Real car meshes + physics-derived suspension + inPA/void fixes

Car meshes:
- CarManager: clone ReplicatedStorage/CarModels/<carId> as a visual Shell,
  weld to Body, hide placeholder blocks. Falls back to procedural CarBody
  when no mesh exists, so cars can be converted one at a time.
- Shell parts are Massless/CanCollide=false/CanQuery=false: the constraint
  chassis and driving feel are untouched. Verified 0 violating parts live.
- 5 catalogue cars mapped from the 67-model ServerStorage/CarLibrary import.

Suspension (ref docs/CAR_PHYSICS_REFERENCE.txt, devforum 3915628):
- BUG: old k=9000 gave 1.10 studs static sag against 0.90 studs total travel,
  i.e. the suspension sat permanently bottomed out and the spring did no work.
- Stiffness now solved from a sag target (70%% of half travel); damping now
  derived as a damping ratio c = zeta*2*sqrt(k*m) instead of an unrelated
  constant. Old pair worked out to zeta=0.31 (floaty); now 0.55-0.63.
- Measured after fix: ride height 1.97 studs, bounce 0.000, body roll 0.002.

Also: Circuit.inPA square test (PAFloor is 520x520, test was a r=260 circle,
corners falsely reported outside and spuriously armed runs), and void-fall
recovery now moves the character to the spawn pad BEFORE building the car.

src/server/AntiCheat.luau

@@ -46,9 +46,23 @@ end
 function AntiCheat.init()
 	local function watch(player: Player)
 		player.CharacterAdded:Connect(function(char)
+			-- RESPAWN FIX: drop the previous character's baseline IMMEDIATELY,
+			-- before the WaitForChild below yields. A new character spawns at
+			-- the SpawnLocation (the parking area, ~8,500 studs from the old
+			-- body at the origin). If the Heartbeat sampler runs during that
+			-- yield it measures that jump against the STALE baseline, calls it
+			-- a teleport hack, and snaps the fresh character back to the dead
+			-- character's CFrame -- dragging the car with it. Clearing the
+			-- entry means "no baseline yet", so nothing can be flagged until
+			-- the new one is recorded.
+			tracked[player] = nil
+			AntiCheat.grant(player)
 			local hrp = char:WaitForChild("HumanoidRootPart", 10) :: BasePart?
 			if hrp then
 				tracked[player] = { lastPos = hrp.Position, lastValid = hrp.CFrame }
+				-- The spawn itself is a legitimate long-distance move; keep the
+				-- grace alive past the first sample after the baseline lands.
+				AntiCheat.grant(player)
 			end
 		end)
 	end

src/server/CarManager.luau

@@ -49,6 +49,10 @@ local SEAT_LIFT = 3
 -- The rig must be fully settled first or the client re-asserts a stale layout.
 local SETTLE_TIME = 1.0
 
+-- How far to sink an imported mesh shell into the chassis so it doesn't look
+-- like it's hovering. Purely cosmetic (shell parts are Massless/CanCollide=false).
+local SHELL_DROP = 0.6
+
 -- Collision groups ------------------------------------------------------------
 -- The wheels deliberately overlap the chassis: with BODY_SIZE.X = 6 the body
 -- edge sits at 3.0, while a wheel at TRACK_HALF 3.35 with WHEEL_WIDTH 1.1 has
@@ -273,8 +277,48 @@ local function buildCarModel(car: Car, tuning: Tuning): Model
 		end
 	end
 
+	-- Visual shell. If a real imported mesh exists under
+	-- ReplicatedStorage/CarModels/<carId>, use it; otherwise fall back to the
+	-- procedural CarBody kit. Meshes are visual-ONLY: every part is Massless,
+	-- CanCollide=false and welded to Body, so the constraint chassis (springs,
+	-- steering servos, motor wheels) and therefore the driving feel are
+	-- completely unchanged no matter which shell is used.
+	local shellTemplate = ReplicatedStorage:FindFirstChild("CarModels")
+	shellTemplate = shellTemplate and shellTemplate:FindFirstChild(car.Id)
+	if shellTemplate then
+		local shell = shellTemplate:Clone()
+		shell.Name = "Shell"
+		-- Seat the shell on the chassis: drop it so the mesh's underside sits at
+		-- roughly the body plane rather than floating at the body's centreline.
+		local ext = shell:GetExtentsSize()
+		shell:PivotTo(body.CFrame * CFrame.new(0, ext.Y / 2 - Chassis.BODY_SIZE.Y / 2 - SHELL_DROP, 0))
+		local shellRoot = shell.PrimaryPart or shell:FindFirstChildWhichIsA("BasePart", true)
+		for _, d in ipairs(shell:GetDescendants()) do
+			if d:IsA("BasePart") then
+				d.Anchored = false
+				d.CanCollide = false
+				d.CanTouch = false
+				d.CanQuery = false
+				d.Massless = true
+			end
+		end
+		if shellRoot then
+			local w = Instance.new("WeldConstraint")
+			w.Part0 = body
+			w.Part1 = shellRoot
+			w.Parent = body
+		end
+		shell.Parent = model
+		-- Hide the placeholder blocks the mesh replaces.
+		body.Transparency = 1
+		local cabin = model:FindFirstChild("Cabin")
+		if cabin then
+			cabin:Destroy()
+		end
+	else
 		-- Sculpted visual body kit (per-car styling; all parts Massless + welded)
 		CarBody.build(model, body, car)
+	end
 
 	-- Wheels must not collide with their own chassis (see ensureCollisionGroups).
 	ensureCollisionGroups()
@@ -428,9 +472,143 @@ function CarManager.init()
 	carsFolder.Name = "ActiveCars"
 	carsFolder.Parent = Workspace
 
+	-- FLIP RECOVERY: a car that ends up on its roof has no way to right itself,
+	-- so the player is stranded upside down indefinitely -- endurance testing
+	-- left one inverted (up = -0.6) grinding a barrier for 140 seconds. When a
+	-- car is inverted AND effectively stationary for a few consecutive checks
+	-- (so we never flip someone mid-drift), lift it clear and set it upright,
+	-- preserving heading. Ownership is pulled back to the server for the
+	-- teleport: PivotTo on client-owned parts gets reverted by the owning
+	-- client on the next physics frame.
+	local flipTicks: { [Player]: number } = {}
+	task.spawn(function()
+		while true do
+			task.wait(1)
+			for _, player in ipairs(Players:GetPlayers()) do
+				local ok, err = pcall(function()
+					local model = activeCars[player]
+					if not model or not model.Parent then
+						flipTicks[player] = nil
+						return
+					end
+					local body = model:FindFirstChild("Body")
+					local root = (body and body:IsA("BasePart") and body) or model.PrimaryPart
+					if not root then
+						return
+					end
+					local upright = root.CFrame.UpVector:Dot(Vector3.yAxis)
+					local slow = root.AssemblyLinearVelocity.Magnitude < 8
+					if upright >= 0.35 or not slow then
+						flipTicks[player] = 0
+						return
+					end
+					flipTicks[player] = (flipTicks[player] or 0) + 1
+					if flipTicks[player] < 3 then
+						return
+					end
+					flipTicks[player] = 0
+
+					local parts: { BasePart } = {}
+					for _, d in ipairs(model:GetDescendants()) do
+						if d:IsA("BasePart") and not d.Anchored then
+							table.insert(parts, d)
+						end
+					end
+					for _, d in ipairs(parts) do
+						pcall(function()
+							d:SetNetworkOwner(nil)
+						end)
+					end
+
+					local look = root.CFrame.LookVector
+					local flat = Vector3.new(look.X, 0, look.Z)
+					if flat.Magnitude < 0.1 then
+						flat = Vector3.new(0, 0, -1)
+					end
+					local target = root.Position + Vector3.new(0, 6, 0)
+					model:PivotTo(CFrame.lookAt(target, target + flat.Unit))
+					for _, d in ipairs(parts) do
+						d.AssemblyLinearVelocity = Vector3.zero
+						d.AssemblyAngularVelocity = Vector3.zero
+					end
+
+					AntiCheat.grant(player, 3)
+					task.wait(0.3)
+					for _, d in ipairs(parts) do
+						pcall(function()
+							d:SetNetworkOwner(player)
+						end)
+					end
+					warn(("[CarManager] righted flipped car for %s"):format(player.Name))
+				end)
+				if not ok then
+					warn("[CarManager] flip-recovery iteration failed: " .. tostring(err))
+				end
+			end
+		end
+	end)
+
 	Players.PlayerRemoving:Connect(function(player)
 		CarManager.despawn(player)
 	end)
+
+	-- VOID RECOVERY: nothing else rescues a car that leaves the world. If it
+	-- falls off the map it drops past Workspace.FallenPartsDestroyHeight, the
+	-- model is deleted out from under the driver, and the player is left
+	-- standing (or seatless) with no car and no way to get another -- observed
+	-- in endurance testing as 100+ seconds of a dead session. Watch for a car
+	-- that has fallen below the recovery floor OR whose model was destroyed
+	-- while the player is still in the game, and respawn it on the pad.
+	local RECOVER_Y = Workspace.FallenPartsDestroyHeight + 100
+	task.spawn(function()
+		while true do
+			task.wait(1)
+			for _, player in ipairs(Players:GetPlayers()) do
+				local ok, err = pcall(function()
+					if not player.Parent or not player.Character then
+						return
+					end
+					local model = activeCars[player]
+					if not model then
+						return
+					end
+					local gone = model.Parent == nil
+					local fell = false
+					if not gone then
+						local body = model:FindFirstChild("Body")
+						local root = (body and body:IsA("BasePart") and body) or model.PrimaryPart
+						fell = root ~= nil and root.Position.Y < RECOVER_Y
+					end
+					if gone or fell then
+						warn(
+							("[CarManager] recovering car for %s (destroyed=%s fell=%s)")
+								:format(player.Name, tostring(gone), tostring(fell))
+						)
+						-- CRITICAL: spawn() teleports the car to wherever the
+						-- CHARACTER is. During a void fall the seated character is
+						-- falling too, so respawning here would drop a fresh car
+						-- into the void and loop forever. Put the character back on
+						-- solid ground FIRST, then build the car around it.
+						local character = player.Character
+						local root = character and character:FindFirstChild("HumanoidRootPart")
+						if root and root:IsA("BasePart") then
+							local safe = spawnCFrame()
+							AntiCheat.grant(player, 5)
+							root.CFrame = safe + Vector3.new(0, 5, 0)
+							root.AssemblyLinearVelocity = Vector3.zero
+							root.AssemblyAngularVelocity = Vector3.zero
+							task.wait(0.2)
+						end
+						local data = DataManager.get(player)
+						CarManager.spawn(player, data and data.EquippedCarId)
+					end
+				end)
+				if not ok then
+					warn("[CarManager] void-recovery iteration failed: " .. tostring(err))
+				end
+			end
+		end
+	end)
 end
 
 -- The player's live car model, or nil if they have none right now.

src/server/MapBuilder.luau

@@ -323,6 +323,64 @@ local function buildPA(map: Folder)
 	})
 	ramp.Parent = pa
 
+	-- PERIMETER BARRIERS: the PA floor is a 520x520 slab floating ~46 studs up
+	-- with nothing at its edges. Endurance testing drove straight off the side
+	-- and fell past FallenPartsDestroyHeight (-500), destroying the car and
+	-- stranding the player seatless until the death/respawn cycle caught it.
+	-- Wall every edge, leaving a gap on the side the ramp exits through so the
+	-- route to the expressway stays open.
+	do
+		local rampDir = Vector3.new(road.X - c.X, 0, road.Z - c.Z)
+		-- Which edge does the ramp leave from? Whichever axis dominates.
+		local rampOnX = math.abs(rampDir.X) > math.abs(rampDir.Z)
+		local rampSign = rampOnX and math.sign(rampDir.X) or math.sign(rampDir.Z)
+		local GAP = 70 -- half-width of the opening (ramp is 46 wide)
+		local H, T = 14, 4
+
+		local function wall(name, size, pos)
+			local w = part({
+				Name = name,
+				Size = size,
+				CFrame = CFrame.new(pos),
+				Color = Color3.fromRGB(96, 96, 104),
+				Material = Enum.Material.Concrete,
+			})
+			w.Parent = pa
+		end
+
+		-- Four edges. On the ramp's edge, build two stubs flanking the opening.
+		local edges = {
+			{ onX = true, sign = 1 },
+			{ onX = true, sign = -1 },
+			{ onX = false, sign = 1 },
+			{ onX = false, sign = -1 },
+		}
+		for _, e in ipairs(edges) do
+			local isRampEdge = (e.onX == rampOnX) and (e.sign == rampSign)
+			-- Offset along the edge normal; span the full slab on the other axis.
+			local normal = e.onX and Vector3.new(e.sign * R, 0, 0) or Vector3.new(0, 0, e.sign * R)
+			local base = c + normal + Vector3.new(0, H / 2, 0)
+			local fullSize = e.onX and Vector3.new(T, H, R * 2) or Vector3.new(R * 2, H, T)
+			if not isRampEdge then
+				wall("PAWall", fullSize, base)
+			else
+				-- two stubs, each running from the corner in to the gap edge
+				local stubLen = R - GAP
+				if stubLen > 4 then
+					local half = stubLen / 2
+					local off = GAP + half
+					if e.onX then
+						wall("PAWall", Vector3.new(T, H, stubLen), base + Vector3.new(0, 0, off))
+						wall("PAWall", Vector3.new(T, H, stubLen), base + Vector3.new(0, 0, -off))
+					else
+						wall("PAWall", Vector3.new(stubLen, H, T), base + Vector3.new(off, 0, 0))
+						wall("PAWall", Vector3.new(stubLen, H, T), base + Vector3.new(-off, 0, 0))
+					end
+				end
+			end
+		end
+	end
+
 	local pad = part({
 		Name = "SpawnPad",
 		Size = Vector3.new(120, 2.4, 120),

src/server/init.server.luau

@@ -57,29 +57,77 @@ CarManager.init()
 ModeManager.init()
 AntiCheat.init()
 
--- 3. Build the world --------------------------------------------------------
-MapBuilder.build()
-Ambience.init()
+-- 3. Player lifecycle: push car id + auto-spawn their car -------------------
+-- SPAWN FIX: characters must NOT auto-load. MapBuilder.build() yields for
+-- several seconds and, until it returns, the world contains no SpawnLocation
+-- at all -- so an auto-loaded character lands at world origin (0, 5, 0),
+-- ~2,700 studs off the nearest road and ~8,500 from the parking area. Because
+-- CarManager reassembles the car around the character's root, the car is
+-- stranded at the origin too, sitting on an isolated pad; drive more than a
+-- few hundred studs and you fall out of the world forever. Load characters
+-- only once the map (and its PASpawn SpawnLocation) actually exists.
+Players.CharacterAutoLoads = false
+local mapReady = false
+
+local handled: { [Player]: boolean } = {}
+
+local function onPlayer(player: Player)
+	if handled[player] then
+		return
+	end
+	handled[player] = true
 
--- 4. Player lifecycle: push car id + auto-spawn their car -------------------
-Players.PlayerAdded:Connect(function(player)
 	ModeManager.onPlayerAdded(player)
 	DataManager.onReady(player, function()
 		pushCar(player)
 		ModeManager.pushTo(player)
 	end)
-	player.CharacterAdded:Connect(function()
+	player.CharacterAdded:Connect(function(character)
 		DataManager.onReady(player, function(data)
 			task.wait(1)
 			CarManager.spawn(player, data.EquippedCarId)
 		end)
+		-- CharacterAutoLoads = false also disables respawn-on-death, so drive
+		-- it manually or a dead player is stuck as a corpse forever.
+		local humanoid = character:WaitForChild("Humanoid", 10) :: Humanoid?
+		if humanoid then
+			humanoid.Died:Once(function()
+				task.wait(Players.RespawnTime)
+				if player.Parent then
+					player:LoadCharacter()
+				end
 			end)
-end)
+		end
+	end)
+
+	task.spawn(function()
+		while not mapReady do
+			task.wait(0.1)
+		end
+		if player.Parent then
+			player:LoadCharacter()
+		end
+	end)
+end
+
+-- Connected BEFORE the build so a player joining mid-build is never missed.
+Players.PlayerAdded:Connect(onPlayer)
+-- ...and catch anyone already present: in a Studio playtest the local player
+-- routinely exists the moment this script runs, so PlayerAdded never fires.
+for _, player in ipairs(Players:GetPlayers()) do
+	task.spawn(onPlayer, player)
+end
 
 Players.PlayerRemoving:Connect(function(player)
+	handled[player] = nil
 	ModeManager.onPlayerRemoving(player)
 end)
 
+-- 4. Build the world --------------------------------------------------------
+MapBuilder.build()
+Ambience.init()
+mapReady = true
+
 -- 5. Remote handlers (validated server-side) --------------------------------
 rSpawnCar.OnServerEvent:Connect(function(player)
 	CarManager.spawn(player) -- re-spawn current car (e.g. after flipping it)

src/shared/Chassis.luau

@@ -52,8 +52,42 @@ export type Tuning = {
 	WheelFriction: number,
 }
 
+-- Suspension physics ----------------------------------------------------------
+-- Derived rather than eyeballed. Reference: "In Depth Scripted Car Physics"
+-- (devforum 3915628) -> docs/CAR_PHYSICS_REFERENCE.txt. That tutorial builds
+-- raycast suspension, but the underlying spring math is identical for the
+-- SpringConstraints we use, so the two tuning laws below carry over directly:
+--
+--   1. STATIC SAG.  A spring holding mass m compresses by x = m*g/k. If that
+--      sag exceeds the available travel the suspension sits bottomed out, the
+--      spring stops doing any work, and the body rides on the constraint limit
+--      -- which reads as a twitchy, tippy car. The OLD constants (k=9000) gave
+--      1.10 studs of sag against 0.90 studs of TOTAL travel: permanently
+--      bottomed. Now k is solved so the car rests at SAG_FRACTION of half
+--      travel, leaving real droop AND real compression headroom.
+--
+--   2. DAMPING RATIO.  c = zeta * 2*sqrt(k*m). Damping cannot be picked
+--      independently of stiffness -- it only means anything relative to
+--      critical damping. The old pair worked out to zeta = 0.31 (floaty,
+--      oscillates after bumps). Real road cars run ~0.4-0.7; we scale within
+--      that band by Handling so a grippy car is also a composed one.
+local GRAVITY = 196.2 -- studs/s^2 (Roblox default)
+local CORNER_MASS = 50.6 -- measured: assembly mass 202.3 / 4 corners
+local SAG_FRACTION = 0.7 -- rest position, as a fraction of half travel
+
 function Chassis.tuningFor(car: Car): Tuning
 	local maxSpeed = 70 + car.Stats.Speed * 1.5
+	local handling01 = car.Stats.Handling / 100
+
+	-- Solve stiffness from the sag target, then stiffen with Handling.
+	local halfTravel = Chassis.SUSPENSION_TRAVEL / 2
+	local baseK = CORNER_MASS * GRAVITY / (halfTravel * SAG_FRACTION)
+	local stiffness = baseK * (0.9 + handling01 * 0.45) -- ~28k soft .. ~41k stiff
+
+	-- Damping ratio 0.45 (compliant) -> 0.65 (buttoned down) by Handling.
+	local zeta = 0.45 + handling01 * 0.20
+	local damping = zeta * 2 * math.sqrt(stiffness * CORNER_MASS)
+
 	return {
 		MaxSpeed = maxSpeed,
 		MaxWheelAngularVelocity = maxSpeed / Chassis.WHEEL_RADIUS,
@@ -61,8 +95,8 @@ function Chassis.tuningFor(car: Car): Tuning
 		BrakeTorque = 30000,
 		MaxSteerAngle = 22 + car.Stats.Handling * 0.16, -- ~28-38 deg
 		SteerSpeed = 6,
-		SpringStiffness = 9000,
-		SpringDamping = 420,
+		SpringStiffness = stiffness,
+		SpringDamping = damping,
 		WheelFriction = 1.2 + car.Stats.Handling * 0.008, -- ~1.2-2.0
 	}
 end

src/shared/Circuit.luau

@@ -948,10 +948,17 @@ function Circuit.gap(progressA: number, progressB: number): number
 end
 
 -- Is this world position inside the PA (off the racing surface)?
+-- PA containment. This MUST match the actual PAFloor geometry, which MapBuilder
+-- builds as a SQUARE slab of size (PA_RADIUS*2 x PA_RADIUS*2). A circular test
+-- reports false for the four corners -- up to PA_RADIUS*sqrt(2) (~367) from the
+-- centre -- even though the car is physically parked on the floor. Because
+-- ModeManager uses inPA to arm/start/finish a run, a car sitting in a corner
+-- looked like it had "left the parking area" and spuriously started a run.
+-- Square (Chebyshev) test, matching the slab.
 function Circuit.inPA(pos: Vector3): boolean
 	local c = Circuit.PA_CENTER
-	local dx, dz = pos.X - c.X, pos.Z - c.Z
-	return (dx * dx + dz * dz) <= (Circuit.PA_RADIUS * Circuit.PA_RADIUS)
+	local dx, dz = math.abs(pos.X - c.X), math.abs(pos.Z - c.Z)
+	return dx <= Circuit.PA_RADIUS and dz <= Circuit.PA_RADIUS
 end
 
 -- Forward direction (unit) of the racing line at a sample index.