The Wheel collider component has a set of properties that simulate a vehicle’s suspension system.
To simulate suspension, the Wheel collider sets a Target Position on the Suspension Distance line that it always tries to return the center to, and has Spring and Damper properties that affect how it moves away from that position and how it returns to that position.
The Target Position of the Wheel collider is the point along the Suspension Distance line that the center of the Wheel collider always returns to when there are no forces (or equal forces) acting upon it.
To set the Target Position, specify a coordinate between 0 and 1 along the Suspension Distance line.
By default the target position is 0.5, exactly midway between the points of maximum suspension spring extension and compression. For most vehicle simulations, a typical value is between 0.3 and 0.7.
In a real-world suspension system, the spring connects the wheel and axle to the vehicle’s frame and body, and sustains the weight of the vehicle body. The spring extends and compresses in response to changes in the terrain, and absorbs some of the upward force from the ground, so that the vehicle’s body doesn’t respond to every bump in the ground’s surface. How well the spring can absorb energy depends on its stiffness.
In the Unity PhysX simulation, the Wheel collider simulates the spring by moving up and down the Suspension Distance line on the vertical Y axis, away from the Target Position. The value of the Spring property represents the spring’s stiffness (in newtons per meter).
A low value simulates a soft, flexible suspension spring that extends and compresses easily, without requiring very much force. In soft suspension, the flexible spring absorbs more bumps and jolts, so the vehicle’s body movement is smoother.
A high value simulates a stiff suspension spring that has more resistance to extension or compression, and therefore requires more force to move. In hard suspension, more bumps and jolts transfer to the vehicle’s body, but the vehicle overall has more responsive handling.
In a real-world suspension system, the damper opposes the suspension spring’s movement and dissipates its stored energy. The strength of the damper’s impact defines how quickly the spring slows down and stops bouncing after being compressed or extended. The damper is often called the shock absorber.
In the Unity PhysX simulation, the Wheel collider simulates the damper or shock absorber by reducing the energy of the suspension Spring. The value of the Damper property represents the rate at which the energy dissipates (in newton seconds per meter).
A high value simulates a hard damper that dissipates the Spring’s energy quickly. Hard damping quickly reduces bounciness and returns the Wheel collider to a steady state at the Target Position.
A lower value simulates a soft damper that dissipates the Spring’s energy slowly. Soft damping allows more bounciness before the Wheel collider settles back into the Target Position.
By default, the Wheel collider’s Spring value is 35000, and the Damper value is 4500. These default values assume that the car’s total mass is 1500 kilograms.
To set the vehicle’s mass, add a Rigidbody component to the vehicle’s root GameObject. For a vehicle to work well with the default suspension settings, set the Rigidbody’s Mass to the recommended value of 1500. You can then test and iterate based on your specific vehicle setup.
PhysX calculates masses and forces proportionally, meaning they depend on the relative distances between each value. If you want to use a lower mass value for your vehicle (to match other Rigidbody masses on different GameObjects in the scene), you must also decrease the Wheel collider’s Spring and Damper values in the same proportion. For example, if you set the vehicle’s Mass to 15, you should also adjust the Spring and Damper to 350 and 45, respectively, instead of 35000 and 4500. This ensures consistent and realistic behavior in your vehicle simulation.