# Traction and friction

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1. Nov 21, 2016

### synMehdi

Hi, I'm simulating the behavior of a car and I have some troubles with the tires. Specially with slip when the car is accelerating.
I've seen many documents over the internet and I understood 2 main things: Wheel slip is necessary to accelerate & the coefficient of friction depends on the slip.
To model a Tyre I'm using Pacejka magic formula which describes the relation between the coefficient of friction and the slip.
If I put torque (car speed=0 & wheel speed=0) in the wheel, there is no slip initially and no force to push the car. The wheel will slip due to the torque, and friction will occur which accelerates the car.This will eventually decrease slip. At some point an equilibrium will occur.
Am I right?
If my torque is bigger than the maximum force of the Pacejka formula (around slip=8%), Am I going to diverge?because friction will decrease when slip increases after that point.
I don't feel that this is what occurs in real life or maybe I didn't understand the wheel model.

Last edited: Nov 21, 2016
2. Nov 21, 2016

### pseudochaos

Wheel slip would take away from acceleration.

3. Nov 21, 2016

### synMehdi

Can you please further explain what do you mean?
If I simply apply the Pacejka model and my understanding of it. If my torque is bigger than the traction force. I will end having no vehicle acceleration and only wheel acceleration.

4. Nov 21, 2016

### Staff: Mentor

Not necessarily. Think of how clutch slip is used to maximize acceleration of a racing car...

5. Nov 21, 2016

### JBA

In an automobile drivetrain the slip at start to prevent instant slip at the wheels or motor stalling at start is provided by either the slipping of the clutch or the the liquid coupling between the engine and the transmission.

6. Nov 21, 2016

### CWatters

Acceleration is due to the friction force between tyre and ground. Simple F=ma.

However the friction force depends on the coefficient of friction. Usually this is either the coefficient of static friction (no wheel slip) or kinetic friction (wheel slip). Usually the coefficient of static friction is higher than that of kinetic friction.

7. Nov 23, 2016

### jack action

As slip increases, the traction force also increases up to a point (Extremum on the next image). That is the normal range for a normal drive. The slip you need, depends on the traction force you need (between 0 and Extremum).

If you reach a wheel torque that can produce a traction force greater than the maximum force, then the «extra» torque is converted into wheel acceleration, which leads to the slip increasing beyond Extremum. Because the traction force decreases with slip at this point, it just means wheel acceleration increases. At some point, the traction force does stabilize (Asymptote).

But because there is a traction force, there is also a vehicle acceleration (as long as there is no opposing forces), although it is not as high as if you had the complete wheel torque converted to traction force.

It is actually the same principle as static and kinetic friction:

Yes, tires always have slip when rolling. There is nothing better than extreme cases to demonstrate the point: