# [Car Physics] - When should be calculated the Traction Torque ?

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Hi everyone.
In my car simulation i have trouble to know when should i calculate the Traction Torque.
I receive my Traction Force from a pacejka curve and then convert it to Traction Torque.

Should i apply the Traction Torque all the time ? Or only when i am accelerating ?

Also, In a Rear Wheels drive car, should the Rear Wheels angular velocity be the same as the car's velocity ?
One example, is when i am accelerating trough a wall, the nose of my car is facing the wall, and in first gear, i am accelerating like a mad. (Car is still 0 km/h because facing the wall).
My slip ratio is then really high, if i suddenly release the accelerator, how fast the rear wheels would go to 0 again ?
Almost instantly ?

Is there a special way to calculate that or like i said when i release the accelerator i can assume that the wheels speed goes at car speed ?

Thank you very much for any help on this, i am having trouble to understand this...
Best regards.

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Is there a special way to calculate that or like i said when i release the accelerator i can assume that the wheels speed goes at car speed ?

no that would be very wrong. The drivetrain is a set of indipendent bodies with their own intertia.
In your example, if you are spinning against a wall and you release the gas the tyres (and with them the entire drivetrain and engine) will slow down depending on the rotational intertia of the entire drivetrain block, the frictional forces developed at the ground and the engine brake torque.
So like everything else in physics this will happen during a period of time, that, if the tyre's angular velocities / inertia is high, can be quite long.

Your "traction torque" is actually a "engine torque", it should be applied at all time, with gas at 0 the engine produces a negative torque (ie. it will try to slow down). The fuction of gas vs. torque vs. rpm can be quite complex in real life but you can approximate it simply by using 2 lookup tables, one for open gas and one for closed gas. The output torque is then the linear interpolation on these 2 curves based on the gas level (0...1) .

Something like:

float powerTorque=powerCurve.getTorque( rpm );
float coastTorque=coastCurve.getTorque( rpm );

float engineTorque= powerTorque*gas + coastTorque* (1.0f-gas);

Finding power curves is trivial, most automotive magazines feature some sort of power curves but nailing a good engine brake curve is one of the black art of racing sims, as there is no documentation and data about this, and it is a very influential element in the behaviour of the car through a corner.

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Hi Kunos !
As always thank you very much for replying and thank you for your interesting input.
As i though it was not that simple haha..

I made some test previously and i was adding my TractionTorque all the time.
To take the example about burning the wheels in front of the wall (stupid example but very easy to understand), After releasing the accelerator, the wheels was spinning for quite some time and slowed down slowly. So, this is supposed to be expected ?...

So i probably was correct when i did my first test.. But it looked visually strange. That is why i had doubt about that.

So to recap, the drivetrain inertia is slowed down by its own mechanism + wheels friction on ground sending negative friction ?

Sorry about this but i did not understand exactly what do you mean by coastTorque ? This is the Torque of the wheels rolling ?
How to calculate it ? Is this coastTorque related to the car's speed ?

Anyway, thank you again Kunos !!
You always save me from my troubles !

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In car dynamics the word "coast" or "coasting" indicates the state when the gas is closed (at zero).
Imagine an engine at 6000rpm, with closed gas, the engine will be generating a negative torque. This is called "engine brake torque" or also coast torque.
Sit in the car with neutral gear engaged, you press the gas and the engine produces positive driving torque that accelerates the engine rpm up... when you release the gas the rpm goes down because, with closed gas, the engine is generating negative torque.
The reason for engine brake is to be researched in friction and air getting "stuck" inside the engine itself.

I suppose in your original test the wheels were spinning for too long because too high intertia and missing engine brake torque.

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Kunos, thank you again, it make a lot of sense.
I think that i get what is exactly this coast "negative" torque.

I suppose in your original test the wheels were spinning for too long because too high intertia and missing engine brake torque.[/quote]
Yes, that was true

Just one last precision though, At cruising speed with gas released, the coast torque and the traction torque should then cancel each other ? Because the coast torque want to brake but the car speed and the traction torque make it go in the other direction right ?

Also, when accelerating the traction torque is supposed to be opposed to the drive torque (so negative torque), but when i am in a coasting state, at 50 km/h, the traction torque is supposed to be positive right ? (I mean in the same direction as the wheel rotation)

Thank you again kunos for your help, very appreciated !
Best regards.

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no they don't cancel them out just as traction "feedback" torque from the wheels doesn't cancel out engine torque. Especially at low gears, the engine has a huge mechanical advance over wheels (higher than 4:1 in most cases) because of gear and final ratio.

And, at cruising speed you have aerodynamic drag wanting to slow the car down.

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Sorry this lead me to a new question, i was facing this lately.
I find it difficult to know how the wheels rotation and engine rotation are related together... It looks like sometimes they are linked together and sometimes work independently...

For example, at X speed, if you brake with lock wheels, the Engine rpm don't go to 0 but slow down slowly..
But when you accelerate full throttle, the wheels go at the same speed as the engine.

This is kind of confusing. I am sorry if i have so many questions, but i research myself on the web and it was kind of hard to find an answer..

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in a real car you have the clutch or a torque converter, that is why if you lock the tyres the engine doesn't go to zero.

if that happens in your simulator and you don't have the clutch modeled yet, then you have a bug

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I see. well, i think that close this topics
It was very interesting reading as always.

I will try my best to improve my simulation.
Thank you again kunos.

Best regards.

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Warning, loud noises. Plus tire physics:
#t=183s

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Rutin
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khawk
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A4L
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