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Thread: The DYNAMICS Part of Vehicle Dynamics

  1. #11
    Senior Member
    Join Date
    Mar 2008
    Brighton, MI


    Quote Originally Posted by DougMilliken View Post
    Bill, Just noticed that the link you provided to zipshare seems to be broken... -- Doug
    Zipshare only holds files for 5 days unless you pay a monthly fee. If I was posting a lot, it would be worth it (maybe).

    The bigger issue for me is the file size and type limit in the Forums (FSAE and TTC). Matlab code posts sometimes get scrambled because of the comments and special characters being used in the programming. The simulation and demo programs and presentations I produce are done with tools (Matlab and PowerPoint) that generate files a bit larger than Forums accept. I formally asked for some additions headroom, but never got an answer. I'd bet there is a selector box for changing this for some users.

    What has worked is private emails that I can send these teaching and demo tools to recipients. But, they then only benefit a few brave souls instead of the entire FSAE forum community.

    For example, I have a TTC data processing system that can produce low res and high res Pacejka and Spline tire model modules along with graphics of the results. Too big to bother trying to post. Can't submit Powerpoint or Excel. So, the 'educational enlightenment' ability aspect of the two forums is absent. Subjects of these posts would be using Matlab as a structured object oriented language instead of a rehash of 1960's Basic or the mucho more complicated Visual Basic Studio, as applied to Vehicle Dynamics solutions.

    I certainly not trying to criticize or complain, just pointing out the reasons I don't usually response to private requests for help. The two posts for VHSIM and NONLIN were just some fun factor demos I did because of a recent snowstorm. Given the processes, more serious and eye opening results can be produced.

  2. #12
    If you want to post code, I recommend pastebin.com or github.com. Pastebin is faster to do, github is less sketchy.
    Penn Electric Racing

  3. #13

    Importance of MOI (I_ZZ)


    Here are the results from the sim. Baseline configuration (first pic) has an I_ZZ of around 150 kg m^2 in the CAD software. The next two pics show the effects if this value gets bumped up four, and ten times.

    Screenshot (302).jpg

    Screenshot (304).jpg

    Screenshot (303).jpg

    There are at least 50 'corner' segments in a 75 segment autocross or endurance lap. They make up around 70% of the length of the lap. So, much more than a 'few seconds slower' if you had your MOI magnified by 10...

    IMO, MOI is important.

  4. #14


    This piece is important to the picture too...

    The moment of inertia to mass ratio.

    I think RCVD deals with this in Chapter 6? The ratio of k^2/(a*b). There's also the CF*CR/m^2 term and the term involving the understeer in the equation....

    Shows why 'heavy' cars with 'high moments of inertia' need not necessarily have slow response times. It's a tire thing...But, because this comp has an energy efficiency category and because more (raw material+power) typically ups my budget, I'd prefer to build light...

    When the front and rear cornering compliances and the inertia to mass RATIO are all fixed, the responses depend only on the distribution of mass, not the amount of.

    Screenshot (306).jpg

    Screenshot (307).jpg

  5. #15
    Quote Originally Posted by Vishnu Sanjay View Post

    Shows why 'heavy' cars with 'high moments of inertia' need not necessarily have slow response times. It's a tire thing...
    In yours examples you increase weight in your simulation but you keep same cornering compliance. This is maybe not possible in the real car, since the change in load on tire will change cornering compliance. So, it is not a correct conclusion in my opinion, maybe not for real world.

  6. #16
    Senior Member
    Join Date
    Mar 2008
    Brighton, MI
    Nope. Weights different, distribution the same, cornering compliances the same means you would need different tire properties to accomplish
    the same steady state gains and response time/bandwidth. Since within any of the popular weight ratings of tires, finding alternate sizes, brands,
    constructions and pressures which allow you to have identical cornering compliances is easy, straight forward and universally done. The ride
    quality may suffer a bit because of pressures, rim widths and aspect ratios needed to accomplish the identity, but for handling matches it's an
    easy deal. Marketting may not want to pay for premium tires, but the ability to engineer the configuration pays well.
    So, your FSAE car with cornering compliances of 3 and 2 or 2 and 1 or 2 and 2 deg/g would have the same steady state gains and dynamics
    as a dump truck with the same wheelbase if tires could be found that produced the required cornering stiffness coefficients. That's why dump trucks
    usually have at lease 4 rear tires and often 8.

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