I tried ported the vehicle_joints3 example from rapier to use bevy_rapier, but the steering move the body instead of the axle.

[ivanceras]

https://github.com/user-attachments/assets/9b92c30a-6c8a-44f4-8bc7-993b7012af49

The other issue is that the collider seems to be off, as the rear wheel of tire just floats instead of touching the surface of the orange obstacle.

Code is here

[Mitchelldscott]

I've been working on a similar setup, it's nice to see some of the same struggles and I wish I saw this example sooner. I spent a good amount of time getting the mass properties and motor parameters to play nice and if positions/axes of colliders and joints didn't correspond perfectly things got weird. Also I definitely had issues when my axle colliders were to small, not sure why that effected the joints (seems like it unlocked locked axes).

in case it helps here's my attempt

use bevy::prelude::*;
use bevy_panorbit_camera::{PanOrbitCamera, PanOrbitCameraPlugin};
use bevy_rapier3d::prelude::*;

fn main() {
    App::new()
        .insert_resource(ClearColor(Color::srgb(0.1, 0.1, 0.2)))
        .add_plugins((
            DefaultPlugins,
            PanOrbitCameraPlugin,
            RapierPhysicsPlugin::<NoUserData>::default(),
            RapierDebugRenderPlugin::default(),
        ))
        .add_systems(Startup, (setup_graphics, setup_environment, setup_physics))
        .add_systems(Update, car_controller)
        .run();
}

fn setup_graphics(mut commands: Commands) {
    // either I'm crazy or moving the camera perturbs the rigid bodies
    // set the chassis to fixed so the car is floating in the air,
    // then swivel the view around and the wheels should start bouncing
    commands.spawn((
        Camera3dBundle {
            transform: Transform::from_xyz(10.0, 3.0, 0.0)
                .looking_at(Vec3::new(0.0, 3.0, 0.0), Vec3::Y),
            ..Default::default()
        },
        PanOrbitCamera::default(),
    ));
}

fn setup_environment(mut commands: Commands) {
    // add other obstacles here later
    let ground_size = 500.0;
    let ground_height = 0.1;

    commands.spawn((
        TransformBundle::from(Transform::from_xyz(0.0, -ground_height, 0.0)),
        Collider::cuboid(ground_size, ground_height, ground_size),
    ));
}

#[derive(Component)]
pub struct Wheel;

#[derive(Component)]
pub enum Axle {
    Steered,
    Fixed,
}

fn setup_physics(mut commands: Commands) {
    let start_height = 1.0;

    // side to side
    let chassis_width = 0.5;
    // front to back
    let chassis_length = 1.0;
    // top to bottom
    let chassis_height = 0.25;

    // range of motion, a very stiff spring will hold the
    // suspension in the middle of this range
    let suspension_height = 0.3;

    // wheel dimensions
    let wheel_radius = 0.2;
    let wheel_width = 0.1;

    // collision group to prevent collision between car parts
    let car_group = Group::GROUP_1;

    // simple cuboid to represent the chassis
    let chassis_collider = (
        Collider::cuboid(
            chassis_width / 2.0,
            chassis_height / 2.0,
            chassis_length / 2.0,
        ),
        ColliderMassProperties::Mass(100.0),
        CollisionGroups::new(car_group, !car_group),
    );

    // bevy entity with rigid body marker and chassis collider
    // set to spawn at start height
    let chassis_id = commands
        .spawn((
            TransformBundle::from(Transform::from_xyz(0.0, start_height, 0.0)),
            RigidBody::Dynamic,
            chassis_collider,
            Sleeping::disabled(),
        ))
        .id();

    // wheel positions are defined relative to the chassis size
    // this was important before using collision groups,
    // now its just for the aesthetic
    let x = 0.6 * (chassis_width + wheel_width);
    let y = -chassis_height;
    let z = 0.45 * chassis_length;
    let wheel_positions = vec![(x, y, z), (-x, y, z), (-x, y, -z), (x, y, -z)];

    for (x, y, z) in wheel_positions {
        // anchors set the joint relative to the collider, this sets the axle
        // joint to be level with the chassis and directly above the axle
        let axle_anchor1 = Vec3::new(x / 2.0, 0.0, z);
        let axle_anchor2 = Vec3::new(0.0, -y, 0.0);

        // sets the locked axes of the axle joint, both are free
        // in linear Y (suspension) and the front is free in angular Y (steer)
        let locked_axle_axes = match z > 0.0 {
            true => JointAxesMask::all() ^ JointAxesMask::ANG_Y ^ JointAxesMask::LIN_Y,
            false => JointAxesMask::all() ^ JointAxesMask::LIN_Y,
        };

        // set the axle marker indicating if an axle is fixed or free to steer,
        // not really used anymore, could just be a steered axle marker
        let axle_marker = match z > 0.0 {
            true => Axle::Steered,
            false => Axle::Fixed,
        };

        // I think a spring joint would work here, but it's pretty simple to define everything
        // as generic. Also where's the into()?
        let axle_joint = GenericJointBuilder::new(locked_axle_axes)
            .local_anchor1(axle_anchor1)
            .local_anchor2(axle_anchor2)
            .limits(
                JointAxis::LinY,
                [-suspension_height / 2.0, suspension_height / 2.0],
            )
            .set_motor(JointAxis::LinY, 0.0, 0.0, 500.0, 500.0)
            .build();

        // hacky jump feature, should really have a second collider fixed
        // to the chassis to actually model the suspension expansion.
        // but this is simpler and fun
        let axle_impulse = ExternalImpulse::at_point(
            Vec3::ZERO,
            Vec3::new(x / 2.0, y + start_height, z),
            Vec3::new(x / 2.0, y + start_height, z),
        );

        // collision for the axle, because it won't move without one
        let axle_collider = (
            // making the ball any smaller makes the joints to weak...
            Collider::ball(y.abs() / 2.0),
            ColliderMassProperties::Mass(50.0),
            CollisionGroups::new(car_group, !car_group),
        );

        // bevy entity for an axle
        let axle_id = commands
            .spawn((
                TransformBundle::from(Transform::from_xyz(x / 2.0, y + start_height, z)),
                RigidBody::Dynamic,
                axle_collider,
                axle_marker,
                axle_impulse,
                Sleeping::disabled(),
                ImpulseJoint::new(chassis_id, TypedJoint::GenericJoint(axle_joint)),
            ))
            .id();

        // Would be nice if RevouteJointBuilder took two axes, instead the collisions
        // it connects need to have the same orientation.
        // Generic joints are fine for now, also more flexible
        let wheel_joint = GenericJointBuilder::new(JointAxesMask::LOCKED_REVOLUTE_AXES)
            .local_anchor1(Vec3::new(x / 2.0, 0.0, 0.0))
            .local_axis1(Vec3::X)
            .local_axis2(Vec3::Y)
            .set_motor(JointAxis::AngX, 0.0, 0.0, 0.0, 1.0)
            .build();

        // cylinder wheel collider, needs to be rotated to face the right way,
        // that means it can't use a revolute joint
        let wheel_collider = (
            Collider::cylinder(wheel_width / 2.0, wheel_radius),
            Friction::coefficient(10.0), // make sure it doesn't slide around
            Restitution::coefficient(0.5), // wheels should bounce right?
            ColliderMassProperties::Mass(10.0),
            CollisionGroups::new(car_group, !car_group),
        );

        // bevy wheel entity with 90 degree rotation
        commands.spawn((
            TransformBundle::from(Transform {
                translation: Vec3::new(x, y + start_height, z),
                rotation: Quat::from_rotation_z(-std::f32::consts::PI / 2.0),
                scale: Vec3::ONE,
            }),
            RigidBody::Dynamic,
            wheel_collider,
            Wheel,
            Sleeping::disabled(),
            ImpulseJoint::new(axle_id, TypedJoint::GenericJoint(wheel_joint)),
        ));
    }
}

fn car_controller(
    _time: Res<Time>,
    keys: Res<ButtonInput<KeyCode>>,
    mut wheel_query: Query<&mut ImpulseJoint, With<Wheel>>,
    mut axle_query: Query<
        (&mut ImpulseJoint, &mut ExternalImpulse, &Transform, &Axle),
        Without<Wheel>,
    >,
) {
    let mut speed = 0.0;
    let mut steer = 0.0;
    let mut jump = 0.0;

    if keys.pressed(KeyCode::KeyW) {
        speed = 30.0;
    }
    if keys.pressed(KeyCode::KeyS) {
        speed = -30.0;
    }
    if keys.pressed(KeyCode::KeyA) {
        steer = std::f32::consts::PI / 6.0;
    }
    if keys.pressed(KeyCode::KeyD) {
        steer = -std::f32::consts::PI / 6.0;
    }
    if keys.just_pressed(KeyCode::Space) {
        jump = 400.0;
    }

    for mut wheel in wheel_query.iter_mut() {
        // Whats a factor?
        wheel
            .data
            .as_mut()
            .set_motor_velocity(JointAxis::AngX, speed, 50.0);
    }

    for (mut joint, mut impulse, tf, axle) in axle_query.iter_mut() {
        match axle {
            Axle::Steered => {
                joint
                    .data
                    .as_mut()
                    .set_motor_position(JointAxis::AngY, steer, 10000.0, 10000.0);
            }
            _ => {}
        }
        // jump is just an impulse on each of the axle colliders.
        // it should be an impulse that expands the suspension to it's limit
        // then the car jumps because of the momentum of the chassis.
        impulse.impulse = tf.rotation * Vec3::Y * jump;
        impulse.torque_impulse = tf.rotation * Vec3::X * -jump / 60.0;
    }
}

[ivanceras]

@Mitchelldscott your code works well, the axle is at the side of wheel which I think fixed the steering part. However, the driving is a bit wonky and the car wobbling side to side. This is fixed by lowering down the stiffness in the set_motor_position function to 1000.0. This is the same value as the original vehicles_join3 in the rapier repo.

.set_motor_position(JointAxis::AngY, steer, 10000.0, 1000.0);

   for (mut joint, mut impulse, tf, axle) in axle_query.iter_mut() {
        match axle {
            Axle::Steered => {
                joint
                    .data
                    .as_mut()
                    .set_motor_position(JointAxis::AngY, steer, 10000.0, 1000.0);
            }
            _ => {}
        }
        // jump is just an impulse on each of the axle colliders.
        // it should be an impulse that expands the suspension to it's limit
        // then the car jumps because of the momentum of the chassis.
        impulse.impulse = tf.rotation * Vec3::Y * jump;
        impulse.torque_impulse = tf.rotation * Vec3::X * -jump / 60.0;
    }

Your also missing the differential speed code which improves the smoothness of the driving.

    let differential_strength = 0.5;
    let sideways_shift = steer.sin() * differential_strength;
    let speed_diff = if sideways_shift > 0.0 {
        f32::hypot(1.0, sideways_shift)
    } else {
        1.0 / f32::hypot(1.0, sideways_shift)
    };

    let ms = [1.0 / speed_diff, speed_diff];

    for (mut impulse_joint, wheel) in wheel_query.iter_mut() {
        let ms = if wheel.is_left{
            ms[0]
        }else{
            ms[1]
        };
        if wheel.is_driving{
            impulse_joint
                .data
                .as_mut()
                .set_motor_velocity(JointAxis::AngX, speed * ms, 50.0);
        }
    }

Thanks to your code, I can continue my progress on the vehicle physics.

[ivanceras]

Correction:

Lowering the friction coefficient of the wheel to 1.0, also removes the driving wobbliness as well, even if the stiffness is still at 10000.

[Mitchelldscott]

I had issues lowering the steering stiffness (the momentum of the car would cause the wheels to turn when accelerating). I didn't want to lower the friction because then the wheels may slip without you knowing, which is pretty unrealistic. I agree that differential kinematics should make it smoother but I had actually reduced the steer stiffness so that the steer angle could perturb, wheels should align with the direction they're going due to torque from friction (which is a sketchy implementation). I should have played a little more with the velocity stiffness because "flexibility" there would help. I did not implement that differential steering because then I would have tried to turn it into a whole module.

Correction:

1. Friction constant for tires should be around 1. F1 cars are closer to 2.0, Baja trucks will have around 0.5 and of course it all depends on the surface.
2. "Flexible" steering can help but can also hurt your steering control, better to use differential kinematics