Working with Tire Models in Modular Vehicle Systems

Although there is no difference in the behavior of the Tire Models being used in a vehicle that has been created from Modular Vehicles Systems or from a Vehicle from Preset, there is slight differences in the way to work with it.

Using the Tire Models

From the Tool Box select the Tire Model group and instantiate a Tire Type extension. Right-click on a tire type and choose one of the available tire models. Next, add any number of Ground Materials field to one of the materials proposed by the drop-down menu (or type an other name if desired). The set of those material name are corresponding to the set of ground material that wheels of a given Tire Type can collide with. Have one entry with 'default' as the model to be used when no entry ...

For a given Tire Type, all ground materials must be different, otherwise a warning message appears.

Any number of this extension can be added to a given mechanism. But please, note that all Ground Materials from all the Tire Type of a given mechanism must be unique. Otherwise a Warning message will appear.

Before running the simulation, check for warning messages.

Important:

When using the Tire Models, the usual material table is ignored. There is only one exception. If among all Tire Type of a given mechanism, none has a material name equal to 'default', and if the Tire Type collides with a ground material that is not defined in its list of Ground Materials then the material table of the scene is used in the usual manner.
The material assigned to a Vehicle System's wheel component is ignored except in case of the exception given above.

Structure of the Tire Model Extension

First key point in setting up the Tire Models of a vehicle is to associate a Tire Type with every wheels of the vehicle. When the wheel travels on some ground surface, the simulation system identifies the Tire Type associated with this wheel and the material associated with the ground at the contact patch, and applies the appropriate ground reaction forces according to the given tire model properties. As such, for each pair of interacting tire type/ground material, the user can define a tire/ground interaction model which should be used to add the necessary reaction forces to the wheel.

The Wheel Adapter

The Wheel Adapter is used to assign specific Tire Type to a given wheel in a vehicle. Wheel Component from the Modular Vehicle Systems has this extension already in place. The Wheel Interface does expose the Tire Type and Wheel Contact does expose Wheel Adapter fields.

Selecting a wheel component from the wheel Modular Vehicle opens up its Properties panel for inspection.

Tire Type: Must be specified by picking one under the Tire Type Collection, otherwise you will see an error message.
Tire Pressure: The tire pressure (in pascals).
Ground Material Name: The name of the ground material in contact with the wheel.
Tire Model Name: The name of the tire model interacting with the ground is formatted as: "Tire Type Name" / "Ground Material Name". "Void" is set when the wheel is not in contact.
Normal Force: Average normal force at the wheel's contact patch (in newtons).
Tire Compliance: The tire compliance at the wheel's contact patch (in m/N).
Ground Compliance: The compliance of the soil at the contact patch (in m/N).
Tire Deflection: The deflection of the tire (zero for infinite tire pressure), in meters.
Ground Deflection: The ground deflection (due to compaction) at the contact patch (zero for hard ground tire models), in meters.
Longitudinal Slip: Longitudinal slip at the previous step.
Transversal Slip: Lateral slip at the previous step expressed in radians.
Longitudinal Friction Bound: The maximum allowed longitudinal friction force according to the current tire model (in newtons).
Longitudinal Friction Ratio: The ratio of the actual longitudinal friction force to the bound.
Lateral Friction Bound: Maximum allowed lateral friction force according to the current tire model (in newtons).
Lateral Friction Ratio: Ratio of the actual lateral friction force to "Lateral Friction Bound".
Alignment Moment: Restoring moment according to the current tire model (in N·m).
Tire Resistance Torque: Rolling resistance torque generated by the tire deformation (in N·m).
Compaction Resistance Torque: Rolling resistance torque generated by the ground compaction. Value is zero in case of hard surface ground interaction (in N·m).
Compaction Resistance Force: Rolling resistance force generated by the ground compaction. Value is zero in case of hard surface ground interaction (in newtons).
Camber: Angle between the normal direction at the contact and the side wall of the wheel (in degrees). A value of zero means that the wheel is vertical.

Tire Model Properties

Each Tire Type gives access to its tire model properties from the property panel.

A tire model defines the way a wheel reacts to the ground taking into consideration ground penetration, traction, side force, and resistance to movement. All tire models introduced in here are further explained in the Tire Models section of the Technical Notes.

There are a few inputs common to all models. The following uses the Coulomb Tire Model as an example.

Ground Material: The name of the Material assigned to the ground.
Rolling Resistance Model: Sets the rolling resistance model.
Damping Ratio: The damping scale factor on the critical damping as computed automatically based on the stiffness at the contact. Stiffness at the contact is computed based on the tire pressure and the width of the tire.

Note More information about the inputs shown above are found in the Tire Models section of the Technical Notes.