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Car Simulation Headaches Part_#2


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#21 PochyPoch   Members   -  Reputation: 158

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Posted 26 September 2012 - 05:29 PM

Hi, bmarci !

I apologize for the long response time, but unfortunately, despite my best efforts, i had not that much interesting answers to bring.

The best progress i made so far was realizing front/rear weight transfer is mainly responsible for indesirable/strange behaviours. Not a great improvement...

At least i worked a lot on it, and cleaned a lot of things and started a better file configuration, and added simultaneous right/front weight transfer. Lateral weight transfer helped a lot in some situations.

Yes, I made a speed sensitive steering which seems to work fine until some heavy intervention is needed, so if my car starts to skid my steering doesn't let me to counteract because of the high speed, and there is only 2-3° of max steering available.

I have kind of the same issue. Maybe simply allow counter steering to go as far as player wants ? I was thinking of something more controled in case of counter-stering / drifting. Like preventing the player from spinning voluntary while cornering.

I use lateral pacejka. The mentioned uncontrollable situation occurs without my special addition.
Earlier I added an extra Vel_long-=wheel_roll_vel; to the contact patch velocity calculation (only for slip angle) since the wheel rolling also affects the contact patch's velocity.
So with that little extra the car is very stable at least on a straight move with lane changes. Maybe it's an "arcadish" tweak for better handling, but without it I have hard time having the car move on a straight line after a little direction change (A simple overtaking is a great challenge, the car is moving off the road and it's a pain in the ...... -somewhere- to put it back on the road again.)

I don't really understand your arcade tweak. But if the car don't want to go straight after a turn, you can try to remove the front/rear weight distribution or tweak a little side weight repartition. I have this issue sometimes because of strange weight balance.

I made a quick force calculations on lateral traction vs. weight transfer; (and noticed some weird result values, see later)

No weight transfer: (steer angle=1°)
Fz_left=3kN
Fy_left=937N
Fz_right=3kN
Fy_right=937N

That's 1874N total lateral force on the front wheels.

With weight transfer, same steer angle (75%R 25%L)
Fz_left=1.5kN
Fy_left=970N
Fz_right=4.5kN
Fy_right=745N

That's 1715N total lateral force which would mean the lateral weight transfer is bad to the stability???
Also my pacejka curve looks good when I display it, but in the last values more load generates less lateral force? I don't get it. Is that normal?
Could you check it with your formula?

I don't directly use the pacejka formula, instead i plot a curve with values. The curve i currently use is the one KJM gave above. So back to your question. Yes you have less response force in the end of the curve, despite maybe higher load.

You can consider pacejka curve and weight influence separatly. You can simply give 1N to your formula, get an output and finally multiply by the weight. So with less weight you can get higher forces if the slip angle is ideal. But ! for 90° slip angle for example, you should always get more force as you add more weight if the slip angle is still 90°. Unless i missed something.

The lateral weight repartition is a good thing if there is not too much and enough. If i have only lateral weight transfer without front/rear, it improves greatly stability, but i am not able to start drifts or to corner faster with controled oversteer. Unless my car naturally produce this oversteer.

The error in my theory is probably somewhere at the two wheels don't have the same slip angle at steering

You mean the two front wheels ? Mine have almost the same slip angle, but it cas vary a little depending on angular velocity for example. Not sure about what you mean ^^

I just got an idea:

Maybe it's time we started using the aligning moment (Mz) from our favourite Pacejka formula Posted Image

I'll try to integrate it into the simulation, so if everything works as I expect the wheels should go back to 0° when I let loose the steering wheel (when moving).
And maybe use steering torques as the drivers hangs on the steering wheel Posted Image and thus at the peak moment the steering and aligning forces should cancel each outher out.

Does it have any sense or absolutely pointless?

I added mouse steering, so any steer angle can be produced instantly. Very unconfortable, but the skidding is much more controllable.

It is intereseting idea, as i understand it this value can be used to produce response forces in racing driving accessories. Maybe you can use it, but i really dont know if it will improve car handling or drift situations/controling. Not a clue, but my guess would be to improve steer control first or to continue experimenting with your mouse until you can do some nice controled drifts.


An other note about stability. Since my car has 60% load on the rear axle, I played with the pacejka coeffs for the rear wheels so now they produce more lateral traction and thus more stable and eliminated the sudden oversteer, just a bit more grip as you mentioned.
BUT!
Now I cannot skid, not even with handbrake, also donuts work only with nose-weighted cars.
So now I have almost everything but they don't work together. If I reduce grip on the rear tyres I can make the parking-lot-show-off-manouvers but cannot take the first corner on the track. Soon I'm gonna be out of ideas.

We are at the same point my friend. I can have all behaviours separatly, but it is more a car which always have this behaviour than a regular car able to drift, etc. + i really offen have sudden car spins, or car starting to roll backward in a turn easily if i try to drift with a "normal" car. I am not out of ideas, but i am very tired to try things blindly. I think we really need an insight to see which parts of our simulations should be improved to allow for controled drifts with a car which is not naturally oversteering.

Weight transfer is really the thing that seems to have the more influence here. I just integrated front/rear & left/right at the same time, but when i do long turn, at some point the front outside wheel get the highest weight load (while inner rear has the lowest), and then my car spins immediatly.

In fact, all my control loss situations and so do make a lot of sense with regards to physics, the problem is not preveting them but making them fun/controlables. The real problem i almost always have is : when i lose control, instead of starting a controled/credible drift situation, car almost always spins immeditatly, which seems really unrealistic. I feel like i should had some kind of "static lateral" friction which would always be there or something like that i may have missed. I use pacejka forces and nothing else, maybe should i ?

If i have a naturally overseering car, it really feels like i am driving on ice, but i have controled drifts situations.

Also, i always need to multiply my normalized (for 1N) pacejka lateral output. If i get 1100N force for 1000N load @20° slip angle for example, my car is really on ice. I need to multiply by 5-10 to be able to turn at 100-150 kmH. I guess it is not normal right ? The car weight is 1500 kg.

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#22 bmarci   Members   -  Reputation: 395

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Posted 30 September 2012 - 11:44 AM

I have kind of the same issue. Maybe simply allow counter steering to go as far as player wants ? I was thinking of something more controled in case of counter-stering / drifting. Like preventing the player from spinning voluntary while cornering.

Yes, this is the point where I need to make more intelligent control. With a steering wheel it would be a piece of cake Posted Image


I don't really understand your arcade tweak. But if the car don't want to go straight after a turn, you can try to remove the front/rear weight distribution or tweak a little side weight repartition. I have this issue sometimes because of strange weight balance.

Yes, this is my problam, that neither do I understand it Posted Image Probably it's a bad idea that helps in some situation which should be solved by proper simulation of other (missing) parts (eg: suspension/diffs)

I don't directly use the pacejka formula, instead i plot a curve with values. The curve i currently use is the one KJM gave above. So back to your question. Yes you have less response force in the end of the curve, despite maybe higher load.

No, I meant less response force is generated by more load at the same SA/SR. Meanwhile I learnt, that it was correct, and it has a name "load sensitivity" Posted Image
And that is what you lose if you use normalized curve.

Weight transfer is really the thing that seems to have the more influence here. I just integrated front/rear & left/right at the same time, but when i do long turn, at some point the front outside wheel get the highest weight load (while inner rear has the lowest), and then my car spins immediatly.

I also calculated with instant weight transfer which shouldn't happen instantly. If I had proper suspension I'd have more transient transfer, and I'm not sure that all "dynamic" weight should be moved because of suspension and anti-roll-bars.

In fact, all my control loss situations and so do make a lot of sense with regards to physics, the problem is not preveting them but making them fun/controlables. The real problem i almost always have is : when i lose control, instead of starting a controled/credible drift situation, car almost always spins immeditatly, which seems really unrealistic. I feel like i should had some kind of "static lateral" friction which would always be there or something like that i may have missed. I use pacejka forces and nothing else, maybe should i ?

I don't think you should add static lateral friction, that should come from the tyre model. At least I would like to avoid all possible magic numbers here and there Posted Image
A haven't tought of speed sensitive downforce that every car generates, yet. And the differential. Now I have rigid rear axle, and I suppose you have that too Posted Image
I've seen in other sims using wings also for street cars to "emulate" downforce generated by the chasis shape. I don't think that has too much significance, though. More likely some addition to a more realistic car, and not to a spin or not to spin situation that we have.

Edited by bmarci, 01 October 2012 - 01:54 AM.





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