[Car Physics] Turbochargers and friends

Math and Physics Programming

Started by bmarci January 14, 2015 02:13 PM

24 comments, last by bmarci 9 years, 3 months ago

bmarci

825

Author

January 23, 2015 09:52 PM

There are a couple of things I managed to figure out;

Watching Edy's video I realized my car is making too much wheelspin, 570Nm and a gear ratio that spins the wheel at 120Km/h in first gear. (Thanks Edy :) )

I tried with "less powerful" setup and voila got much better.

My keyboard control was also too slow for drifting and took too much time to countersteer, that also didn't help get things better.

Yet, my pacejka combiner is still weird, I doublechecked the original Beckman's paper and everything seems ok, the situation is the following;

Starting the "drift maneuver" the rear end gets some angular momentum, and when it start skidding (still spinning tires) because the slip ratio is high the lateral force "disappears" and I'm unable to stop spinning out, and while the engine is revving at max it takes about a second to get back the traction.

Edy, would you like to discuss your pacejka combining method? ;)

Here is what I use:

Normalized slip parameters

s=slip_ratio/optimal_sr

a=slip_angle/optimal_sa

Normalized slip functions

Fx(optimal_sr) = Ox(1)

Fy(optimal_sa) = Oy(1)

p=sqrt(s^2 + a^2)

Combined forces:

Fx(s,a)=(s/p)*Ox(p)

Fy(s,a)=(a/p)*Oy(p)

With taking care of original and cimbined slip signs, it should be the famous Beckman's model :)

Chris_F

3,030

January 23, 2015 11:35 PM

I have no idea on how to do that but that is overkill for a game.

Unless you were making this game for gear heads who are very enthusiastic about this sort of thing...

Edy

373

January 26, 2015 08:45 PM

@bmarci Nice to have helped in some way!

The vehicle in my video doesn't use pacejka curves, but a constant friction coefficient. You can imagine it as a flat friction line instead of a curve. It uses isotropic friction (same friction properties in all directions) so it doesn't even use a friction combining method.

The actual development version supports several ways of configuring the friction curves, from a flat line to Simplified Pacejka (I've documented the simplified version at the end of this article). Still, Simplified Pacejka with isotropic friction doesn't require any combination method. I plan to add Pacekja'94 once the anisotropic friction is implemented.

While Beckman's is considered the best combination method it contains strong deficiencies. Let me illustrate the problem graphically. Consider these two simplified "friction curves":

[attachment=25658:2015-01-26_210535.png]

Using the Beckman's method to combine them results in this force graphic:

[attachment=25659:2015-01-26_210857.png]

While the curves are correctly combined at the peak position (traction ellipse) the transitions outside the ellipse are badly combined, producing distorted forces. Beckman + Pacejka would generate a similar graphic but with smooth edges.

I've designed a better combination method inspired on Beckman's that produces a correctly uniform combination out of both curves. It looks perfect on paper but I can't confirm if it works as expected until I implement the anisotropic friction in my simulation.

Edy / @VehiclePhysics
vehiclephysics.com

bmarci

825

Author

January 27, 2015 04:19 PM

Hi Edy, your graphs look interesting, I didn'd know earlier why I had to use a "random" factor, usualy 1.2 to scale Fy after combining, to have better result, but that also affcted the not-over-the-peak values too.

If I wanted to make a 3d combined graph plot function like yours, are the heights (z) the sum of Fx & Fy ?

Edy

373

January 27, 2015 05:47 PM

No, z is the modulus of the resulting final force, same as described in Beckman's book, chapter 25:

$$\sqrt{(F{_{x}})^{2}+(F{_{y}})^{2}}$$

Beckman captured the region around the traction circle so it looks fine at first sight. But if he would have plotted a wider area, similar distorted forces would have been exposed.

Edy / @VehiclePhysics
vehiclephysics.com