vehicle-parts-mod-creation

name	vehicle-parts-mod-creation
description	Create MotorTown vehicle part mods — tires (with custom physics DataAssets) and intakes (inline sub-struct tuning). Covers VehicleParts DataTable mechanics, PartType sub-structs, dot-path patching, and compat-mod support.

Vehicle Parts Mod Creation

Create mod PAKs that add new vehicle parts to MotorTown. This skill covers tire mods (with separate physics DataAssets) and intake mods (with inline sub-struct tuning), plus the shared VehicleParts DataTable mechanics that apply to all part types.

Part Types Overview

Every VehicleParts row contains ALL part sub-structs (Tire, Intake, Turbocharger, Suspension, etc.), with PartType determining which the engine reads:

PartType: Tire
├── Tire.TirePhysicsDataAsset → import reference to separate physics asset
├── VehicleTypes: [Small, Medium]
├── VehicleKeys: [Elisa_Police, ...]
├── LevelRequirementToBuy: {CL_Police: 10}
├── Cost, MassKg, Name2.Texts, etc.
└── ... (all other sub-structs present but inactive)

PartType: Intake
├── Intake.Slope → torque curve slope
├── Intake.BaseRPMRatio → RPM ratio where effect begins
├── Intake.IntakeSpeedEfficencyMultiplier → overall HP multiplier
├── Common fields (VehicleTypes, VehicleKeys, Cost, etc.)
└── ... (all other sub-structs present but inactive)

PartType: Turbocharger
├── Turbocharger.bIsValid → must be true
├── Turbocharger.TorqueMultiplier, BaseTorqueMultiplier
├── Turbocharger.TurbineWeight → spool time
├── Turbocharger.IntakePressureMultiplier, TurbineAspectRatio
├── Turbocharger.HeatingMultiplier, FuelConsumptionMultiplier
├── Common fields
└── ...

[!IMPORTANT] Only patch VehicleParts0 (50 rows), NOT VehicleParts (713 rows). This minimizes conflict surface with other mods. See general-modding skill for details.

---

Tire Mods

Quick Start

# Standalone
nix develop --command bash -c 'python3 scripts/mods.py build police-tyres'

# Or directly:
nix develop --command bash -c '
python3 scripts/create_tirepack.py \
  --config mods/police-tyres/tire_entries.json \
  --output ASEAN_PoliceTyres_P.pak
'

# Compatible with another mod
nix develop --command bash -c '
python3 scripts/create_tirepack.py \
  --config mods/police-tyres/tire_entries.json \
  --output ASEAN_PoliceTyres_MoreTuningCompat_P.pak \
  --compat-mod path/to/qxZap_MoreTuning_P.pak
'

Pipeline

Step	Tool	Input	Output
0. Extract compat base	Rust mod_explore	--compat-mod PAK	VehicleParts0 template
1. Patch tire physics	C# --patch-tire	tire_entries.json + BasicTire_45.uasset	New tire .uasset/.uexp
2. Add to VehicleParts0	C# --add-tire-parts	tire_entries.json + VehicleParts0.uasset	Modified VehicleParts0.uasset
3. Assemble PAK dir	Python	All outputs	Directory structure
4. Build PAK	Rust mod_pack	Directory	.pak file

Configuration (tire_entries.json)

{
  "tires": [
    {
      "tire_physics": {
        "name": "APF_79_Tire",
        "template": "BasicTire_45",
        "static_mu": 1.6,
        "sliding_mu": 1.3,
        "offroad_friction": 1.4,
        "spring_x": 150000
      },
      "tire_part": {
        "row_name": "APF_79",
        "display_name": ["AMC Police 79"],
        "cost": 2000,
        "mass_kg": 10,
        "vehicle_types": ["Small"],
        "vehicle_keys": ["Elisa_Police", "Muhan_Police", "Zydro_Police"],
        "level_requirement": {"CL_Police": 10},
        "tire_asset_path": "/Game/Cars/Parts/Tire/APF_79_Tire/APF_79_Tire"
      }
    }
  ]
}

Tire Physics Fields

Field	Default	Description
name	required	Internal asset name (becomes .uasset filename)
template	required	Base game tire to clone from (filename in out/)
static_mu	inherited	Static friction coefficient (grip at rest)
sliding_mu	inherited	Sliding friction coefficient
offroad_friction	inherited	Off-road friction multiplier
spring_x	inherited (180000)	Lateral stiffness — lower reduces rollover
spring_y	inherited	Longitudinal stiffness
damping_x	inherited	Lateral damping
damping_y	inherited	Longitudinal damping

Tire Part Fields

Field	Default	Description
row_name	required	DataTable row name
display_name	required	Array for in-game display (Name2.Texts)
cost	required	Purchase price
mass_kg	10	Tire mass
vehicle_types	required	Small, Medium, Large, HeavyMachine, MotorCycle
vehicle_keys	[] (all)	Restrict to specific vehicles
level_requirement	{} (none)	Career level gate
tire_asset_path	required	UE path: /Game/Cars/Parts/Tire/{Name}/{Name}

Base Game Tire Physics Reference

Tire	StaticMu	SlidingMu	OffroadFriction
BasicTire_45 (standard)	1.1	1.0	—
BasicTire_65 (wider)	1.1	1.0	—
PerformanceTire	—	~1.0	—
DriftTire	—	0.85	—
OffroadTire	0.95	0.9	1.4
HeavyDutyFront/Rear	—	0.9	—
HeavyDuty_Load1	0.97	0.87	—
HeavyMachine_20Ton	0.95	0.88	1.2

Recommended Tire Value Ranges

Feel	StaticMu	SlidingMu	OffroadFriction	SpringX
Stock-like	1.0–1.1	0.85–1.0	—	180000
Cruiser (mild upgrade)	1.3–1.4	1.1–1.2	1.3	180000
Anti-rollover pursuit	1.5–1.6	1.2–1.4	1.4	130000–150000
High-grip pursuit	1.6–1.8	1.4–1.6	1.6	180000
Racing slick	1.8–2.0	1.5–1.7	—	180000
Unrealistic max grip	2.0+	2.0+	2.0+	200000+

[!WARNING] Rollover risk: High StaticMu (1.8+) on tall vehicles (SUVs, police cars) generates so much lateral G-force that the car rolls over instead of sliding. Fix: lower SpringX to ~150000, or reduce StaticMu to ~1.5.

Bike Tires — Dual Physics Assets

Unlike car tires which use a single physics asset, bike tires reference two separate assets — one for the front wheel and one for the rear:

Property	Car Tire	Bike Tire
Tire.TirePhysicsDataAsset	Front + Rear (same)	Front only
Tire.TirePhysicsDataAsset_BikeRear	null (unused)	Rear only
Asset count per tire	1	2

Stock bike (MotorCycleTire_01) references:

• Front: Motorcycle_Front — SpringX=25000, SpringY=2500, DampingY=40, MaxWeightKg=200
• Rear: Motorcycle_Rear — SpringY=3000, DampingY=50, MaxWeightKg=300

The rear is slightly stiffer and supports more weight. When creating custom bike tires, you MUST clone and patch both assets.

Builder support: The create_tirepack.py script auto-detects bike mode when tire_physics has front and rear sub-dicts. It automatically sets both import references and nulls out the car-only rear ref for non-bike tires.

{
  "tire_physics": {
    "front": {"name": "APF_Cruiser_78_Front", "template": "Motorcycle_Front", ...},
    "rear":  {"name": "APF_Cruiser_78_Rear",  "template": "Motorcycle_Rear",  ...}
  }
}

Tire Gotchas

1. PAK Path — FLAT, No Subfolder: Tire physics assets MUST be in Cars/Parts/Tire/ directly, not in a subfolder. The engine maps /Game/Cars/Parts/Tire/{Name} to Content/Cars/Parts/Tire/{Name}.uasset.

2. NameMap[0] — Must Rename: When cloning a tire, NameMap[0] has the old package path. Must update it or the game uses default physics. For cooked assets, --clone-asset MUST have patch_namemap_0: true.

3. FName Number Suffix: BasicTire_45 is stored as FName("BasicTire", Number=46). Always use new FName(asset, name, 0) to avoid stale Number suffixes.

4. VehicleParts0 Override: Game loads both VehicleParts (713 rows) and VehicleParts0 (50 rows). VehicleParts0 supersedes for shared categories. Add tire to BOTH.

5. Template Row Selection: Use the last tire row (Offroad), not MotorCycleTire_01 which has bike-only properties.

6. Display Name: In-game name comes from Name2.Texts, NOT Name or Desciption.

7. Rename Order: Exports → Imports → NameMap. Renaming NameMap first causes silent corruption.

8. Pure Digit Row Names: APF_78 gets parsed as FName("APF", Number=79), causing save/load mismatches and disappearing tires. Append a letter: APF_78A.

9. Combining mods manually requires merging all assets: The tire pack builder only stages its own assets. If combining PD Parts (which has 6 car tire physics assets) with bike tires, you must extract BOTH PAKs, merge directories (so all tire physics assets are present), patch VehicleParts0 for any cross-mod changes, then repack. See general-modding skill for the manual merge workflow.

---

Intake Mods

Quick Start

# Standalone
nix develop --command bash -c 'python3 scripts/mods.py build police-sc'

# Or directly:
nix develop --command bash -c '
python3 scripts/create_intakepack.py \
  --config mods/police-sc/intake_entries.json \
  --output AMC_PDParts_P.pak
'

# Compatible with other mods
nix develop --command bash -c '
python3 scripts/create_intakepack.py \
  --config mods/police-sc/intake_entries.json \
  --output AMC_PDParts_MoreTuningCompat_P.pak \
  --compat-mod path/to/qxZap_MoreTuning_P.pak
'

Pipeline

Step	Tool	Input	Output
0. Extract compat base	Rust mod_explore	--compat-mod PAK	VehicleParts0 template
1. Add rows to VehicleParts0	C# --add-rows	intake_entries.json + VehicleParts0.uasset	Modified VehicleParts0.uasset
2. Assemble PAK dir	Python	DataTable output	Directory structure
3. Build PAK	Rust mod_pack	Directory	.pak file

No transform_assets() step — intake parts don't need separate physics assets like tires.

How Intake Parts Work

An Intake row has 3 tunable fields in the Intake sub-struct:

Field	Effect
Slope	Torque curve slope — higher positive = more torque bias at peak RPM
BaseRPMRatio	RPM ratio where effect kicks in — lower = earlier power delivery
IntakeSpeedEfficencyMultiplier	Overall efficiency multiplier — higher = more horsepower

Supercharger Simulation

Intake parts can simulate superchargers by tuning for instant response and high top-end power:

Characteristic	Supercharger tuning	Turbocharger comparison
Response	Low BaseRPMRatio (0.1–0.3) = instant boost	Turbo has lag (spool via TurbineWeight)
Torque	Moderate Slope (0.05–0.15)	Higher TorqueMultiplier (1.1–1.2)
Horsepower	High EfficiencyMult (2.0–2.5+)	High IntakePressureMultiplier (0.5–0.8)

[!TIP] A supercharger's defining characteristic is immediate response (low BaseRPMRatio) with linear power delivery (steady Slope). Turbos have delayed response (spool) but higher peak torque.

Vanilla Game Values (v0.7.18+1)

Intake rows:

Row	Slope	BaseRPMRatio	EfficiencyMult	Cost
Row 201 (GUID)	0.1	0.7	1.5	300
Row 202 (GUID)	-0.1	0.8	0.7	300

Turbocharger rows (for comparison):

Row	TorqueMult	BaseTorqueMult	IntakePressMult	TurbineWeight	Cost
Stock (Small)	1.1	0.98	1.0	30	3,000
Stage1 (Small)	1.2	0.95	0.5/0.8	100	10,000
Stock (HeavyDuty)	1.1	0.98	1.0	30	10,000
Stage1 (HeavyDuty)	1.2	0.95	0.5/0.8	100	30,000
Stock (HeavyMachine)	1.1	0.98	1.0	30	50,000

Recommended Intake Tuning Ranges

Feel	Slope	BaseRPMRatio	EfficiencyMult
Mild upgrade	0.05	0.3	1.5–2.0
Sport intake	0.1	0.2	2.0–2.2
Supercharger (street)	0.1	0.1–0.15	2.0–2.3
Supercharger (pursuit)	0.15	0.1	2.3–2.5
Extreme	0.2+	0.05	2.5+

[!WARNING] Negative Slope (like vanilla Row 202 with -0.1) reduces torque at high RPM. For "strongest" superchargers, use positive Slope with high EfficiencyMult — this gives both torque AND horsepower.

Configuration (intake_entries.json)

{
  "intakes": [
    {
      "row_name": "PD_SC_Standard",
      "display_name": ["PD SC Standard"],
      "cost": 5000,
      "mass_kg": 5.0,
      "vehicle_types": ["Small"],
      "vehicle_keys": ["Elisa_Police", "Muhan_Police"],
      "level_requirement": {"CL_Police": 10},
      "intake": {
        "Slope": 0.05,
        "BaseRPMRatio": 0.3,
        "IntakeSpeedEfficencyMultiplier": 2.0
      }
    }
  ]
}

Intake Entry Fields

Field	Default	Description
row_name	required	DataTable row name
display_name	required	Array for in-game display
cost	required	Purchase price
mass_kg	5.0	Part mass
vehicle_types	required	Vehicle class filter
vehicle_keys	[] (all)	Restrict to specific vehicles
level_requirement	{} (none)	Career level gate
intake.Slope	inherited	Torque curve slope
intake.BaseRPMRatio	inherited	RPM ratio where effect begins
intake.IntakeSpeedEfficencyMultiplier	inherited	Overall HP multiplier

Intake Gotchas

1. No Separate Physics Asset: All tuning is inline in the VehicleParts row. PAK only contains VehicleParts0.uasset + .uexp — no extra files.

2. PartType Must Be Intake: The template_row_match ensures the cloned row has PartType = Intake. Wrong template = game ignores the Intake sub-struct.

3. Positive Slope = Stronger: Higher positive Slope = more torque at peak RPM. Negative Slope reduces high-end torque. For strongest superchargers, use positive Slope + high EfficiencyMult.

4. BaseRPMRatio Below 0.5 = Instant Response: Vanilla has 0.7–0.8 (boost at 70–80% RPM). Supercharger feel: 0.1–0.3 (boost from 10–30% RPM).

5. VehicleKeys Must Include All Target Vehicles: If an intake row has VehicleKeys: ["Elisa_Police", "Muhan_Police"], ONLY those vehicles can equip it. Adding a new vehicle (e.g., Gunthoo_Police) requires patching the row to include it. Use --patch-rows with set_name_array to update the whitelist.

---

Shared Mechanics

Dot-Path Resolution Through Sub-Structs

The C# --add-rows command uses ResolvePropertyWithContainer to resolve dot-separated paths through StructPropertyData containers:

"Intake.Slope" → finds "Intake" StructPropertyData → traverses into it → finds "Slope" FloatPropertyData

Works with set_or_add_float, set_or_create_name, set_or_create_int, etc. Only ONE level of dot-path nesting is needed for VehicleParts sub-structs (e.g., Intake.Slope, Turbocharger.bIsValid).

Template Row Matching

--add-rows uses template_row_match to find an existing row as the clone template. The * wildcard does a substring match on the field value:

// For intakes
"template_row_match": {"PartType": "*Intake*"}

// For tires (handled internally by --add-tire-parts)
// Uses "last tire row found" strategy → Offroad

[!IMPORTANT] If no row matches, the C# tool falls back to the last row in the DataTable, which may be a completely different part type. Always verify the template row in build output.

Row Chaining

Rows are added sequentially — each new row is appended to the output of the previous step:

VehicleParts0 (45 rows) → +Row1 → (46) → +Row2 → (47) → +Row3 → (48)

Compat-Mod Support

Both tire and intake mods support --compat-mod for building on top of other mods' VehicleParts0:

# Chain: AMC Tires MoreTuningCompat PAK already has both mods' rows
# → adding intakes on top produces PAK with all three mods
python3 scripts/create_intakepack.py \
  --config mods/police-sc/intake_entries.json \
  --output zzz_AMC_PDParts_MoreTuningCompat_P.pak \
  --compat-mod zzz_ASEAN_PoliceTyres_v0.2.0_MoreTuningCompat_P.pak

[!CAUTION] Users must install only one variant. Both standalone + compat causes double-override on VehicleParts0.

Display Name (Name2.Texts)

The in-game display name comes from the Name2 struct's Texts array, NOT from Name or Desciption:

• Name — localization hash (GUID). Generate random for uniqueness.
• Name2.Texts — actual display strings shown in the shop UI.
• Desciption — fallback / localization key.

If Name2.Texts is empty and Desciption still has the template's hash, the part displays the template's name (e.g., "Offroad").

---

Verifying a Built PAK

# List PAK contents
cargo run --release --bin mod_explore --quiet -- MyMod_P.pak --list

# Dump asset internals
cd csharp/UAssetTool
dotnet run --configuration Release --verbosity quiet -- \
  --dump /path/to/extracted/asset.uasset

# Binary scan for stale template references
python3 -c "
import re
with open('asset.uasset', 'rb') as f:
    data = f.read()
for m in re.finditer(rb'BasicTire|OldTemplate', data):
    print(f'Stale ref at offset {m.start()}: {m.group()}')
"

Diagnostic Checklist

Symptom	Likely Cause
Part not in list	Missing from VehicleParts0, or wrong PartType
Shows wrong template name	Name2.Texts not set
Tire has no friction values	Asset in subfolder, NameMap[0] stale, _NN suffix, or physics asset missing from PAK
Intake doesn't feel different	Dot-path not patched correctly
Only on wrong vehicles	VehicleTypes/VehicleKeys mismatch
Conflicting with another mod	Rebuild with --compat-mod
Tire disappears on restart	Pure digit row name (use APF_78A not APF_78)
Bike tire has no grip	Only front OR rear physics asset present — bike tires need BOTH
Combined mod loses car tire grip	Other mod's tire physics assets not merged — use manual merge, not just --compat-mod

Key Files

File	Purpose
scripts/create_tirepack.py	Tire mod build script
scripts/create_intakepack.py	Intake mod build script
scripts/modbase.py	Shared ModBuilder base class
csharp/UAssetTool/Program.cs	C# tool: --add-rows, --patch-tire, --add-tire-parts, --dump
out/VehicleParts0.uasset	Base game VehicleParts0 DataTable template
out/VehicleParts.uasset	Full VehicleParts DataTable (713 rows)
mods/police-tyres/tire_entries.json	Tire physics + part config
mods/police-sc/intake_entries.json	Intake tuning config