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FFB Effects & Customization Guide

This document is not up to date, it is based on an older version of the app.

This document details the Force Feedback effects implemented in LMUFFB, how they are derived from telemetry, and how to customize them.

1. Understeer (Front Grip Loss)

  • Goal: To communicate when the front tires are losing grip and sliding (pushing).
  • Telemetry: Derived from mGripFract (Grip Fraction) of the Front Left (FL) and Front Right (FR) tires.
  • Mechanism: Modulates the main steering force.
    • Output = GameForce * (1.0 - (1.0 - FrontGrip) * UndersteerGain)
    • As front grip drops, the wheel becomes lighter ("goes light"), simulating the loss of pneumatic trail and self-aligning torque.
  • Customization:
    • Understeer Effect (Slider): Controls the intensity of the lightening effect.

2. Oversteer (Rear Grip Loss)

  • Goal: To communicate when the rear tires are losing grip (loose/sliding), allowing the driver to catch a slide early.

  • Current Multi-Effect Implementation (v0.6+): The oversteer system uses multiple distinct effects that work together:

    1. Lateral G (SoP): Injects chassis lateral acceleration into wheel torque, providing "weight" cues during cornering. Calculated in calculate_sop_lateral().

    2. Lateral G Boost (Slide): Amplifies the SoP force when rear grip is lower than front grip (oversteer condition). Formula: SoP *= (1.0 + (FrontGrip - RearGrip) * OversteerBoost * 2.0).

    3. Rear Align Torque (SoP Self-Aligning): Calculates a synthetic aligning torque for the rear axle using slip angle and estimated load. Provides directional counter-steering pull during slides.

    4. Yaw Kick: Sharp momentary impulse at the onset of rotation. Derived from mLocalRotAccel.y with configurable activation threshold (m_yaw_kick_threshold, default 0.2 rad/s²) to filter road noise.

3. Braking Lockup (Progressive Scrub)

  • Goal: To signal when tires have stopped rotating during braking (flat-spotting risk), allowing the driver to find the threshold.
  • Current Dynamic Implementation (v0.2.2+):
    • Progressive Vibration: Signal is derived from SlipRatio deviation.
    • Range: -0.1 (Peak Grip) to -0.5 (Locking).
    • Frequency: Transitions from High Pitch (60Hz) at the limit to Low Pitch (10Hz) at full lock.
    • Amplitude: Scales linearly with severity.
  • Customization:
    • Lockup Rumble (Toggle): Enable/Disable.
    • Lockup Gain (Slider): Intensity of the vibration.

4. Wheel Spin (Traction Loss)

  • Goal: To signal when the driven wheels are spinning under power.
  • Current Dynamic Implementation (v0.2.2+):
    • Torque Reduction: As rear wheel slip increases, the total FFB force is reduced (simulating "floating" rear end).
    • Vibration: Frequency scales with wheel speed difference (Slip Ratio), giving a "revving up" sensation through the rim.
  • Customization:
    • Spin Traction Loss (Toggle): Enable/Disable.
    • Spin Gain (Slider): Intensity.

5. Road & Slide Texture

  • Slide Texture: Adds "scrubbing" vibration when any tire is sliding laterally (high Slip Angle).
  • Road Texture: Adds "bumps" based on suspension velocity changes (High-Pass Filter).

Signal Interference & Clarity

A critical challenge in FFB design is managing the "Noise Floor". When multiple effects are active simultaneously, they can interfere with each other or mask the underlying physics.

1. Signal Masking

  • The Issue: High-frequency vibrations (like Lockup Rumble or Road Texture) can physically overpower subtle torque changes (like Understeer Lightness or SoP). If the wheel is vibrating violently due to a lockup, the driver might miss the feeling of the rear end stepping out (SoP).
  • Mitigation:
    • Priority System (Planned, Not Yet Implemented): A future version will implement "Side-chaining" or "Ducking" to dynamically reduce lower-priority effects when higher-priority signals need headroom. For example, if understeer occurs during curb contact, the system would reduce Road Texture to preserve the grip information signal.
    • Frequency Separation: Ideally, "Information" (Grip/SoP) should be low-frequency (< 20Hz), while "Texture" (Lockup/Spin/Road) should be high-frequency (> 50Hz). This helps the human hand distinguish them.

2. Clipping

  • The Issue: Summing multiple effects (Game Torque + SoP + Rumble) can easily exceed the 100% force capability of the motor.
  • Result: The signal "clips" (flattens at max force). Information is lost. E.g., if you are cornering at 90% torque and a 20% SoP effect is added, you hit 100% and lose the detail of the SoP ramp-up.
  • Mitigation:
    • Master Limiter: A soft-clip algorithm that compresses dynamic range rather than hard-clipping.
    • Tuning: Users are advised to set "Master Gain" such that peak cornering forces hover around 70-80%, leaving headroom for dynamic effects.

3. Ambiguity (Texture Confusion)

  • The Issue: Lockup and Wheel Spin often use similar "Synthetic Rumble" effects. In the heat of battle, a driver might confuse one for the other if relying solely on the tactile cue without context (pedal position).
  • Mitigation:
    • Distinct Frequencies: Future updates will tune Lockup to be "Sharper/Higher Pitch" (square wave) and Wheel Spin to be "Rougher/Lower Pitch" (sawtooth or randomized).
    • Context: Since the driver knows if they are braking or accelerating, this ambiguity is usually resolved by context, but distinct tactile signatures help subconscious reaction times.

4. Interaction of Spin vs SoP Effects

  • The Spin effect reduces total_force (Torque Drop).
  • The SoP effect boosts force during oversteer.
  • Result: These two will fight slightly during a power slide. This is actually a good "natural" balance—the wheel tries to self-align (SoP), but the loss of traction makes it feel lighter/vaguer (Spin Drop). This should feel intuitive to the driver.

5. Robustness & Telemetry Health

LMUFFB includes a "Sanity Check" layer that protects effects against telemetry glitches (common in some game builds).

  • Missing Load: If the game reports 0 Load on tires, texture effects (Slide/Road/Lockup) will use a fallback value instead of going silent.
  • Missing Grip: If Grip data is missing, the Understeer effect defaults to "Full Grip" so you don't lose FFB entirely.
  • Note: If these fallbacks are triggered, a Red Warning will appear in the Telemetry Inspector GUI.

Comparison of Implementation with iRFFB and Marvin's AIRA

Effect iRFFB (iRacing) Marvin's AIRA (iRacing) LMUFFB (LMU/rF2)
Oversteer SoP (Lateral G) + Yaw logic Layered Effect: Separate "Slip" channel. Rear Aligning Torque + SoP: Synthetic rear-axle torque integration.
Lockup Not explicit (part of "Understeer" feel in iRacing logic) Pedal Haptics (often sent to pedals, but can be on wheel) Progressive Wheel Scrub: Dynamic frequency/amplitude based on slip ratio.
Wheel Spin Not explicit Pedal Haptics / Wheel Rumble Torque Drop + Vibration: Simulates traction loss + progressive rumble.

Legacy Implementation Notes (Pre-v0.2.2)

  • Old Oversteer: Relied solely on Grip Delta between Front/Rear to boost SoP.
  • Old Lockup: Binary rumble triggered when SlipRatio < -0.2.
  • Old Wheel Spin: Binary rumble triggered when SlipRatio > 0.2.