Tire Models

Vortex® Studio includes the Tire Model feature in Vehicle Systems to accurately capture the interaction between the wheels of a vehicle and the ground surface material.

Different Types of Tire Models

Tire models can be divided in two main categories: hard grounds and soft grounds. These models allow you to realistically capture the friction forces between the tire and the ground, taking into account both the material in contact and the driving maneuvers (i.e., whether the vehicle is accelerating, breaking or steering, the longitudinal and the lateral friction forces are changing). In case of soft ground models, the deflection of the ground is responsible for resistance to movement. This effect is important to capture, for example, in case of off-road mobility analysis fuel consumption or driver training ability.

Hard ground

The hard ground category of models is used in cases where the ground is infinitely rigid. This category accurately captures the friction in the forward and the lateral direction of the tire movement which depends on the vehicle's maneuvers. Therefore, the traction available and the resistance to sideways movements are more realistic. Hard ground models also capture the alignment moment.

Vortex provides the following hard ground models:

  • Magic Formula
  • Pacejka Magic Formula 2002
  • Composite Slip
  • Fiala
  • Coulomb

Note The Coulomb model is not a tire model per se. However, it has been included for any users who still want to use it. It allows the user to treat all wheel interaction models in an unified way, particularly for the treatment of the tire pressure and rolling resistance.

Soft Ground

As with the hard ground models, friction forces are accurately computed, including the fact that the friction depends on the size of the contacting tire patch. In addition, the compliance of the soil includes resistance to the wheel movement as a resistive force and a resistive torque.

Two fundamental mathematical relations have been developed for soft ground models: one that relates the pressure on the ground and the sinkage, and the other that defines the shear stress to the shear deformation (strain). Because of the wide variety of soil characteristics that can be found in nature, a single unique mathematical model that can describe all of the cases does not exist. For this reason, a few different models have been implemented.

Pressure sinkage models

  • Bekker
  • Wong
  • Reece
  • Muskeg
  • Snow

Shear stress- strain models

  • Exponential
  • Hump
  • Wong

Lateral force Model

In the case of hard ground models, the lateral friction force is given directly from the models. In case of soft ground, the lateral force is obtained by a side force model that uses an exponential function of the lateral slip. More details can be found in the tire model's technical notes.

Tire Pressure and Rolling Resistance

Tire pressure is used to determine the tire stiffness which results in tire deflection. Such deflection, among other, is responsible of rolling resistance. Few models of rolling resistance that depends on the tire pressure are included. In addition, if a soft ground tire model is used, the tire deflection changes the size of the tire patch which will have a consequence on the compaction resistance force and possibly on the friction forces ( which happens if the ground property is not cohesionless). Please, take note that, changes in tire patch size is implemented only for Wong and Bekker pressure sinkage models.

Working with Tire Models

There is slight differences in working with Tire Models depending on if a vehicle has been created from the Modular Vehicle Systems or the from a Vehicle from presets. Please follow the link that corresponds to the case of interest.