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# [Car Physics] - Traction Torque problem

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### #1jujunosuke  Members   -  Reputation: 172

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Posted 25 April 2012 - 03:22 AM

Hi guys, i need your help one more time. I am having issue i can't solve without ugly hacks.
Sometimes, the traction torque is stronger than the drive torque making my car go backward !

Let's say that at standstill i push the accelerator, i then get 200 of Torque.
the car speed is still 0 at this time so because of the wheel angular velocity generated by the drive torque, my slip ratio is very high.

So sometimes the resulting force from Pacejka i get is higher than the drive torque !

So, the hack i found is if the traction torque is higher than the drive torque i set the traction torque = drive torque / 2;
This kind of solve the problem, but it is very ugly.

Could someone can explain me the steps to calculate the traction torque the good way ?
Any help appreciated.

### #2kunos  Members   -  Reputation: 1312

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Posted 25 April 2012 - 06:36 AM

You'll need hacks at low speed, with speed going to zero, slip ratio goes to infinity. This is a common problem for every tyre model based on slip ratios.
There is a huge amount of informations on the internet about this problem.. check the old newsgroup rec.auto.simulators on google groups, it is full of informations about developing a racing sim.

The usual approaches include:

- Switch to a simpler "velocity based" tyre model at low speed. This is what I use in netKar under ~9 km/h . The simpler model is derived from the tyre model used for the game RARS. Again, google should point you to a good explanation of the model, it is very very simple.

- Implement a relaxation length approach with low speed damping. There is a SAE paper about it, also Ruud Van Gaal implemented this in his "Racer". There is an open source version of Racer available and it would be interesting for you to check it out. I am using relaxation lengths for my new simulator combined with a small hack for velocities under 1 m/s .

- Complete velocity based tyre model. This can include some amount of FEM.
Stefano Casillo
AssettoCorsa - netKar PRO - Kunos Simulazioni

### #3jujunosuke  Members   -  Reputation: 172

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Posted 25 April 2012 - 08:49 PM

Hum i see, i didn't know it was a common problem at low speed.

I think i will have some good reading for next month

Best regards.

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