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Member Since 05 Jul 2002
Offline Last Active Yesterday, 02:35 AM

Posts I've Made

In Topic: Car Physics: Questions about traction and grip

04 September 2015 - 03:23 AM

f=min(1, slip*10) * fz * mu

of course, this works only one direction, you must trim the slip value between -1 and 1 !!!

In Topic: Car Physics: Questions about traction and grip

04 September 2015 - 03:20 AM

Hi, for a 2d car you can start with a simple linear model with some limit at max grip. And scaling it with normal load and a magic number, let's call it grip factor.

f=min(1, slip*10) * fz * mu

f - resultant force
fz - weight on the wheel
mu - grip factor
slip - slipangle or ratio

the *10 is just a scale, so the tire reaches the limit at slip=0.1 you get the idea ;)

if you separate it to two different versions, for longitudinal and lateral, you can have a pretty nice handling

the fz can be an importanf factor if you calculate dynamic weight transfer, this will decrease grip on inside wheels when turning, so on rear wheels when braking, thus makink it spin out easier...etc.

hope this helped a bit

In Topic: [Car Physics] Turbochargers and friends

28 January 2015 - 02:13 PM

So, I made a plot of my Beckman's pacejka, and looks quite ... so as expected :)




No fancy 3d plots, just a heightmap made of sqrt(fx^2 + fy^2)

The upper area is for the negative slip ratio


The interesting thing is that my better working method looks pretty unexpected (at the left-right sides):




I'll keep experimenting...

In Topic: [Car Physics] Turbochargers and friends

27 January 2015 - 10:19 AM

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 ?

In Topic: [Car Physics] Turbochargers and friends

23 January 2015 - 03:52 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




Normalized slip functions

Fx(optimal_sr) = Ox(1)

Fy(optimal_sa) = Oy(1)


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


Combined forces:




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