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Thread: Testing, Testing 1,2 ...

  1. #31
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    Compliance with the Steering Rules.

    I am disappointed but not surprised at the lack of response, comments or interest in the steering compliance results posted here by Vishnu. (Commendable job, BTW, and very brave to reveal your data and model coefficients). I mean, of course, the form, traits, values and character of the test results.

    No response because the hunch I have has the probability of being 100% correct. You see, chances are good that there are no regular members here who have any ideas on what to do with this information. It's simple, really, the books, the forums, the papers and the plethora of software(s) have no "window" to acknowledge, accept, and incorporate this compliance effect. Words like "tire stiffness", "bump steer" (really when you do the bump???) are apparently the only real and necessary factors which influence vehicle dynamics expectations and results. Right ? Gotta have that "load transfer" and "TLLTD" vernacular probably, too.

    So, what's the Big Deal about this data and information? Where does it come from, why is it so important (BTW: it's VERY important no matter how many g's you are pulling). what are the manifestations and infestations of it, how do you incorporate it into simulation tools and how may have it contributed to this being a "winning" car ? (They said it was a winner).

    Anybody still awake? Can we read your take on this information, how should it get used and evaluated, why is it so important, and what did it do for this car (if anything) ? No further points for absent answers from the Soprano Prima Donnas. Let's discuss vehicle dynamics instead of wheel studs vs. nuts, dental floss for carbon fibre wings or what kind of wax to use for that 4th decimal place aero drag coefficient. Jump into the TTC Forum if you feel its necessary to continue...

  2. #32
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    I'm with Bill on this. Once you get past the stage of keeping the wheels on the car, then it is time to figure out where they are, relative to the sprung mass. Also, which way they are really pointing (steer and camber) ... under realistic loading conditions. Until you have done some kinematic and compliance testing, you won't have a clue.

    For anyone not familiar with the term compliance in this context it is 1/stiffness, the deflection for a given force. For example inches/pound or, more conveniently, "inches/1000 lb" (possibly "M/1000N"). Rotational compliance is in "deg/100 lb-ft" ("deg/100 N-M"). Morse Measurements have some nice tech articles on their site, but you don't need their expensive test time to get some rough data in your own shop.

  3. #33
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    Thank You, Doug !

    Thanks, Doug !. It's been so boring here, I went out and cut some hay and baled it.

  4. #34
    And thank you both for sparking a very interesting topic! Now where's Z, Geoff, Rob Woods and the rest of the guys? Missed a good old time thread like this. Seems that most of the guys nowadays spend their time on faceplace instead.

  5. #35
    Bill,

    I was hoping for some contributions as well. When I said my data was from a 'winning' car, I was also being a bit ironical. Somebody like Michael, from Monash, who posted on the thread earlier will probably have values more representative of a winner (They've won 7 of the past 8 Aussie comps, I think?). I was hoping for some feedback from someone like him (although he has mentioned earlier in the thread that he doesn't have any values for these at the moment).

    For starters, I thought I'd post a few results from the edited version of a simple sim that you had uploaded on:

    http://www.fsae.com/forums/showthrea...icle-Dynamics&

    I haven't changed much, just the way the inertia is represented (this way it's easier for a team to read the value directly from their CAD package).

    The two pics show what happens when your design (or not designed for) compliance is not the same as your manufactured compliance. The 'car' still has the same 'balance' or 'understeer' though (DF-DR=1, in both cases)

    Screenshot (302).jpg

    Screenshot (301).jpg

    The sim is, as you have noted elsewhere, is a simple one of a linear stiffness mechanism. But it does tell you (a lot of) what to expect and what not to leave out.

    As you have mentioned, the TTC Forums and data, can build on this.....

    You might also get a sense of what the "reasonable loading conditions" that Doug mentioned actually are.

    Hopefully this brings in some new inputs.

  6. #36
    This is probably when the carpet plots come in handy, and a chirp/pulse/square steer test that was briefly discussed earlier on.

    Imagine if you could combine this steady-state force/moment analysis to visualize the evolution of the transients with different initial/operating conditions.

  7. #37
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    Enf is Enough- II

    Prologue:
    I use the term "ENF" to describe steer ("E") from aligning moments and tierod forces (N) at
    the 'F'ront axle not resulting from any steerig wheel movement. Other compliances (inverse of stiffnesses)
    recognised, simulated and tested for are attributed to lateral force ("Y"), as in EYF and EYR, camber ("G")
    i.e. lateral force (GYF and GYR), and roll steer (EF and ER) and roll camber GR and GR. The force,
    moment or displacement induces the movement. The effect of a compliance is a direct slip or camber angle
    modifier. These compliances are designed for, managed, accounted for and verified in the specification
    of handling performance in virtually all productio vehicles. This is what K&C machines are used for.

    In the case of ENF, slip angle is attenuated because the steered wheels yield to the tire induced aligning
    moments as well as the effects of loads on tirerods from all sources (caster, caster offset, overturning
    moment, etc). The loading of the steering mechanism is usually the biggest contributor to this
    characteristic either by design or ignorance. Intermediate shaft cross section, isolators, Cardan joint expansion,
    steering gear mounting, rack push-away, T-bar stiffness in hydraulic power assist systems, wheel bearings and
    wheel stiffness are all players in this term.

    Even if the ENF term could be made to be zero, there is still a set of MZ moments from each tire that act
    on the vehicle as a rigid body. They can't be ignored as they are contributors to total vehicle
    understeering traits.

    ALL vehicles have the "features" of the steering compliance situation revealed here by Vishnu:
    it is a stiffening spring. It can get ugly, as you should all know, because tires are softening springs.
    Thus the flow of forces and moments becomes a dance to a certain extent that the vehicle processes
    to balance all of the forces and moments acting on it (usually input by YOU). Considering that the tires
    do not react instantaneously, your can be assured that the computation of all the interactions between
    these characteristics stirred together is mostly beyond the technology of most handwaving scientologists.
    Thus, they are ignored, and your computer simulated car handling is shown as nothing more than axle weights,
    wheelbase and a tire lateral force generation element. Say it ain't so, Rembrandt!

    As you would know from being TTC members, tire Mz's go NEGATIVE after a spell and so much of this
    disertation has the opposite effect: Your understeering effects become oversteering. When too much is
    enough, drivers loose control and you "speeeen out". At low sideforce levels your car is "tight" and
    higher up its "loose".

    A saving grace to all you FSAE designers, optimizers and wrenches, is that your tires have very
    low Mz to FY stiffness ratio (am I allowed to say that ???) and the tires are very load sensitive (Most are
    radials) so your rearward weight bias is livable, AND you also don't really go that fast so the effects
    of low understeer and some limit oversteer are second fiddle.

    BUTT, now the reality: Low tire pressure, + compliance, then higher pressure from heat makes the car
    "non-stationary" in engineering terms. i.e. it can be different every time you examine it, just like
    photons in the Michaelson-Morley demonstration.

    OK, still awake ?

    Most of you would be horrified to know that the list of favorite cars from Road & Track etc have gobs of
    this ENF stuff because there's no other way to make it safe (understeering) for Grandma when she takes
    the Ferrari out for a spin (so to speak). (Me thinks I have posted a scatterplot of the statistics on
    here previously). This quickens the response of the car by raising its natural frequency and settles the
    steering gain down as you increase speed. This loss of steering gain is often patched with a lower numerical
    overall steer ratio, but guess what: she-it happens: the lower gear ratio increases the loads on the
    gear mechanism so you can wind up with a car that's actually "slower" (less responsive) because ENF jumps
    up when you have steering parts made up from the wrong kind of pasta !!!



    OK. So, to recap:

    1) I've posted previously a 2 term log type math function that captures the essence of aligning moment/
    kingpin compliance. The B term regulates the initial stiffness while the C term indicates the 67% range
    of influence. Actual ENF has another useful property. Toyota refers to it as "viscous feeling" which is
    a 1st order steer velocity response trait happening because of the kinds of parts used. This affects the hysteresis you
    would measure as part of an ENF test. It implies that the steer velocity you apply affects the shape
    and width of the hysteresis area. A Simulink model of this makes for cool wall art.

    2) Tuning a car for constant understeer ("soft" steering stiffness with "hard" tires, merged with "hard"
    steering stiffness with "softening" tires as you corner harder) can produce an exilerating feeling vehicle.
    Most uniformed/naive evaluators decry understeer as a blemish on a car, but actually the departure from
    linearity is the hated understeer trap. Hence Derivative of Understeer wrt Ay is the actual flaw.

    3) The "best" cars feel good. It's a haptic realization that many drivers use. Steering MOMENTS are used
    as control inputs, not angles. When all is said and done, the steering wheel moment running through all
    the friction, gears and geometry transforms is how you control a car with a closed loop driver and a
    path constraint. The bandwidth and transmissibility qualities of steering controls can make a vehicle fun,
    safe, easy to drive and last a long time. If you have to wrestle a car to make it behave, the car wins.
    ENF is THE major player in this behavior. The car accepts your moment and cancels it.

    There you go ! Now let's have some twit bump us off this thread with an inquiry as to whether they
    should use psi or kPa for tire pressure settings... Ain't it the truth ?

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