Arbitrary steer inputs to ChassisSim
So, therefore, one could input a System's Engineering 'chirp' (time dependent frequency responce input), run the play, recover the appropriate data logger channels for steer, yaw velocity, roll angle, lateral acceleration and sideslip (for example) and recover the system dynamics in Bode form to further analyze gain, damping, stability, understeer, and phase margin metrics. Since these tests are easily run with actual vehicles, a comparison would be worth seeing. Yes, the test is considered a 'linear range' performance region evaluation, but there are ways to go beyond this because car nonlinearities are usually just softening springs.
Here is your signal for Excel SWA: = SWGain * =SIN((0.5*(time-1))^2). (use a gain of 1.0 or 0.1 at the road wheels if you don't have SWA)for starters.
Start the sim with zero steer up to time of 1.00 seconds and zero steer 1.00 second before a 40.96 second run segment finish time. Output sampling at .01 seconds will produce 4096 data points per channel for an ideal constant PSD, no aliasing, no filter required FFT process input.
Then compute the transfer functions for AY by steer, Yawrate by steer, sideslip by AY and roll by AY. You can do this Matlab in just a few statements.
The Bode plots will indicate quite a few revelations including goodness of the model and the simulation. Its a reality check for sure.
Even better: fit the Bode responses to s-plane transfer function form and deliver system gain, damping stability margin and phase margin from Matlab's built-in Systems Aanalysis toolbox.
Get an Indian student to do this and get +10 extra points and a complment from Z !!
Good cars test well...
Assumptions make an ASS of U and ME
Quote:
Originally Posted by Z;
Assuming "reasonably rigid bodies"....
Quote:
Originally Posted by
BillCobb
...unless your car body is made of gluten free pasta.
Unfortunately, Kraft mac and fsae is still a thing. So, you need to check the front axle and rear axle to see if your frame compliance (or suspension mounting, or anything else in the corner compliances along the load paths) is acceptable and not having undesirable dynamic effects (frame stiffness =< roll stiffness, oh no!). There are still many cars built for this competition today that would have better luck with some lard, sticks of pasta, and sheets of lasagna to hold the damn thing together. Assuming rigid anything would mean that the those with a 0.5" rear toe base and 3" of caster trail would also be sound logic. Even at the top, there is a reason ETS named their 2013(?) car "La Fromage". In the chase for minimum weight, finding the limits of what is considered "not-reasonably-rigid-but-maybe-"acceptably"-rigid-but-even-then-it's-barely-acceptable-do-you-think-that's-too-much?" for every system becomes a game in itself.
"In the final analysis, every engineering material is rubber"
- Sir Henry Royce, Rolls Royce
