you have some way to record suspension movement, these can be turned into math channels instead, and recorded on your logger.

    Word of warning: These kinematic measurements neglect the compliance occurring unless you have an estimate for that from Bill's lab measures as well.


    If you do have data acquisition, preferably an IMU (3 axis accel, 3 axis gyro) or even just lateral accel and yaw rate sensor, I would suggest the increasing radius, slaloms of various spacing, and a "tire scrubbing" maneuver.
    Tire scrubbing, as in driving at slow speed in a straight line and sawing the wheel back and forth from limit to limit.
    This might not be an intuitive test, but the tire scrubbing maneuver can show you your "torque reserve" (analogous to engine controls) as I've started to think of it lately.
    This is essentially how much total aligning torque (Mz) your car is able to produce at that particular speed that is not being used up by the tire to produce lateral acceleration or longitudinal acceleration, which can be important for the short, technical sections of the track.
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    Last edited by MCoach; 05-15-2017 at 05:20 PM. Reason: words
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  3. #13
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    And the answer is ....

    "at the roadwheel" is a common oversimplification and may be the reason so many cars are mischaracterised and so many simulations come out wrong.
    You may be missing the compliance (or lack of) at the tierods (buckling), at the rack (mounts and rack/[imion pushaway) and the intermediate shaft (Cardan joint out of plane moments),
    Last I saw, a driver uses the steering wheel to drive and all measurements are universally referenced to it.

    By "lazy", I mean slow, takes a relatively long time to evolve. Usually slower than the preferred perception tim of an experienced driver ( < .28 seconds or better). Unsersteer doesn't reduce damping, it raises the plant's natural frequency. Look a where (DF -DR) appears in the transfer function denominators.

    The oversteering vehicle is stable at a specific speed until the the amount of oversteer exceeds the so-called Ackermann Gradient. Then the denominator goes negative and all your state variables go south (or north depending which hemisphere you are in. The Ackermann Gradient components tell you that a longer wheelbase is good, a lower speed is good, and I suppose smaller values of PI would help.

    Since the Ackermann gradient goes down squared with speed, there's not a long of help from it when you are going REALLY fast.

  4. #14
    Quote Originally Posted by MCoach View Post
    While running these tests, if you place an O-ring on your damper shaft and push it up to the face of your body
    A cable-tie works just as well and it much easier to fit & remove.

  5. #15
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    Reality Check

    Quote Originally Posted by turtle View Post
    I need to convince myself of a few things. Lets plot the lateral acceleration steering gain of an understeering, oversteering and neutral steer bicycle with respect to speed.




    By lazy, do you mean that the response looks almost first order?



    Hmm that's interesting. I've convinced myself that at oversteer does not necessarily mean open-loop unstable. However, there is a point where if I increase my rear cornering compliance too much, the step response blows up. The response of my oversteering example is definitely sluggish and appears to be dominated by the exponential response (no overshoot), but I would have thought the transition between the open-loop stable oversteer and open-loop unstable oversteer to be a little bit more exciting. Is there any significance to this transition?
    Take a look at what is probably more like a FSAE car:

    DF=2, DR=1

    and
    DF=2, DR=2

    and DF=1, DR=2.

    Total wgt = 200 kg, distributions 40/60 and 50/50, wheelbase = 1600 mm, steering ratio 4:1, Max speed 100 kph.

    These factors are likely good up to 1.0 g or so, then you gotta take charge.

  6. #16
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    And the answer is: SYNTHESIS

    Quote Originally Posted by turtle View Post
    I assume, if I had good confidence in my design-time parameters, the difference between the steady-state simulation and the test result (if I had any) could be attributed to some sort of compliance. How would I pick my understeer target in the first place?
    You would not pick the understeer as a target for a FSAE car. That would be a constraint, goal or target if you were designing a vehicle with large payload changes or anticipating forseeable misuse by owners replacing tires with aftermarket skins, etc.
    In this case, you would want to pick front and rear cornering compliances to deliver an optimum transient response suitable, acceptable and comfortable to your driver. That means you would choose the system dynamics of your car to work within their skills, reflexes and reactions.

    Here is a simple textbook kind of example of the technique.

    Lets say you have some sort of foundation of your car's architecture: Total weight, weight distribution, and speed range to be encountered.

    Plop them into a synthesis tool which can map out transient responses from hypothetical top tier chassis + tire subsystem specs:
    VHSYNTH.jpg

    Run a play which produces an array of response time vs. front and rear cornering compliance. Lateral acceleration response time is clearly the specific trait correlated to pleaseability. A crude display of this is also shown.
    DFDRTAY.JPG

    Now you need the response time target for your driver and a list of possibilities for one end of the car as far as the tires and K&C recipe:

    Based on cost, availability, performance and tuneability, let's say your team could produce a front cornering compliance (DF) of 3.00 deg/g. Then your requirement for a rear cornering compliance (DR) would be 2.47 deg/g on a 0.30 sec response time requirement, leaving you with an underteer of 0.53 deg/g.

    If your front tires and chassis can produce a DF of 2.00 deg/g (more likely), then your DR could be 2.017 deg/g and a slightly oversteering car. Does this all start to sink in ?
    This changes a bit if your driver has better reactions (let's say .24 response time more likely for Fangio) then your DF of 2.00 deg/g would need a DR of 1.79 deg/g to queue with such a driver.

    Keep in mind there are a few other issues here but the process is the same. Here I chose DF as a starting reference. Most of the time it would be DR, but you get the picture. And there is consideration for what happens as your g levels rise. FSAE car tires are so large, that their properties really don't change that much
    over the range of useage as I see. Wild guesses as to great nonlinearity are from wild imaginations. That's why the TTC is necessary.

    The next drill down would be to evaluate all the tire data to find tire properties that can deliver your DF when you add some sort of steering system. That's not always easy because of tire availability, steering mechanism limitations and packaging, etc. Finally, given the gain of the car in the speed range you are working with, you choose a steering ratio that minimizes hand movement and broken arms and shoulders from excessive steering torque. (did someone mention power steering for low speed maneuverability ???)

    Let me also remind you that as you reduce the steer ratio, reactions loads go up big time, so steering compliances get jacked up. It's very possible (and common) for a 'quicker' gear to produce a 'slower' car because the increased compliance slows the car's gain down faster than the quicker gear speeds it up. Sheet Happens !

  7. #17
    It seems to be a ghost town here, but there is a lot of useful information to digest. I will just go right ahead and ask more questions even if I don't fully understand everything just yet. Maybe some homework is in order

    Quote Originally Posted by DougMilliken
    You might take a look through SAE 205A "Motions of Skidding Automobiles", Radt & Milliken. 1960. If you can't get the paper from your library or SAE, let me know.
    Looks like the library archive goes to 1967 on microfiche (!!). The paper preview starts with a presentation of the model though I would be interested in what is presented after the model.

    Quote Originally Posted by MCoach
    While running these tests, if you place an O-ring on your damper shaft and push it up to the face of your body, you can that to find your max damper travel during each event.
    This is a very clever trick that I've got to try now.

    Quote Originally Posted by BillCobb View Post
    You would not pick the understeer as a target for a FSAE car. That would be a constraint, goal or target if you were designing a vehicle with large payload changes or anticipating forseeable misuse by owners replacing tires with aftermarket skins, etc.
    In this case, you would want to pick front and rear cornering compliances to deliver an optimum transient response suitable, acceptable and comfortable to your driver. That means you would choose the system dynamics of your car to work within their skills, reflexes and reactions.
    Wow! Lots of information to break down here. I understand the motivation of trying to design an FSAE for 'optimum' transient response. What I do not understand is how you design for variation in your vehicle model like in the road vehicle case you mentioned. Is there any reason to consider something like payload variation (heavier driver??) for an FSAE? And if so, how do you 'design' for these variations?

    Returning to the theme of simulation/testing, let's go back to the scenario of being broke without a fancy data acquisition system. Synthesis is one part of the puzzle, but what about characterization? Is there a simple way to invert this process from some sort or road test information to derive how far off the car was from it's design targets? I know it's discussed a bit in the other threads you have but I might as well bring it up.

    Another piece of the puzzle is finding the right driver. As much as I would like to think that I am a karting champion, the reality is that many of us are not the greatest driver. How can the transient response specs be selected early in the process such that the vehicle design accommodates the wide range of amateur drivers that I have to work with?

  8. #18
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    Ghost Buster

    It's a ghost town because so few members know what 'simulation' actually is. They confuse simulation with 'tools' . In that case, my best simulations here on the Farm are my Milwaukee battery impact driver and a 1/2" (13mm) box wrench. The catch all is a pair (2) of vice grips and a large flat head screw driver for mounting tires and fixing flats. These are my highly correlated simulations. I get good lap times around the fields with good tires.

    In fact, this FORUM is a TOOL, thus, I'd guess its a simulation !

    Meanwhile, 'Design Variation' usually involves 'Build Variation' because of all the slotted holes teams think they need. Instead, a well done suspension parameter modeling TOOL which include not only geometry but compliant bodies is used to produce a variation analysis. You fire thousands of random misbuilds at the 'tool' (Monte Carlo Analysis) and recover the SDF's (Suspension Design Factors). That include ride/Roll steer, compliance steer and camber, roll centers (force method), asymmetries, etc.
    This is then analyzed in bulk to identify the points and member elements with the highest participation is misbuild conditions. THOSE condition get the most emphasis during construction and part selection. You ought then to be able to 'net build'. You don't need all those drill holes that show only that you have no idea what you are doing, just wandering around, lost in the uncertainty forest. That's Monte Python Analysis. An alternative to the Monte Carlo technique is the BOB and WOW process. Best of the Best and Worst of the Worst. Build boxes around values for your key design parameters and evaluate your System Model with the 4 values in each position.

    For testing, search elsewhere in the forum for a "Characteristic Speed Test" procedure I submitted. Knowledgeable readers may recall that I measured the understeer of my speed boat. It's understeering with a 3 blade propeller, neutral with a 4 blade and oversteering with a 5 bladed Mercury Hi-Five ported prop.

    The right driver ? Eliminate the wrong driver(s). Its a show-me contest usually involving several cars: good, bad and ugly. A good driver produces the best times, saves the car parts (and tires), is smooth (smoove), and can tell you why. SCCA Autocross might be a good starting qualification, good technical background, a good balance of left and right brain functions (need the haptic elements). has had early childhood exposure to more than 0.1 g lateral force, and has not had severe head injuries from crashing their hot air balloon.

    That is all.

  9. #19
    From Merriam-Webster

    Tool: a handheld device that aids in accomplishing a task
    Simulation: the imitative representation of the functioning of one system or process by means of the functioning of another

    Tools with bad simulations. Simulations with bad tools. Maybe the definition of usefulness is when you have both!

    Couldn't find the characteristic speed test procedure, but I did find some references to it on another forum dating 2007.

    Vbox or equivalent process for fixed steer understeer test:
    Measure speed and yawrate or speed and lateral acceleration while SLOWLY increasing speed. Be careful about shifting gears so pick them wisely. Do this for several radii. Then from ay compute yawrate. A Vbox actually only measures steady state g so its perfect. Then in Excel or Matlab or whatever, make up a function with lateral acceleration on x axis and curvature (yawrate/speed) on y axis. Compute the slope of these multi-radii functions in both directions. The understeer is the negative slope of this function times the wheelbase in radius units. There is a pi thing in there, 2.
    If only I could extract DF/DR from a test as simple as the fixed steer understeer test

  10. #20
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    Smart App

    Several recent papers out there showing use of raw data from an Android phone (GPS, gyro, and ay) to calculate vehicle dynamics test values. Some may snicker because they can afford to use a $20,00 sensor, but what have you got to lose ?

    All you need is sisu.

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