# Custom Tire Force and Torque

Compute interactions and spatial relationships between tire and ground surface

*Since R2024a*

**Libraries:**

Simscape /
Multibody /
Forces and Torques

## Description

The Custom Tire Force and Torque block computes the interactions and spatial relationships between a tire and the ground surface.

To model a custom tire, use the outputs from the block to compute the tire force and
torque. Then, loop these signals back into the block as inputs. The tire force and torque must
be calculated relative to the contact frame of the tire. The contact frame is located at the
contact point and the *z*-axis of the frame is perpendicular to the contact
plane. The tire force must be nonnegative; or the block clips the input force to zero.
Additionally, you must maintain consistent units for force and torque throughout the
simulation. The image shows the diagram of a custom tire model.

The block has two methods to calculate the data that characterizes the interactions and spatial relationships between the tire and the ground. The closest point method determines the contact point by finding the point on the ground surface that is closest to the center of the tire and lies in the plane of the tire. The contact normal vector is located at the contact point and perpendicular to the contact patch at the contact point.

For scenarios where tires experience multi-point contact, such as off-road terrain or obstacles like speed bumps, use the weighted-penetration method. To simplify the computation due to the irregularities of the contact surface, this method computes an equivalent contact plane at each simulation time step to approximate the actual contact area. The contact point is the nearest point on this equivalent plane to the center of the tire. The contact normal vector is located at the contact point and perpendicular to the equivalent plane. For example, the image shows how the weighted-penetration method computes the equivalent plane, contact point, and normal vector when a tire encounters a ramp.

The weighted-penetration normal vector, *n*, is orthogonal to the
equivalent contact plane.

**Note**

When using the weighted-penetration method, the contact point may not lie on the actual ground geometry. For example, the contact point may be below or above the ground surface if the contact area is locally convex or concave.

## Ports

### Geometry

### Frame

### Input

### Output

## Parameters

## Version History

**Introduced in R2024a**