Roblox Custom Vehicle Physics: 4-Wheel Raycast Suspension, Tire Friction & Luau Springs

Creating responsive, arcade-smooth or hyper-realistic driving in Roblox requires ditching built-in CylindricalConstraints and SpringConstraints for competitive games. Collision glitches and network desync quickly destroy the driving experience.

By implementing a 4-wheel raycast suspension model based on Hooke's Law, custom Pacejka tire slip-angle friction, dynamic anti-roll bars, and client network ownership, developers can build buttery-smooth cars capable of high-speed drifting and off-road stability.

1. Why Raycast Suspension Outperforms Physics Constraints

Overcoming the limitations of physical collision wheels in Roblox:

• Zero Terrain Clipping: Physical wheel parts frequently penetrate complex mesh roads and terrain triangles at high speeds. Raycasts detect collisions instantaneously regardless of velocity.
• Custom Compression Physics: You directly control spring stiffness, damping coefficient, rest length, and bump stops with mathematical precision in Luau.
• Lower Network Overhead: Instead of synchronizing 4 physical spinning cylinders and constraint states over the network, only the central chassis root CFrame requires replication.

2. The 4-Wheel Raycast Suspension Equation (Hooke's Law + Damping)

Calculating physical upward forces applied per wheel per frame:

• Raycast Direction & Hit Distance: Cast a vertical ray downwards from each wheel attachment point: Length = RestLength + WheelRadius.
• Spring Force Calculation: Compression = RestLength - HitDistance; SpringForce = Stiffness * Compression.
• Damping Force (Shock Absorption): Calculate the relative vertical velocity of the wheel attachment against the ground: DampingForce = Damping * RelativeVelocity.
• Total Upward Vector: Apply VectorForce at the attachment point oriented along the chassis UpVector: Force = (SpringForce + DampingForce) * RayNormal.

3. Tire Friction Models: Longitudinal Grip & Lateral Drift (Pacejka)

Simulating realistic cornering, drift counter-steer, and burnouts:

• Lateral Slip Angle: Measure the angle between the wheel's pointing direction and its actual ground travel vector. As slip angle increases, lateral cornering force builds up.
• Pacejka Curve Approximation: Tire grip peaks at 6–10 degrees of slip angle; pushing past this threshold smoothly transitions the tire into a controlled kinetic drift rather than an abrupt spinout.
• Longitudinal Traction: Calculate drive torque and wheel spin acceleration, clamping max forward acceleration to available normal load (Downforce * FrictionCoefficient).

-- Raycast Suspension Calculation Function
local function CalculateWheelForce(wheelAttachment, restLength, stiffness, damping, wheelRadius)
    local origin = wheelAttachment.WorldPosition
    local direction = -wheelAttachment.WorldCFrame.UpVector * (restLength + wheelRadius)
    
    local raycastParams = RaycastParams.new()
    raycastParams.FilterType = RaycastFilterType.Exclude
    raycastParams.FilterDescendantsInstances = {wheelAttachment.Parent}
    
    local result = workspace:Raycast(origin, direction, raycastParams)
    if not result then return Vector3.zero end
    
    local hitDistance = result.Distance - wheelRadius
    local compression = restLength - hitDistance
    if compression <= 0 then return Vector3.zero end
    
    -- Calculate vertical relative velocity
    local pointVelocity = wheelAttachment.Parent.AssemblyLinearVelocity
    local suspensionVelocity = wheelAttachment.WorldCFrame.UpVector:Dot(pointVelocity)
    
    -- Hooke's Law with Damping: F = (k * x) - (c * v)
    local springForce = stiffness * compression
    local dampingForce = damping * suspensionVelocity
    local totalForceMagnitude = math.max(0, springForce - dampingForce)
    
    return result.Normal * totalForceMagnitude
end

4. Weight Transfer & Anti-Roll Torsional Bars

Stabilizing the vehicle chassis during heavy cornering and braking:

• Chassis Pitch & Squat: Hard braking shifts vehicle weight forward onto front springs, while acceleration squats the rear; calculate dynamic wheel load to vary front/rear grip.
• Anti-Roll Bars (Sway Bars): Measure the difference in compression between left and right wheels on the same axle; apply equal and opposite balancing forces to eliminate excessive body roll.
• Aerodynamic Downforce: Apply dynamic downward force proportional to Velocity^2 to increase cornering grip at high speeds without adding dead mass.

5. Network Ownership & Authoritative Client Prediction

Delivering zero-input-lag steering across all network conditions:

• Driver Network Ownership: Grant chassis:SetNetworkOwner(driverPlayer) the moment a player enters the driver seat, allowing local 120 FPS physics simulation.
• Unreliable Remote Input Replication: Broadcast steering angles and throttle state via UnreliableRemoteEvent for smooth third-party visual wheel rotation.
• Anti-Teleport Server Sanity Checks: The server checks vehicle velocity vectors every 500ms; reject any position updates exceeding the vehicle's theoretical top speed by more than 15%.

Frequently Asked Questions