Car Physics for rFactor 2

Introduction to rFactor 2 Car Physics

Car physics in rFactor 2 represent the culmination of decades of racing simulation development. The physics engine calculates every aspect of vehicle dynamics in real-time: engine power delivery, suspension geometry changes through travel, weight transfer, aerodynamic forces, and how all these systems interact through the tires.

Professional rFactor 2 mod development requires understanding multiple file types that work together to create authentic vehicle behavior. This guide focuses on two critical areas: engine development and suspension geometry.

File Types Overview

• HDV (Vehicle) Files: The master file containing all vehicle parameters including mass, inertia, aerodynamics, suspension settings, and references to other physics files
• Engine Files: Define power delivery, torque curves, fuel consumption, and thermal characteristics
• UltraChassis INI Files: Modern suspension geometry system using INI format (replaces legacy PM files)
• Gear Files: Transmission ratios for all forward gears, reverse, and final drive

Required Research Data

Creating accurate vehicle physics requires extensive real-world data. Unlike simplified simulation approaches that estimate values, professional rFactor 2 development demands research and validation. The following data is essential:

Mass and Inertia Data

• Curb Weight: Total vehicle mass in kilograms (for Mass value in HDV)
• Center of Gravity Height: Height above reference plane (for CGHeight in HDV) - often not published but can be estimated from similar vehicles
• Dimensions: Length, width, height minus ride height for accurate inertia calculations
• Weight Distribution: Front/rear and left/right percentages (for CGRearRange and CGRightRange in HDV)
• Fuel Tank Position: Location relative to rear axle (for FuelTankPos in HDV) - requires underside photos or technical diagrams

Aerodynamic Data

• Drag Coefficient: Cd value (for BodyDragBase in HDV)
• Frontal Area: Cross-sectional area for drag calculations
• Downforce Data: Front and rear wing positions relative to axles, downforce values if racing car

Chassis and Suspension Data

• Track Width: Front and rear track measurements (for FrontWheelTrack/RearWheelTrack in HDV)
• Wheelbase: Distance between front and rear axle centers (for LeftWheelBase/RightWheelBase in HDV)
• Suspension Travel: Maximum bump and rebound travel distance (for BumpTravel/ReboundTravel in HDV)
• Ride Height: Ground clearance at factory height (for RideHeightRange in HDV)
• Spring Rates: Factory spring stiffness values (for SpringRange in HDV)
• Anti-Sway Bar Diameters: Front and rear sway bar sizes (for FrontAntiSwayRange/RearAntiSwayRange in HDV)
• Alignment Specifications: Factory toe, caster, and camber settings
• Suspension Geometry Photos: Critical for UltraChassis creation - need clear photos of suspension arms, mounting points, and pushrod/shock positions

Brake System Data

• Brake Disc Thickness: Front and rear rotor thickness (for BrakeDiscRange in HDV)
• Brake Bias: Front/rear brake force distribution percentage
• Pad Type Information: Operating temperature ranges if available

Powertrain Data

• Wheel Torque Dyno Graph: Essential for engine file creation - shows actual power at the wheels
• Gear Ratios: All forward gears, reverse, and final drive ratio (for Gear file creation)
• Engine Specifications: Displacement, configuration, compression ratio, redline
• Differential Type: Open, viscous LSD, clutch-type LSD with locking percentages

Tire Data

• Factory Tire Sizes: Width, aspect ratio, diameter for front and rear
• Tire Brand and Model: Specific tire used for accurate tire physics development
• UTQG Ratings: Treadwear, traction, and temperature ratings
• Test Results: Skidpad G-force, braking distances for grip validation

Engine Development

Engine files define the heart of vehicle performance: power delivery characteristics, fuel consumption, thermal behavior, and engine braking. Unlike simple power/torque multipliers, rFactor 2 engine files model the complete torque curve with compression losses and thermal management.

Engine File Structure

Engine files use a plain text format with RPM/Torque data points and various operational parameters:

Understanding RPMTorque Lines

Each RPMTorque line contains three values: RPMTorque=(RPM, Compression_Torque, Power_Torque)

Power Torque (Third Value)

This is the actual torque produced by combustion at this RPM. These values should come from a wheel horsepower dyno chart, not crank horsepower. Using wheel horsepower eliminates the need to model driveline friction losses separately.

To convert dyno data to RPMTorque format:

1. Obtain wheel HP dyno chart with RPM on X-axis and torque on Y-axis
2. Sample data points every 250 RPM from idle to redline
3. Enter torque values in Newton-meters (convert from lb-ft if necessary: Nm = lb-ft x 1.356)
4. Use rFactor 2 Physics Editor to visualize the curve and ensure smoothness

Compression Torque (Second Value)

This represents engine braking force - the resistance from compressing air in the cylinders when the throttle is closed. Compression torque increases steadily with RPM and depends on:

• Cylinder Count: More cylinders = more compression events = more braking
• Compression Ratio: Higher compression = more engine braking
• Displacement: Larger engines have more compression resistance

As a reference point, a typical 4-cylinder naturally aspirated engine with 9.5:1 compression might have compression torque values ranging from -12.7 Nm at 250 RPM to -75.0 Nm at redline. A 6-cylinder engine would have roughly 50% higher values; an 8-cylinder would have double.

Critical Engine Parameters

Engine Inertia

Rotational inertia of all rotating engine components (crank, flywheel, pistons, rods). Higher values mean the engine rev changes more slowly. This affects:

• How quickly the engine spins up when blipping throttle
• How quickly RPM drops during upshifts
• Engine braking effectiveness

Typical values: Small 4-cylinder = 0.10-0.15, Large V8 = 0.18-0.25, with heavy flywheel = +0.05-0.10

Fuel Consumption

FuelConsumption: Liters of fuel consumed per Nm of torque per second. This value is affected by throttle position and RPM. Smaller, more efficient engines have lower values (~3.0e-05), while large displacement or forced induction engines have higher values (~4.5e-05).

FuelEstimate: Multiplier used by AI for pit strategy calculations. Typically 1.0-1.1 for most engines.

Idle and Rev Limit

IdleRPMLogic=(min, max): The ECU attempts to maintain idle speed between these RPM values. Most street cars idle at 600-900 RPM; race cars at 1200-1800 RPM.

RevLimitRange=(limit, increment, steps): Maximum RPM and adjustability. For stock cars, typically only one value (no adjustment). Race cars may have adjustable limits.

RevLimitLogic: RPM range for limiter operation. Lower values = harder bouncing off limiter; higher values = softer limiting. A value of 0 means immediate hard cut.

Launch Control

LaunchRPMLogic: Even if the vehicle doesn't have launch control (LaunchEfficiency=0), this value is used by AI for launch RPM. Set this to the engine's peak torque RPM or slightly above for optimal launches.

Thermal Management

Engine thermal model affects oil temperature, which influences engine performance and longevity:

• OptimumOilTemp: Temperature (Celsius) for peak performance. Typical range: 90-110 °C
• CombustionHeat: Degrees added per liter of fuel burned. Higher for turbocharged engines (lower compression = less heat), lower for high-compression NA engines. Add ~5 degrees per additional cylinder beyond 4.
• EngineSpeedHeat: Heat generated linearly with RPM. Higher revving engines typically have slightly higher values.
• RadiatorCooling: Base cooling rate and per-setting multiplier. Affects how radiator/grille tape adjustments impact cooling.

Engine Life and Failure

These parameters model engine wear and failure over time:

• LifetimeEngineRPM=(base, range): At base RPM (3000), engine life is normal. Every "range" (4000) RPM higher, lifetime is halved. Keeping engine at 7000 RPM constantly would be (7000-3000)/4000 = 1 doubling, meaning 50% lifetime.
• LifetimeOilTemp=(base, range): Same concept for oil temperature. At 120 °C base, lifetime is normal. Every 20 °C higher, lifetime is halved.
• LifetimeAvg=1296000: Average engine life in seconds (360 hours for this example)
• LifetimeVar=2000: Random variance - some engines fail earlier, some later

HDV File - General Section

The HDV (Vehicle) file [GENERAL] section contains fundamental vehicle properties that affect all other systems. These values define the vehicle's mass, inertia, center of gravity, and fuel system.

Mass and Inertia

Mass

Mass=1250: Total vehicle curb weight in kilograms, excluding fuel. Use manufacturer's published curb weight.

Inertia Calculation

Inertia=(Pitch, Yaw, Roll): Rotational resistance in each axis. These values determine how quickly the vehicle rotates around each axis:

• Pitch: Resistance to nose-up/nose-down rotation (under braking/acceleration)
• Yaw: Resistance to left/right rotation (turning)
• Roll: Resistance to body roll (cornering)

For a vehicle approximated as a rectangular mass distribution, calculate inertia using:

• Pitch = (Mass/12) x (Length² + Height²)
• Yaw = (Mass/12) x (Length² + Width²)
• Roll = (Mass/12) x (Height² + Width²)

Important: Height should be the body height MINUS the ride height, because rFactor 2's reference plane is the bottom of the chassis, not the ground.

Material construction affects these values:

• Heavy reinforcement: Old steel/iron construction - add 10-15% to calculated values
• Standard construction: Modern steel unibody - use calculated values
• Lightweight construction: Aluminum/carbon fiber - subtract 5-10% from calculated values

Center of Gravity

CGHeight=0.350: Height of the vehicle's center of mass above the reference plane (bottom of chassis), in meters. This value critically affects weight transfer and handling:

• Lower CG = less weight transfer = more stable, better cornering
• Higher CG = more weight transfer = more dynamic, potentially unstable

To calculate: Find the vehicle's published CG height (often around crankshaft centerline), then subtract the ride height. If unavailable, typical values: Sports cars 0.30-0.35m, Sedans 0.35-0.42m, SUVs 0.45-0.55m, Race cars 0.25-0.32m.

CGRightRange=(0.500, 0.005, 1): Lateral weight distribution as fraction on left tires. 0.500 = perfect 50/50 left/right. The increment (0.005) and number of steps (1) define adjustability - in this case, not adjustable.

CGRearRange=(0.450, 0.005, 1): Longitudinal weight distribution as fraction on rear tires. 0.450 = 45% rear, 55% front. This is critical for handling balance. Use manufacturer's published weight distribution.

Fuel System

FuelTankPos=(left/right, up/down, front/back): Fuel tank location relative to the center of the rear axle in the reference plane. Requires photos of vehicle underside or technical diagrams. Fuel positioning affects weight distribution as fuel load changes.

FuelTankMotion=(spring_rate, damping): Simple model of fuel sloshing. Higher spring rate = fuel moves less freely; higher damping = fuel settles more quickly. These values rarely need adjustment from defaults.

FuelRange=(starting_amount, increment, max_units): Fuel capacity in liters. In this example: starts at 1L, increments of 1L, max 72L capacity.

HDV File - Suspension Parameters

The HDV [SUSPENSION] section and wheel-specific sections ([FRONTLEFT], [FRONTRIGHT], [REARLEFT], [REARRIGHT]) define spring rates, damping, geometry, and all adjustable parameters. These work in conjunction with the UltraChassis INI file to create the complete suspension system.

Basic Geometry

UltraChassis: References the modern INI file containing suspension geometry. For legacy vehicles, you may see PhysicalModelFile=Vehicle.pm instead.

Front/RearWheelTrack: Distance between left and right wheel centers in meters. Wider track = more lateral stability but potentially slower direction changes.

Left/RightWheelBase: Distance between front and rear axle centers. Usually identical unless the vehicle has asymmetric geometry.

Anti-Sway Bars (Anti-Roll Bars)

SpringBasedAntiSway=0: Use diameter-based calculation (0) or spring-based (1). For stock vehicles with known sway bar diameters, use 0.

AllowNoAntiSway=1: When enabled, the first setting represents "no sway bar" (0mm diameter). This is useful for vehicles where sway bars are removable upgrades.

FrontAntiSwayRange=(diameter, increment, steps): Base diameter in meters, size increment, number of increments. Example: (0.025, 0.0, 2) means starts at 25mm diameter, 0mm increment, for 2 total positions - so 0mm (no bar) and 25mm (stock bar).

FrontAntiSwayRate=(base, power): Stiffness calculation formula: rate = base x (diameter ^ power). For hollow/tubular steel sway bars, use base=6.2e10, power=4. For solid steel bars, use base=1.1e11.

Alignment Settings

ToeInRange=(start_degrees, increment, steps): Toe angle range. Negative values = toe-out, positive = toe-in. Example: (-0.45, 0.01, 56) provides range from -0.45,to +0.55,in 0.01,increments.

ToeInSetting=44: Default setting index. Calculate actual toe: -0.45 + (44 x 0.01) = -0.01,(slight toe-out).

CasterRange=(start_degrees, increment, steps): Caster angle for each front wheel. Higher caster increases self-centering force and can affect camber gain in steering. Most street cars: 3-7°, performance cars: 6-9°, race cars: 7-12°.

Per-Wheel Parameters

Each wheel corner (FRONTLEFT, FRONTRIGHT, REARLEFT, REARRIGHT) has identical parameter structures but different values. Here's an explanation using FRONTLEFT as the example:

Suspension Travel

BumpTravel=-0.10: Maximum compression distance (negative value) in meters before hitting the bump stop. When the suspension compresses 0.10m (100mm) from ride height, it contacts the bump stop.

ReboundTravel=-0.320: Maximum extension distance (negative value) before reaching full droop. Total suspension travel = |BumpTravel| + |ReboundTravel| = 0.10 + 0.320 = 0.420m (420mm).

The ride height setting must fall between these two values, typically closer to rebound than bump to allow more compression travel for bumps.

Bump Stops

Bump stops are progressive - they get stiffer as they compress:

• BumpStopSpring: Initial spring rate when first contacting bump stop (N/m)
• BumpStopRisingSpring: Progressive rate increase (multiplied by deflection²). Very high values create hard stops.
• BumpStopDamper/RisingDamper: Same progressive concept for damping. Prevents bounce after bottoming.

Pushrod Attachment Points

Critical for motion ratio calculation: These coordinates define where the spring/damper attaches to the suspension.

PushrodSpindle=(X, Y, Z): Attachment point on the spindle/suspension arm, relative to wheel center.

PushrodBody=(X, Y, Z): Attachment point on the chassis, relative to wheel center.

The distance and angle between these points determines motion ratio - how much the spring compresses for a given wheel movement. If the spring is 70% of the distance from the body mount to the spindle, the motion ratio is 0.7, and SpringMult should be 0.7.

Springs

SpringMult=1.00: Motion ratio multiplier. If spring is directly on the spindle (perpendicular to travel), use 1.0. If spring is inboard, calculate the geometric motion ratio.

SpringRange=(rate, increment, steps): Spring rate in N/m. Factory springs are typically non-adjustable (increment=0, steps=1). This example: 17862 N/m ≈ 1.82 kg/mm, typical for a 1250kg street car.

Dampers (Shock Absorbers)

BumpStage2/ReboundStage2: Shaft velocity (m/s) where the damper transitions from "slow" to "fast" valving. At velocities below ±0.025 m/s, slow damping applies; above this, fast damping applies. This models the actual blow-off behavior of shock valving.

DamperMult: Same as SpringMult - motion ratio for damper if it's not directly on the spindle.

Damper force = Rate x Velocity, so these are damping coefficients in N/(m/s):

• SlowBump: Low-speed compression damping (hitting a gradual bump)
• FastBump: High-speed compression damping (hitting a sharp edge)
• SlowRebound: Low-speed extension damping (controls weight transfer rate)
• FastRebound: High-speed extension damping (prevents harsh rebound)

Typical relationship: SlowRebound ≈ 2-3× SlowBump, FastRebound ≈ 1.5× FastBump. This creates the classic "firm on compression, controlled on extension" shock behavior.

Ride Height

RideHeightRange=(height, increment, steps): Height of inner suspension pivot points above reference plane, in meters. This is NOT ground clearance - it's the actual height of the suspension geometry attachment points on the chassis.

To determine this value: Take the vehicle's ground clearance, add the distance from the ground to the lower suspension pivot point. For most cars, this is 0.15m to 0.25m.

Camber

CamberRange=(start, increment, steps): Static camber adjustment range in degrees. Negative camber tilts the top of the tire inward. Example: (-0.5, 0.5, 1) means -0.5,camber with no adjustment range (steps=1).

Tire Pressure

PressureRange=(start_kPa, increment, steps): Tire pressure adjustment range in kilopascals. Example range: 120 to 380 kPa (17.4 to 55.1 PSI).

Brakes

BrakeDiscRange=(thickness, increment, steps): Brake rotor thickness in meters. For stock vehicles, typically not adjustable.

BrakeResponseCurve=(cold_temp, min_optimal, max_optimal, hot_temp): Temperature curve defining brake performance. At cold_temp (-20 °C), brakes are at 50% effectiveness. Between min_optimal and max_optimal (20-160 °C), brakes are 100% effective. At hot_temp (200 °C), brakes fade to 50%. These values should match brake pad specifications.

BrakeTorque=1682.0: Maximum brake torque in Nm at optimal temperature and zero wear. Adjust by feel to allow threshold braking without locking up (with proper tires).

BrakeHeating/Cooling: Thermal model parameters. Vented rotors cool faster (higher cooling values); track pads generate more heat (higher heating values).

UltraChassis - Modern Suspension Geometry

UltraChassis is the modern suspension geometry system in rFactor 2, replacing legacy PM files. It uses INI file format and is referenced in the HDV with UltraChassis=filename.ini. If both UltraChassis and PhysicalModelFile lines exist in the HDV, UltraChassis takes precedence.

UltraChassis File Structure

UltraChassis INI files define suspension geometry using a more accessible format than legacy PM files. The system calculates camber curves, bump steer, roll centers, and all geometric effects throughout suspension travel.

Coordinate System

rFactor 2 uses this coordinate system for all position values:

• +X = LEFT (positive values move left, negative move right)
• +Y = UP (positive values move up, negative move down)
• +Z = REAR (positive values move rearward, negative move forward)

All measurements are relative to the wheel center position in the reference plane (ride height).

HDV Integration

Reference the UltraChassis file in your HDV [SUSPENSION] section. The system automatically handles spindle masses, wheel connections, and suspension geometry calculations.

Key Advantages of UltraChassis

• INI Format: Easier to read and edit than PM files
• Better Integration: Works seamlessly with HDV suspension parameters
• More Flexible: Simpler to adjust geometry without complex constraint definitions
• Modern Standard: All new Studio 397 vehicles use UltraChassis

Suspension Types in rFactor 2

UltraChassis supports all common suspension types:

Fully Supported:

• Double Wishbone: Most accurate representation, full control over geometry
• Multi-Link: Complex geometry with multiple control arms
• MacPherson Strut: Can be properly modeled in UltraChassis
• Solid Axle: Both live axles and beam axles with proper constraints

Important Limitations:

• Rubber Bushings: rFactor 2 doesn't model bushing compliance. All joints are rigid connections. You must approximate compliance effects through geometry or slight adjustment of anti-squat/dive characteristics
• Torsional Flexibility: Chassis flex is not modeled - all chassis mounting points are rigidly fixed

Measuring Suspension Geometry

Creating accurate UltraChassis files requires real-world suspension measurements:

Required Data:

1. Suspension Photos: High-quality photos from multiple angles showing all control arm mounting points, with wheels removed
2. Factory Service Manual: Often contains suspension diagrams with critical measurements
3. Technical Drawings: Manufacturers sometimes provide suspension geometry specifications in homologation documents
4. Physical Measurements: If you have vehicle access, measure from wheel center to each mounting point

Critical Measurements:

• Control Arm Pickup Points: X, Y, Z coordinates of each arm's chassis and spindle mounting points
• Steering Arm Geometry: Affects Ackermann angle and bump steer characteristics
• Spring/Damper Mounting: Position and angle affect motion ratios
• Anti-Roll Bar Mounting: Pickup points determine effective rate

Best Practices for Car Physics Development

Research Methodology

1. Start with Official Sources: Manufacturer specs, technical manuals, racing homologation documents
2. Cross-Reference Multiple Sources: One source may have errors; verify critical values from multiple publications
3. Use Engineering Calculations: When exact data isn't available, calculate from known values (e.g., calculate inertia from dimensions)
4. Study Similar Vehicles: If your target vehicle lacks data, find a similar vehicle (same platform, similar weight) with published specs
5. Document Everything: Keep notes on where each value came from and why you chose it

Development Workflow

1. Gather All Required Data: Don't start building until you have at least 80% of required specifications
2. Start with Basic HDV: Get the car driving with basic values - worry about perfection later
3. Validate Against Known Benchmarks: Real-world 0-60 times, braking distances, skidpad G-force, top speed
4. Iterate One System at a Time: Don't change engine, suspension, and tires simultaneously - you won't know what fixed or broke the behavior
5. Test at Multiple Tracks: A car that feels good at one track might expose problems elsewhere
6. Get Driver Feedback: Experienced drivers can identify unrealistic behavior that numbers can't reveal

Validation Techniques

Performance Validation

• Acceleration: 0-60 mph, 0-100 mph, quarter-mile times should match real-world data within 5%
• Braking: 60-0 mph braking distance should match (adjust BrakeTorque to achieve this)
• Cornering: Skidpad lateral G should match published tests (validates tire grip levels)
• Top Speed: Should match real data (validates drag coefficient and engine power)

Dynamic Behavior Validation

• Weight Transfer: Does the car dive/squat under braking/acceleration as expected?
• Balance: Is the handling balance (understeer/oversteer) appropriate for the weight distribution?
• Damping: Does the car settle after hitting bumps in a realistic timeframe?
• Steering Feel: Is the steering weight appropriate? Does it self-center properly?

Telemetry Comparison

For professional development, compare simulation telemetry to real-world data:

• Tire temperatures and pressures throughout a stint
• Brake temperatures and brake bias usage
• Gear shift points and RPM traces
• Suspension travel usage (should use full range without bottoming excessively)
• Fuel consumption rates

Common Mistakes to Avoid

1. Using Crank HP Instead of Wheel HP

Always use wheel horsepower dyno data for engine files. If you use crank HP, you'll need to model driveline friction separately, which is complex and error-prone. Wheel HP data already includes all losses.

2. Incorrect Inertia Calculations

Remember to subtract ride height from vehicle height before calculating rotational inertia. Also account for construction type - a carbon fiber chassis has less inertia than steel for the same dimensions.

3. Misunderstanding Motion Ratios

If the spring isn't directly on the spindle, you must calculate and apply the motion ratio with SpringMult/DamperMult. Ignoring this makes the suspension too soft or too stiff.

4. Unrealistic Bump Stop Settings

Bump stops should be progressive and firm but not infinitely stiff. If BumpStopRisingSpring is too high, the car will violently bounce when bottoming. If too low, the car will bottom harshly.

5. Copying Suspension Geometry

Every vehicle's suspension geometry is unique. Copying another car's UltraChassis file and adjusting only the track width will create unrealistic camber curves and roll center behavior. Always measure the actual vehicle's geometry.

6. Ignoring Thermal Models

Engine oil temperature, brake temperature, and tire temperature all affect performance. A car with poor cooling setup should overheat and lose performance - this adds realism and strategic depth.

7. Overly Simplified Testing

Testing only at one track or in one car configuration won't expose problems. Test with different fuel loads, different tracks (smooth and bumpy), different weather conditions, and different setup extremes.

Advanced Development Topics

For teams pursuing the highest level of accuracy:

Aerodynamic Mapping

Beyond basic drag coefficient, map downforce and drag at different ride heights, pitch angles, and speeds. This requires either CFD data or wind tunnel testing from the real vehicle.

Tire Model Correlation

Work with tire development specialists to ensure tire behavior matches real-world data. The interaction between vehicle dynamics and tire physics is critical for realism.

Suspension Compliance

While rFactor 2 doesn't model rubber bushings, you can approximate compliance effects by slightly adjusting anti-squat/dive characteristics through geometry to match real vehicle behavior under acceleration and braking.