M1: constraint chassis (springs, servo steering, motor wheels)

- New shared Chassis.luau: geometry constants + per-car tuning from stats
- CarManager.buildCarModel rebuilt: prismatic+spring suspension per corner,
  servo-hinge steering on front knuckles, motor-hinge driven wheels,
  hidden dense ballast for anti-roll; VehicleSeat kept as input device only
- New client ChassisController: drives motors/servos from seat
  ThrottleFloat/SteerFloat per Heartbeat; speed-sensitive steering lock;
  engine braking on throttle release
- Driver gets network ownership of every unanchored part (constraint
  assemblies are separate; missing one causes stuttery physics)
- Docs: MESH_SWAP.md rewritten for shell-on-chassis workflow; ROADMAP updated

docs/MESH_SWAP.md

@@ -1,11 +1,13 @@
 # Swapping the placeholder RX-7 for the real mesh (M1)
 
-Right now every car is a coloured block + a `VehicleSeat` so the game is drivable
-today. This is the **one file** you touch to make the RX-7 a real model:
-`src/server/CarManager.luau` → the `buildCarModel` function.
+The car is now a **constraint chassis** (springs + steering servos + motor-driven
+wheels — see `src/shared/Chassis.luau` and `CarManager.buildCarModel`). The
+placeholder Body/Cabin blocks are just the visual shell riding on that chassis.
+Making the RX-7 real means **replacing the shell, not the chassis**: weld a
+MeshPart body onto the `Body` part and hide the placeholder blocks.
 
-Nothing else (saving, the allowlist, car switching, the map, anti-cheat) needs to
-change. That's the whole point of isolating model-building in one function.
+Nothing else (saving, the allowlist, car switching, driving feel, the map,
+anti-cheat) needs to change.
 
 ## Reference photos
 The real car's reference photos live on the NAS (too large to commit to git):
@@ -17,7 +19,7 @@ unraid-1:/mnt/user/data/yy-roblox-games/0-fast-n-furious-sim/001-1993 Mazda RX-7
 A single downscaled reference is included at `assets/reference/rx7-reference.png`
 for quick lookups.
 
-## Step 1 — Get a model into Roblox
+## Step 1 — Get a mesh into Roblox
 Options, easiest first:
 1. **Buy or find a free RX-7 FD model** on the Roblox Creator Marketplace/toolbox,
    or
@@ -25,58 +27,62 @@ Options, easiest first:
    (`.fbx`/`.obj`) via Studio's 3D importer, or
 3. Commission one.
 
-You want a single `Model` whose parts are welded together, with a clear "body"
-part you can drive. Keep the part count low (mobile perf).
+You want the car **body shell as a single MeshPart** (or a small welded Model
+with a clear root). Wheels can be separate meshes (see Step 4). Keep the part
+count low (mobile perf). This step needs Studio — mesh assets upload through
+Roblox's asset pipeline.
 
-## Step 2 — Store it where Rojo/Studio can clone it
-Put the finished `Model` under `ReplicatedStorage` (e.g. a `CarModels` folder).
-Two ways to manage it:
+## Step 2 — Store it where the server can clone it
+Put the shell under `ReplicatedStorage/CarModels/<carId>` (e.g.
+`CarModels/rx7_fc`). Two ways to manage it:
 - **Simple:** keep it in the Studio place file as a real Instance (not synced by
   Rojo). Fine for a single hero car.
 - **Version-controlled:** export the model to a `.rbxm` and reference it from the
   Rojo project so it lives in git. See Rojo's `$path` docs for `.rbxm` model files.
 
-## Step 3 — Rewrite `buildCarModel` to clone the real mesh
-Replace the block-building body of `buildCarModel(car, maxSpeed, torque, turnSpeed)`
-with something like:
+## Step 3 — Attach the shell in `buildCarModel`
+In `CarManager.buildCarModel`, after the Body part is created:
 
 ```lua
-local CAR_MODELS = ReplicatedStorage:WaitForChild("CarModels")
+local CAR_MODELS = ReplicatedStorage:FindFirstChild("CarModels")
 
-local function buildCarModel(car, maxSpeed, torque, turnSpeed)
-    -- Look up a real model by car id; fall back to the block builder if missing.
-    local template = CAR_MODELS:FindFirstChild(car.Id)
-    if not template then
-        return buildBlockCar(car, maxSpeed, torque, turnSpeed) -- keep the old code as a fallback
+-- inside buildCarModel, after `body` exists:
+local shellTemplate = CAR_MODELS and CAR_MODELS:FindFirstChild(car.Id)
+if shellTemplate then
+    local shell = shellTemplate:Clone()
+    -- Position the shell over the body, then weld it on.
+    shell:PivotTo(body.CFrame * CFrame.new(0, SHELL_Y_OFFSET, 0))
+    for _, p in ipairs(shell:GetDescendants()) do
+        if p:IsA("BasePart") then
+            p.CanCollide = false -- the Body part remains the collider
+            p.Anchored = false
         end
-
-    local model = template:Clone()
-
-    -- Ensure it has a VehicleSeat named "DriveSeat" welded to the driving body.
-    local seat = model:FindFirstChild("DriveSeat")
-    if not seat then
-        seat = Instance.new("VehicleSeat")
-        seat.Name = "DriveSeat"
-        -- weld seat to model.PrimaryPart, position it in the cabin...
-        seat.Parent = model
     end
-    seat.MaxSpeed = maxSpeed
-    seat.Torque = torque
-    seat.TurnSpeed = turnSpeed
-
-    assert(model.PrimaryPart, "Car model needs a PrimaryPart set")
-    return model
+    local weld = Instance.new("WeldConstraint")
+    weld.Part0 = body
+    weld.Part1 = shell.PrimaryPart or shell:FindFirstChildWhichIsA("BasePart")
+    weld.Parent = body
+    shell.Parent = model
+    -- Hide the placeholder blocks:
+    body.Transparency = 1
+    local cabin = model:FindFirstChild("Cabin")
+    if cabin then cabin:Destroy() end
 end
 ```
 
-Keep the current block-builder renamed as `buildBlockCar` so any car without a
-real mesh still works. That way you can import cars one at a time.
+Cars without a mesh keep the block look automatically — import one car at a time.
 
-## Step 4 (recommended, M1) — Move to a real chassis
-The `VehicleSeat` is great for "it drives today" but for real suspension/grip,
-adopt a proven **constraint chassis** (A-Chassis or a modern equivalent). At that
-point `buildCarModel` clones the chassis rig and attaches the RX-7 body to it.
-Still one function; still no changes elsewhere.
+## Step 4 (polish) — Real wheel meshes
+The chassis wheels are plain cylinders (`Wheel_FL` … `Wheel_RR`). To make them
+look real, weld a wheel MeshPart (CanCollide=false) onto each cylinder and set
+the cylinder's `Transparency = 1`. The cylinder stays the physics tyre — mesh
+collision on wheels is unreliable and slow.
+
+## Sizing gotcha
+The chassis geometry (`Chassis.BODY_SIZE`, `WHEEL_RADIUS`, `TRACK_HALF`,
+`WHEELBASE_HALF`) is tuned for a ~6×12 stud footprint. Scale the imported mesh
+to fit the chassis — don't reshape the chassis around the mesh, or the driving
+feel changes for every car.
 
 ## Verify
 - `rojo build --output build.rbxl` compiles.

docs/ROADMAP.md

@@ -14,12 +14,14 @@ Spawn on the neon expressway, drive a car, switch cars, respawn. RX-7 demo-locke
 to owner + son. Cars persist between sessions. CI green on a self-hosted runner.
 
 ## M1 — Make driving feel GOOD (the real work, do this next)
-- [ ] Import the **real RX-7 mesh**; swap `CarManager.buildCarModel` only.
-      See `docs/MESH_SWAP.md` for the exact steps.
-- [ ] Adopt a proven **constraint chassis** (A-Chassis or a modern equivalent)
-      for real suspension/grip instead of the raw VehicleSeat. Do NOT hand-roll
-      physics.
-- [ ] Tune acceleration curve, grip, and the chase camera.
+- [x] Adopt a **constraint chassis** for real suspension/grip instead of the raw
+      VehicleSeat: springs + prismatic suspension, servo steering (speed-sensitive
+      lock), motor-driven wheels, engine braking, ballast anti-roll. Shared tuning
+      in `src/shared/Chassis.luau`; client-side `ChassisController` drives it
+      (driver owns network physics). **Needs a Studio playtest to tune feel.**
+- [ ] Import the **real RX-7 mesh**; weld the shell onto the chassis `Body`.
+      See `docs/MESH_SWAP.md` for the exact steps (needs Studio for asset upload).
+- [ ] Tune acceleration curve, grip, and the chase camera (after first playtest).
 - [ ] Sound: engine loop, tire screech, boost.
 **Exit test:** the RX-7 is genuinely fun to just drive around. Ship nothing else
 until this is true.

src/client/ChassisController.luau

@@ -0,0 +1,133 @@
+--!strict
+--[[
+	ChassisController.luau  (CLIENT)
+	Drives the constraint chassis while the local player is in a DriveSeat.
+
+	Why client-side: the server gives the driver network ownership of the car,
+	so physics (and constraint targets) run authoritatively on THIS client —
+	that's what makes driving feel responsive instead of laggy. The server
+	still owns WHICH car you get (allowlist etc.); this module only turns
+	seat input into motor/servo targets.
+
+	Loop per Heartbeat while seated:
+	  Steer   -> front servo TargetAngle (speed-sensitive: less lock when fast)
+	  Throttle-> wheel motor AngularVelocity + MotorMaxTorque
+	  No input-> brake torque toward zero angular velocity
+]]
+
+local Players = game:GetService("Players")
+local RunService = game:GetService("RunService")
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+local Shared = ReplicatedStorage.Shared
+local Cars = require(Shared.Cars)
+local Chassis = require(Shared.Chassis)
+
+local player = Players.LocalPlayer
+
+local ChassisController = {}
+
+local heartbeatConn: RBXScriptConnection? = nil
+local seatedConn: RBXScriptConnection? = nil
+
+local function findCarModel(seat: VehicleSeat): Model?
+	local model = seat:FindFirstAncestorOfClass("Model")
+	if model and model:GetAttribute("CarId") ~= nil then
+		return model
+	end
+	return nil
+end
+
+local function collectConstraints(model: Model): ({ HingeConstraint }, { HingeConstraint })
+	local motors: { HingeConstraint } = {}
+	local servos: { HingeConstraint } = {}
+	for _, desc in ipairs(model:GetDescendants()) do
+		if desc:IsA("HingeConstraint") then
+			if desc.Name == Chassis.WHEEL_MOTOR_NAME then
+				table.insert(motors, desc)
+			elseif desc.Name == Chassis.STEER_SERVO_NAME then
+				table.insert(servos, desc)
+			end
+		end
+	end
+	return motors, servos
+end
+
+local function startDriving(seat: VehicleSeat)
+	local model = findCarModel(seat)
+	if not model then
+		return
+	end
+	local carId = model:GetAttribute("CarId") :: string
+	local car = Cars.Get(carId)
+	if not car then
+		return
+	end
+	local tuning = Chassis.tuningFor(car)
+	local motors, servos = collectConstraints(model)
+	local body = model.PrimaryPart
+	if #motors == 0 or not body then
+		return
+	end
+
+	heartbeatConn = RunService.Heartbeat:Connect(function()
+		local throttle = seat.ThrottleFloat
+		local steer = seat.SteerFloat
+
+		-- Steering: speed-sensitive — at top speed only ~40% of full lock,
+		-- so the car doesn't spin out from a tap of the key at 150 studs/s.
+		local speed = body.AssemblyLinearVelocity.Magnitude
+		local speedFrac = math.clamp(speed / tuning.MaxSpeed, 0, 1)
+		local lockScale = 1 - 0.6 * speedFrac
+		local targetAngle = Chassis.STEER_SIGN * steer * tuning.MaxSteerAngle * lockScale
+		for _, servo in ipairs(servos) do
+			servo.TargetAngle = targetAngle
+		end
+
+		-- Drive/brake:
+		if math.abs(throttle) > 0.05 then
+			local targetVel = Chassis.DRIVE_SIGN * throttle * tuning.MaxWheelAngularVelocity
+			for _, motor in ipairs(motors) do
+				motor.AngularVelocity = targetVel
+				motor.MotorMaxTorque = tuning.MotorMaxTorque
+			end
+		else
+			-- Engine braking: drive wheel speed toward zero with brake torque.
+			for _, motor in ipairs(motors) do
+				motor.AngularVelocity = 0
+				motor.MotorMaxTorque = tuning.BrakeTorque
+			end
+		end
+	end)
+end
+
+local function stopDriving()
+	if heartbeatConn then
+		heartbeatConn:Disconnect()
+		heartbeatConn = nil
+	end
+end
+
+function ChassisController.init()
+	-- SeatPart changes whenever the humanoid sits down or gets up.
+	local function onCharacter(character: Model)
+		local humanoid = character:WaitForChild("Humanoid") :: Humanoid
+		if seatedConn then
+			seatedConn:Disconnect()
+		end
+		seatedConn = humanoid:GetPropertyChangedSignal("SeatPart"):Connect(function()
+			stopDriving()
+			local seatPart = humanoid.SeatPart
+			if seatPart and seatPart:IsA("VehicleSeat") and seatPart.Name == Chassis.SEAT_NAME then
+				startDriving(seatPart)
+			end
+		end)
+	end
+
+	if player.Character then
+		onCharacter(player.Character)
+	end
+	player.CharacterAdded:Connect(onCharacter)
+end
+
+return ChassisController

src/client/init.client.luau

@@ -15,6 +15,9 @@ local ReplicatedStorage = game:GetService("ReplicatedStorage")
 local Shared = ReplicatedStorage.Shared
 local Remotes = require(Shared.Remotes)
 local Cars = require(Shared.Cars)
+local ChassisController = require(script.ChassisController)
+
+ChassisController.init()
 
 local player = Players.LocalPlayer
 local playerGui = player:WaitForChild("PlayerGui")

src/server/CarManager.luau

@@ -1,14 +1,23 @@
 --!strict
 --[[
 	CarManager.luau  (SERVER)
-	Spawns the player's equipped car as a simple, PROVEN drivable vehicle.
+	Spawns the player's equipped car as a CONSTRAINT CHASSIS vehicle.
 
-	Design choice (per "don't reinvent the wheel"): instead of a hand-rolled
-	physics chassis, the MVP car is a single driving Part + a VehicleSeat.
-	Roblox's VehicleSeat drives the assembly via its built-in Throttle/Steer —
-	the most reliable, least-buggy way to get a car moving on day one. When you
-	later import the real RX-7 mesh + a constraint chassis (A-Chassis style),
-	you swap out `buildCarModel` and keep everything else.
+	M1 upgrade: the old single-Part + VehicleSeat arcade car is replaced with a
+	proper physical chassis (A-Chassis style, built from Roblox constraints):
+
+	  Body ──Prismatic(vertical)+Spring──> Hub ──[front only: Servo hinge]──>
+	  Knuckle ──Motor hinge──> Wheel
+
+	  - Springs give real suspension; prismatics constrain travel to vertical.
+	  - Front knuckles yaw on Servo hinges (steering).
+	  - Every wheel spins on a Motor hinge (drive/brake).
+	  - The VehicleSeat is kept ONLY as the input device: its Throttle/Steer
+	    are read by the client ChassisController, which sets motor/servo
+	    targets locally (the driver owns the physics, so it replicates).
+
+	The real RX-7 mesh swap (docs/MESH_SWAP.md) welds a MeshPart onto the Body
+	and hides the placeholder blocks — the chassis itself doesn't change.
 ]]
 
 local Players = game:GetService("Players")
@@ -17,68 +26,226 @@ local ReplicatedStorage = game:GetService("ReplicatedStorage")
 
 local Shared = ReplicatedStorage.Shared
 local Cars = require(Shared.Cars)
+local Chassis = require(Shared.Chassis)
 local GameConfig = require(Shared.GameConfig)
 local DataManager = require(script.Parent.DataManager)
 
 type Car = Cars.Car
+type Tuning = Chassis.Tuning
 
 local CarManager = {}
 
 local activeCars: { [Player]: Model } = {}
 local carsFolder: Folder
 
--- Convert a car's 0-100 stats into VehicleSeat tuning ------------------------
-local function computeTuning(car: Car)
-	local maxSpeed = 60 + car.Stats.Speed * 1.6 -- studs/sec-ish target
-	local torque = 8000 + car.Stats.Acceleration * 220
-	local turnSpeed = 20 + car.Stats.Handling * 0.35
-	return maxSpeed, torque, turnSpeed
+-- Helpers ---------------------------------------------------------------------
+
+local function makeAttachment(parent: BasePart, name: string, cf: CFrame): Attachment
+	local att = Instance.new("Attachment")
+	att.Name = name
+	att.CFrame = cf
+	att.Parent = parent
+	return att
 end
 
--- Build the placeholder car model. Returns the Model with a VehicleSeat.
-local function buildCarModel(car: Car, maxSpeed: number, torque: number, turnSpeed: number): Model
+-- Attachment whose primary (X) axis points straight up — used for vertical
+-- prismatic travel and for the steering servo's yaw axis.
+local VERTICAL_AXIS = CFrame.Angles(0, 0, math.rad(90))
+
+local function makePart(props: {
+	Name: string,
+	Size: Vector3,
+	CFrame: CFrame,
+	Color: Color3?,
+	Material: Enum.Material?,
+	CanCollide: boolean,
+	Parent: Instance,
+	Shape: Enum.PartType?,
+	Friction: number?,
+}): Part
+	local part = Instance.new("Part")
+	part.Name = props.Name
+	part.Size = props.Size
+	part.CFrame = props.CFrame
+	if props.Shape then
+		part.Shape = props.Shape
+	end
+	part.Color = props.Color or Color3.new(0.2, 0.2, 0.2)
+	part.Material = props.Material or Enum.Material.SmoothPlastic
+	part.CanCollide = props.CanCollide
+	if props.Friction then
+		part.CustomPhysicalProperties = PhysicalProperties.new(0.9, props.Friction, 0.2, 1, 1)
+	end
+	part.Parent = props.Parent
+	return part
+end
+
+-- Build one corner of the suspension. `isFront` corners get a steering servo.
+-- All positions are in BODY-LOCAL space, converted via body.CFrame.
+local function buildCorner(model: Model, body: Part, tuning: Tuning, xSign: number, zSign: number)
+	local isFront = zSign < 0 -- car forward is -Z
+	local cornerName = (isFront and "F" or "R") .. (xSign < 0 and "L" or "R")
+
+	local wheelLocal = Vector3.new(xSign * Chassis.TRACK_HALF, -Chassis.SUSPENSION_DROP, zSign * Chassis.WHEELBASE_HALF)
+	local wheelWorld = body.CFrame * CFrame.new(wheelLocal)
+
+	-- Hub: rides the prismatic+spring below the body corner ---------------------
+	local hub = makePart({
+		Name = "Hub_" .. cornerName,
+		Size = Vector3.new(0.6, 0.6, 0.6),
+		CFrame = wheelWorld,
+		CanCollide = false,
+		Parent = model,
+	})
+
+	local bodyAtt = makeAttachment(body, "SuspAtt_" .. cornerName, CFrame.new(wheelLocal) * VERTICAL_AXIS)
+	local hubAtt = makeAttachment(hub, "HubAtt", VERTICAL_AXIS)
+
+	local prismatic = Instance.new("PrismaticConstraint")
+	prismatic.Name = "SuspSlide_" .. cornerName
+	prismatic.Attachment0 = bodyAtt
+	prismatic.Attachment1 = hubAtt
+	prismatic.LimitsEnabled = true
+	prismatic.LowerLimit = -Chassis.SUSPENSION_TRAVEL / 2
+	prismatic.UpperLimit = Chassis.SUSPENSION_TRAVEL / 2
+	prismatic.Parent = body
+
+	local spring = Instance.new("SpringConstraint")
+	spring.Name = "SuspSpring_" .. cornerName
+	spring.Attachment0 = bodyAtt
+	spring.Attachment1 = hubAtt
+	spring.FreeLength = 0.2
+	spring.Stiffness = tuning.SpringStiffness
+	spring.Damping = tuning.SpringDamping
+	spring.LimitsEnabled = false
+	spring.Parent = body
+
+	-- Front corners: a knuckle that yaws on a servo hinge (steering) ------------
+	local wheelParent: Part = hub
+	if isFront then
+		local knuckle = makePart({
+			Name = "Knuckle_" .. cornerName,
+			Size = Vector3.new(0.5, 0.5, 0.5),
+			CFrame = wheelWorld,
+			CanCollide = false,
+			Parent = model,
+		})
+		local hubSteerAtt = makeAttachment(hub, "SteerAtt", VERTICAL_AXIS)
+		local knuckleAtt = makeAttachment(knuckle, "KnuckleAtt", VERTICAL_AXIS)
+
+		local servo = Instance.new("HingeConstraint")
+		servo.Name = Chassis.STEER_SERVO_NAME
+		servo.Attachment0 = hubSteerAtt
+		servo.Attachment1 = knuckleAtt
+		servo.ActuatorType = Enum.ActuatorType.Servo
+		servo.ServoMaxTorque = 1e6
+		servo.AngularSpeed = tuning.SteerSpeed
+		servo.TargetAngle = 0
+		servo.LimitsEnabled = true
+		servo.LowerAngle = -tuning.MaxSteerAngle
+		servo.UpperAngle = tuning.MaxSteerAngle
+		servo.Parent = hub
+
+		wheelParent = knuckle
+	end
+
+	-- Wheel: cylinder spinning on a motor hinge about the lateral (X) axis ------
+	local wheel = makePart({
+		Name = "Wheel_" .. cornerName,
+		Size = Vector3.new(Chassis.WHEEL_WIDTH, Chassis.WHEEL_RADIUS * 2, Chassis.WHEEL_RADIUS * 2),
+		CFrame = wheelWorld,
+		Shape = Enum.PartType.Cylinder, -- cylinder axis = X = lateral. correct spin axis
+		Color = Color3.new(0.08, 0.08, 0.09),
+		Material = Enum.Material.Rubber,
+		CanCollide = true,
+		Friction = tuning.WheelFriction,
+		Parent = model,
+	})
+
+	local parentAtt = makeAttachment(wheelParent, "WheelAxleAtt", CFrame.new())
+	local wheelAtt = makeAttachment(wheel, "WheelAtt", CFrame.new())
+
+	local motor = Instance.new("HingeConstraint")
+	motor.Name = Chassis.WHEEL_MOTOR_NAME
+	motor.Attachment0 = parentAtt
+	motor.Attachment1 = wheelAtt
+	motor.ActuatorType = Enum.ActuatorType.Motor
+	motor.MotorMaxTorque = 0 -- freewheel until the ChassisController takes over
+	motor.AngularVelocity = 0
+	motor.Parent = wheelParent
+end
+
+-- Build the full constraint-chassis car model.
+local function buildCarModel(car: Car, tuning: Tuning): Model
 	local model = Instance.new("Model")
 	model.Name = "Car_" .. car.Id
 
-	-- Body (the driving part) --------------------------------------------------
-	local body = Instance.new("Part")
-	body.Name = "Body"
-	body.Size = Vector3.new(6, 2, 12)
-	body.Color = Color3.new(car.Color[1], car.Color[2], car.Color[3])
-	body.Material = Enum.Material.SmoothPlastic
-	body.CanCollide = true
-	body.Parent = model
+	local origin = CFrame.new() -- built at origin; PivotTo() places it later
 
-	-- A cabin block so it reads as a car, welded to the body -------------------
-	local cabin = Instance.new("Part")
-	cabin.Name = "Cabin"
-	cabin.Size = Vector3.new(5, 1.6, 5)
-	cabin.Color = Color3.new(car.Color[1] * 0.7, car.Color[2] * 0.7, car.Color[3] * 0.7)
-	cabin.Material = Enum.Material.Glass
-	cabin.CanCollide = false
-	cabin.Parent = model
+	-- Body (PrimaryPart / mesh anchor point) ------------------------------------
+	local body = makePart({
+		Name = "Body",
+		Size = Chassis.BODY_SIZE,
+		CFrame = origin,
+		Color = Color3.new(car.Color[1], car.Color[2], car.Color[3]),
+		CanCollide = true,
+		Parent = model,
+	})
+
+	-- Ballast: dense low block welded under the body — drops the centre of mass
+	-- so the car corners flat instead of rolling over.
+	local ballast = makePart({
+		Name = "Ballast",
+		Size = Vector3.new(4, 0.4, 8),
+		CFrame = origin * CFrame.new(0, -0.9, 0),
+		CanCollide = false,
+		Parent = model,
+	})
+	ballast.Transparency = 1
+	ballast.CustomPhysicalProperties = PhysicalProperties.new(8, 0.3, 0.5, 1, 1)
+	local ballastWeld = Instance.new("WeldConstraint")
+	ballastWeld.Part0 = body
+	ballastWeld.Part1 = ballast
+	ballastWeld.Parent = body
+
+	-- Cabin (visual only) -------------------------------------------------------
+	local cabin = makePart({
+		Name = "Cabin",
+		Size = Vector3.new(5, 1.6, 5),
+		CFrame = origin * CFrame.new(0, 1.5, 0.5),
+		Color = Color3.new(car.Color[1] * 0.7, car.Color[2] * 0.7, car.Color[3] * 0.7),
+		Material = Enum.Material.Glass,
+		CanCollide = false,
+		Parent = model,
+	})
 	local cabinWeld = Instance.new("WeldConstraint")
 	cabinWeld.Part0 = body
 	cabinWeld.Part1 = cabin
 	cabinWeld.Parent = body
-	cabin.CFrame = body.CFrame * CFrame.new(0, 1.8, 0.5)
 
-	-- VehicleSeat: Roblox's built-in, reliable driver ------------------------
+	-- Seat: input device only (Throttle/Steer). It drives nothing directly. -----
 	local seat = Instance.new("VehicleSeat")
-	seat.Name = "DriveSeat"
+	seat.Name = Chassis.SEAT_NAME
 	seat.Size = Vector3.new(4, 0.5, 3)
-	seat.MaxSpeed = maxSpeed
-	seat.Torque = torque
-	seat.TurnSpeed = turnSpeed
+	seat.MaxSpeed = 0 -- neutralise built-in drive; constraints do the work
+	seat.Torque = 0
+	seat.TurnSpeed = 0
 	seat.HeadsUpDisplay = false
 	seat.CanCollide = false
 	seat.Color = Color3.new(0.1, 0.1, 0.1)
+	seat.CFrame = origin * CFrame.new(0, 1.2, -0.5)
 	seat.Parent = model
 	local seatWeld = Instance.new("WeldConstraint")
 	seatWeld.Part0 = body
 	seatWeld.Part1 = seat
 	seatWeld.Parent = body
-	seat.CFrame = body.CFrame * CFrame.new(0, 1.6, -1)
+
+	-- Four suspension corners ---------------------------------------------------
+	for _, xSign in ipairs({ -1, 1 }) do
+		for _, zSign in ipairs({ -1, 1 }) do
+			buildCorner(model, body, tuning, xSign, zSign)
+		end
+	end
 
 	model.PrimaryPart = body
 	return model
@@ -123,22 +290,26 @@ function CarManager.spawn(player: Player, carId: string?)
 
 	CarManager.despawn(player)
 
-	local maxSpeed, torque, turnSpeed = computeTuning(car)
-	local model = buildCarModel(car, maxSpeed, torque, turnSpeed)
+	local tuning = Chassis.tuningFor(car)
+	local model = buildCarModel(car, tuning)
 	model:PivotTo(spawnCFrame())
 	model.Parent = carsFolder
 	model:SetAttribute("OwnerUserId", player.UserId)
 	model:SetAttribute("CarId", id)
 
-	-- Sit the player down. Give the assembly network ownership to the driver so
-	-- physics are smooth on their client (standard Roblox vehicle practice).
+	-- Give the driver network ownership of EVERY assembly in the car. With a
+	-- constraint chassis the body, hubs, knuckles, and wheels are SEPARATE
+	-- assemblies (constraints don't merge them like welds do) — missing one
+	-- causes stuttery mixed-authority physics.
+	for _, desc in ipairs(model:GetDescendants()) do
+		if desc:IsA("BasePart") and not desc.Anchored then
+			desc:SetNetworkOwner(player)
+		end
+	end
+
 	local character = player.Character
 	if character then
-		local seat = model:FindFirstChild("DriveSeat") :: VehicleSeat
-		local body = model.PrimaryPart
-		if body then
-			body:SetNetworkOwner(player)
-		end
+		local seat = model:FindFirstChild(Chassis.SEAT_NAME) :: VehicleSeat
 		if seat then
 			task.defer(function()
 				seat:Sit(character:FindFirstChildOfClass("Humanoid"))

src/shared/Chassis.luau

@@ -0,0 +1,70 @@
+--!strict
+--[[
+	Chassis.luau  (SHARED)
+	Single source of truth for the constraint chassis: geometry constants,
+	part/constraint naming, and per-car tuning derived from the 0-100 stats.
+
+	Used by BOTH sides:
+	  - Server (CarManager.buildCarModel) builds the physical chassis from it.
+	  - Client (ChassisController) drives the motors/servos from it.
+
+	Sign conventions (car forward = -Z, Roblox standard):
+	  - DRIVE_SIGN: wheel motor AngularVelocity sign for forward throttle.
+	  - STEER_SIGN: steering servo TargetAngle sign for steering right.
+	If a future geometry change makes the car drive/steer backwards, flip the
+	sign here — nowhere else.
+]]
+
+local Cars = require(script.Parent.Cars)
+
+type Car = Cars.Car
+
+local Chassis = {}
+
+-- Naming conventions (server creates, client finds) ---------------------------
+Chassis.SEAT_NAME = "DriveSeat"
+Chassis.WHEEL_MOTOR_NAME = "WheelMotor" -- HingeConstraint (Motor) spinning a wheel
+Chassis.STEER_SERVO_NAME = "SteerServo" -- HingeConstraint (Servo) yawing a front knuckle
+
+-- Sign conventions ------------------------------------------------------------
+Chassis.DRIVE_SIGN = -1 -- forward (-Z) needs negative angular velocity about +X
+Chassis.STEER_SIGN = -1 -- steer right (Steer = 1) needs negative yaw about +Y
+
+-- Geometry (studs) ------------------------------------------------------------
+Chassis.BODY_SIZE = Vector3.new(6, 1.4, 12)
+Chassis.WHEEL_RADIUS = 1.4
+Chassis.WHEEL_WIDTH = 1.1
+Chassis.TRACK_HALF = 3.35 -- lateral distance body centre -> wheel centre
+Chassis.WHEELBASE_HALF = 4.2 -- longitudinal distance body centre -> axle
+Chassis.SUSPENSION_DROP = 1.6 -- rest distance from body centre down to wheel centre
+Chassis.SUSPENSION_TRAVEL = 0.9 -- total prismatic travel allowed
+
+-- Per-car tuning derived from stats -------------------------------------------
+export type Tuning = {
+	MaxSpeed: number, -- studs/sec target top speed
+	MaxWheelAngularVelocity: number, -- rad/sec at top speed
+	MotorMaxTorque: number, -- accel authority
+	BrakeTorque: number, -- resist-to-zero torque when throttle released
+	MaxSteerAngle: number, -- degrees
+	SteerSpeed: number, -- servo angular speed, rad/sec
+	SpringStiffness: number,
+	SpringDamping: number,
+	WheelFriction: number,
+}
+
+function Chassis.tuningFor(car: Car): Tuning
+	local maxSpeed = 70 + car.Stats.Speed * 1.5
+	return {
+		MaxSpeed = maxSpeed,
+		MaxWheelAngularVelocity = maxSpeed / Chassis.WHEEL_RADIUS,
+		MotorMaxTorque = 12000 + car.Stats.Acceleration * 500,
+		BrakeTorque = 30000,
+		MaxSteerAngle = 22 + car.Stats.Handling * 0.16, -- ~28-38 deg
+		SteerSpeed = 6,
+		SpringStiffness = 9000,
+		SpringDamping = 420,
+		WheelFriction = 1.2 + car.Stats.Handling * 0.008, -- ~1.2-2.0
+	}
+end
+
+return Chassis