Thread: Physics, Customization and Manual

This thread is erased a bit and renamed to Physics, Customization and Manual.

I’m going to post here something info about vehicle dynamics so anyone could read and understand to tweak vehicle physics and understand updates. Maybe I will compile these posts and make online manual or something like this.
I begin with tires.
When a vehicle drives it contact with a road by tires so tires is the most important thing. It’s logically start from tires and some theory.

At the picture below you can see Tire Axis System that XMR uses.



You can compare it with SAE Axis that is attached.



A vehicle accelerates and brakes because of applied longitudinal tire force (Z axis).
If Z > 0 then tire accelerate a vehicle. If Z < 0 then tire brakes a vehicle.
A vehicle turns because of applied Lateral tire force (X axis).
If X > 0 then tire turns a vehicle to the right, else if X < 0 then tire turns a vehicle to the left.
A tire provides longitudinal and lateral forces if Normal force (Y) > 0. If Normal force is equal 0 then X and Z must be 0 also.


You can see tire forces on this video.

Longitudinal and lateral forces depend on slip ratio (slip) and slip angle. Camber thrust adds lateral force if camber angle is not zero.
Longitudinal force depends on slip ratio. XMR uses Sakai slip definition:

Traction Slip = V * cos (A) / w*R – 1
Braking Slip = 1 – (w*R)/(V*cos(A))


V – speed of the tire over the roadway
A – slip angle of the tire.
w – angular velocity of the tire
R – effective radius of the tire

Slip angle is an angle between a rolling tire’s direction and the direction of the tire heading.
You can see that Traction Slip < 0 and Braking Slip > 0.
Slip is limited by [-1..1]
For example, traction slip is -0.5 when the tire spinning (2X free rolling).
-1 Slip when tire spinning (infinite).
At this picture you can see a typical longitudinal forces (tractive or braking) diagram when a slip angle is zero and tire load is equal to 4 kN.



You can see when the tire has the maximum tractive and braking forces. Tractive force at -1 Slip is less than Tractive force at -0.5 Slip. The same behavior for braking forces.

On the picture below you can see a typical lateral force diagram when a slip is zero and tire load is equal to 4 kN.



You can see that lateral force is max at 7 degree of the slip angle. Lateral force is decreasing with the increasing of the slip angle. So you will have less grip at large slip angle.

And a short video to see slip and slip angle

Combined slip Pacejka Magic Formula is implemented as an alternative tire model.
This model gives good results and allows using data from the real tires.
To read more about pure-slip Pacejka model (not combined slip) click here.

Before this you could see lateral forces diagrams when slip = 0.
Lateral forces are different with the different slips.
On the picture you can see lateral forces when slip = 0, 0.1 and 0.25. This is just an example of lateral tire force variations.

VehiclePhysics displays tire forces more obviously.
You could change longitudinal slip real-time for the lateral tire forces to see its behavior.
On the pictures you can see lateral forces for different longitudinal slips.


Or the full 3D diagram where you can see forces variations with longitudinal slip and slip-angle.


On the short video you can see forces variations and its influences on the vehicle.

It’s nice to see static tire forces diagrams on the VehiclePhysics.
But it’s better to study them real-time under load, tire pressure, temperature, longitudinal slip, slip angle, velocity etc.
Telemetry Tire system allows to study tire model for the each tire (Left Front, Right Front etc).
This is something like tire “debugging”.
Each column consists of longitudinal and lateral tire forces and their components.
There are four columns for the four tires - LF, RF, RR, LR.
On a short video you can see telemetry tire system under different tire conditions like accelerating, braking, turning etc.

Camber thrust (or camber force) this is a lateral force in the direction of the tilt.
Camber thrust occurs on the pneumatic-tire wheel. Camber thrust depends on the camber and slip angles, load, pressure, construction etc. Camber thrust may add to or reduce from the total lateral force, depending on the camber angle. When a car turns around a corner camber thrusts for the front and rear tires can be in a different direction and lead to oversteer or understeer. It depends on the default camber angle, suspension kinematics and road surface tilt.
You can see camber thrust on the pictures below.




A short video that displays camber angles tweaking

The dependence of grip on the temperature is one the most tire characteristics.
Temperature affects the lateral and longitudinal grip.
For example when tire is cold it has minimum of grip.
If temperature increases, grip increases and reach its maximum on something value of tire temperature.
It’s very easy to setup temperature behavior in VehiclePhysics utility.
You just need to tire (front or rear) and set a graph of grip change on temperature.
You can watch current tire temperature in telemetry system - right bottom corner.
Thus, you can specify any change of lateral/longitudinal grip on temperature and watch them in XMR real time.

The same way you can specify tire properties on load, pressure and speed in VehiclePhysics utility.
There one additional characteristic for the lateral properties – lateral speed.
It specifies lateral properties on the lateral speed of the tire.
When the tire is ready you can save or export it as a tire file.
Ready tire can be to applied for any other car on the pit stop or VehiclePhysics utility.

XMR can use two tire models.
- Pacejka tire model. This model is well known and needs no introduction.
- 100% Customizable tire model. You can specify any tire properties you like. This model is more complicated, but gives almost absolute freedom to specify tire properties.

You should select in VehiclePhysics utility what tire model you want to use.
You can see how to toggle between combined-slip Pacejka and customizable tire model.