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Thread: FSAE Data Swap Consortium

  1. #11
    Senior Member
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    I've noticed that FSAE cars don't tend to have a peak on their yawrate frequency response.

    Never got around to finding out why though...

  2. #12
    Senior Member
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    Yawn

    Quote Originally Posted by Tim.Wright View Post
    I've noticed that FSAE cars don't tend to have a peak on their yawrate frequency response.
    FSAE cars usually wind up will very little or no linear range understeer. As a result, their state variable response characteristics (yaw velocity and sideslip angle) have only first order (exponential) traits in the time domain and the peak yaw velocity gain is at zero Hz. in the frequency response domain.

    While the upside of this is the likelihood of higher maximum lateral acceleration capability in the limit, the downsides usually detract heavily from the car's performance. A first order response is sluggish and painfully slow. In the example I showed, the predicted lateral acceleration response times are about 0.34 seconds. This is well within the range of perception for even a novice driver so the sense of a lagging response to inputs is evident. BTW: this response time is about the same as a sport version of a pickup truck or production Boy Racer something. Most if not all serious production sports cars are well below this response time. They accomplish this, not necessarily with a lot of added understeer, but with higher cornering stiffness tires. Such a luxury is apparently not available to FSAE participants unless TTC data is used for tire selection.

    The low understeer also causes the steering gain (g's/100 deg SWA) to be quadratic with speed: The faster you go, the higher it gets in spades. This makes a car "twitchy" and awkward to drive because the inputs necessary to maneuver must be very small and controlled extremely well.

    A neutral steer car has another quirk that makes it a puzzle to design and evaluate. It is extremely sensitive to minor variations in build tolerances. A millimeter here and there, misplacement in an attachment point, a few N/mm stiffness change in a bushing or spring, and a few revs of hard work on a tire will greatly alter the gain and response times of the car. That usually means the car is different on every single lap and not much fun to drive over a course with many laps as the car wears out. Or, just changing out a suspension piece with itself will readjust the car because of slop in the holes.

    But all that really matters is that max-lat number, right ? The value nobody ever gets to enjoy.

    If you have test ability(s) a great experiment with such a car is to add another set of wheels and tires to it (it's a dually, cowboys). This immediately lowers the rear cornering compliance which gives you some linear range understeer, shorter response times and a more manageable steering gain. You'll probably get a yaw velocity by steer gain plot that peaks above 1. Hz. too.

    Looks funny, though (the dually)...

  3. #13

    Robin the Hood

    Quote Originally Posted by BillCobb View Post
    If you have test ability(s) a great experiment with such a car is to add another set of wheels and tires to it (it's a dually, cowboys). This immediately lowers the rear cornering compliance which gives you some linear range understeer, shorter response times and a more manageable steering gain. You'll probably get a yaw velocity by steer gain plot that peaks above 1. Hz. too.

    Looks funny, though (the dually)...
    To try to increase cornering stiffness, you could also mount the tires on the widest wheels they unreasonably fit on.
    Still looks funny though...although much more inner city and less outback.
    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

  4. #14
    Quote Originally Posted by MCoach View Post
    To try to increase cornering stiffness, you could also mount the tires on the widest wheels they unreasonably fit on.
    Still looks funny though...although much more inner city and less outback.
    It might be time for some of you boys to start shaving.

  5. #15
    Bill... Thanks for your valuable and professional contributions to this forum!

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