How WheelColliders Work

01Quick Start

To change how a car grips and rides:

1. Suspension: select the vehicle, open the Suspension tab, pick Front Axle or Rear Axle, and edit spring, damping and distance.
2. Grip on every car and surface: open Tools → BoneCracker Games → Realistic Car Controller → Configure → Configure Ground Materials and change Forward Stiffness / Sideways Stiffness for a surface. See Ground Physics.
3. Less grip on one wheel or axle: in the vehicle's Wheels tab, click Edit next to a WheelCollider, open Friction & Grip, and lower Forward Grip or Sideways Grip.
4. Change the whole driving style: use a behavior preset. See Behavior Presets.

Warning: Raising Stiffness or Wheel Damping Rate on Unity's WheelCollider component does nothing. RCC overwrites both every physics step (see below).

02How a Wheel Is Built

Each wheel is a child of the vehicle's Wheel Colliders object. It has two components: Unity's WheelCollider (suspension, friction, raycast) and RCC_WheelCollider (grip, drivetrain flags, skidmarks, sounds, particles, flat tires, wheel-model alignment). Create WheelColliders adds both. See How to Create New Vehicles.

Figure 01Open full size ↗

Create WheelColliders writes these values to each new WheelCollider. Most come from RCC_InitialSettings.asset (Tools → BoneCracker Games → Realistic Car Controller → Configure → Configure Initial Vehicle Setup Settings). Changing that asset affects only WheelColliders you create afterwards.

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03What RCC Overwrites at Runtime

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How grip is calculated

Every physics step, each wheel sets its friction stiffness from the surface under it:

• Forward stiffness = ground Forward Stiffness × Forward Grip
• Sideways stiffness = ground Sideways Stiffness × Sideways Grip × handbrake factor × traction helper factor

Figure 05Open full size ↗

So the only ways to raise grip are the ground material's stiffness values. Forward Grip and Sideways Grip range from 0 to 1, so they can only lower grip.

Drift preset: grip that drops while sliding

Behavior presets with applyExternalWheelFrictions on also lower each wheel's friction curve while it slips. The only shipped preset with it on is Drift. While sliding, sideways grip falls to as low as 65%, and forward grip to as low as 75%. Front wheels lose more forward grip as they slide sideways. Rear wheels on an RWD car also get a sideways push at 0.8 throttle or more when sideways slip is 0.3 or more. All other presets leave the curves alone.

04The Friction Curve

Unity models tire grip as a curve of slip (how much the tire slides) against force:

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Forward friction handles throttle and braking. Sideways friction handles cornering. They are separate curves: a tire can spin forward while still gripping sideways.

05RCC Wheel Collider Inspector

Select a wheel under Wheel Colliders, or click Edit next to it in the vehicle's Wheels tab.

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Wheel State & Geometry

The four Can toggles are set by RCC unless Override All Wheels is on in the vehicle's Wheels tab:

The drivetrain is Wheel Type Choise (Engine & Gears tab). To set the toggles yourself, for example for a rear-steer forklift, turn on Override All Wheels.

Friction & Grip

Deflation Settings

Ackerman Steering

RCC uses these values to give the left and right steered wheels different steer angles.

06Skidmarks, Smoke and Tire Sounds

Each wheel reads the ground material under it. When the wheel's slip is above that material's Slip value:

• It plays the material's Wheel Sound. The volume rises with slip, up to the material's Volume.
• It emits the material's Wheel Particles.
• It draws skidmarks with the material's Wheel Skidmarks prefab, Wheel Width wide.

Figure 08Open full size ↗

Terrains are matched by splatmap layer. See Ground Physics.

To turn these off for every vehicle, use RCC Settings → Optimization → Do Not Use Any Particle Effects and Do Not Use Skidmarks. How many marks stay on screen is set by Max Marks on each RCC_Skidmarks prefab. The shipped prefabs use 1024, and the oldest marks are overwritten first.

Changed in V6: Skidmarks no longer allocate memory every frame. A wheel that drives onto a different surface starts a new skidmark chain, so marks no longer disappear when wheels are on different surfaces.

07Flat Tires

A tire goes flat only when it drives on a ground material with Deflate on (Configure Ground Materials), or when a script calls Deflate(). Collisions don't flatten tires. A flat tire:

• shrinks its radius by Deflate Radius Multiplier,
• multiplies its grip by Deflated Stiffness Multiplier,
• plays the deflate and flat-tire sounds and the deflate particles set in RCC Settings.

To fix a flat tire, call Inflate(). RCC.Repair(vehicle) also inflates the tires, but only when the vehicle has collision damage to repair.

using UnityEngine;

public class InflateAllTires : MonoBehaviour {

    public RCC_CarControllerV4 vehicle;

    public void InflateAll() {

        foreach (RCC_WheelCollider wheel in vehicle.GetComponentsInChildren<RCC_WheelCollider>())
            wheel.Inflate();

    }

}

08Tuning Levers That Work

09Performance

• RCC_LOD does not change wheels: wheel physics, wheel-model alignment and skidmarks keep running at every distance. What it does turn off is covered in Mobile Setup.
• Do Not Use Any Particle Effects and Do Not Use Skidmarks (RCC Settings → Optimization) remove those costs for every vehicle.
• A lower Max Marks on the RCC_Skidmarks prefabs uses less memory.

10See Also

• Vehicle Inspector Reference
• Behavior Presets
• Ground Physics
• How to Create New Vehicles
• Troubleshooting