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The Physics of Feel: LMUFFB Driver's Guide

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

This guide explains how LMUFFB translates raw telemetry data into tactile sensations. It details what you should feel in specific driving situations and provides a blueprint for visualizing these relationships through telemetry graphs.

Part 1: Individual Driving Phenomena

1. Understeer (Front Grip Loss)

Plot A (FFB Output): Final FFB Signal showing a drop in amplitude despite increased steering. Plot B (Input): Steering Angle increasing. Plot C (Physics): Front Tire Grip Fraction dropping from 1.0 to < 0.8. Plot D (Physics): Lateral G-Force plateauing or dropping.

Additional understeer telemetry visualization showing the relationship between steering input, grip fraction, and FFB output.

The Situation: You turn the wheel, but the car continues straight. The front tires have exceeded their slip angle limit and are scrubbing across the asphalt.

The FFB Sensation: As you turn the wheel further, instead of the resistance increasing (as it would with a spring), the wheel suddenly goes light. The weight falls out of the steering, signaling that adding more steering angle is futile.

How it Works: LMUFFB monitors the GripFraction of the front tires. As this value drops below 1.0 (100%), the application reduces the global steering force.


2. Oversteer (Rear Grip Loss / SoP)

Plot A (FFB Output): SoP/Oversteer Force Component spiking. Plot B (Physics): Lateral G-Force high amplitude. Plot C (Physics): Grip Delta (Front Grip minus Rear Grip). Plot D (Input): Steering Angle showing the driver counter-steering in response.

Additional oversteer telemetry visualization showing SoP effect and rear grip loss dynamics.

The Situation: The rear tires lose grip, and the back of the car rotates (yaws) faster than the front. The car is sliding sideways.

The FFB Sensation: You feel a distinct pull in the direction of the slide, urging you to counter-steer. Simultaneously, you feel the "weight" of the car shifting sideways through the rim, giving you an early warning before your eyes even detect the rotation.

How it Works: This is the "Seat of Pants" (SoP) effect. LMUFFB injects Lateral G-force (mLocalAccel.x) into the steering signal. Additionally, it calculates a synthetic "Aligning Torque" based on the rear axle's lateral forces, boosting the signal when the rear grip drops below the front grip.


3. Braking Lockup (Threshold Braking)

Plot A (FFB Output): Lockup Rumble Signal showing frequency change. Plot B (Input): Brake Pedal Position at 100%. Plot C (Physics): Wheel Slip Ratio dropping below -0.1. Plot D (Physics): Car Speed decreasing, correlating with the changing frequency in Plot A.

Additional brake lockup telemetry visualization showing the relationship between speed, slip ratio, and vibration frequency.

The Situation: You stomp on the brakes. One or more tires stop rotating while the car is still moving. The rubber is dragging along the road surface.

The FFB Sensation: You feel a vibration that changes pitch based on your speed.

  • High Speed: A high-frequency "screeching" buzz (approx 60-80Hz).
  • Low Speed: A low-frequency "judder" or "grinding" (approx 10-20Hz).
  • Note: The vibration is stronger when the tire is heavily loaded (downforce/weight transfer) and fades if the tire is unloaded.

How it Works: LMUFFB detects when SlipRatio is less than -0.1. It generates a sine wave where the Frequency is linked to Car Speed (mLocalVel.z) and the Amplitude is linked to Vertical Tire Load (mTireLoad).


4. Traction Loss (Power Wheel Spin)

Plot A (FFB Output): Total Force showing a sudden drop/notch plus Vibration overlay. Plot B (Input): Throttle Position at 100%. Plot C (Physics): Rear Wheel Slip Ratio spiking > 0.2. Plot D (Physics): Slip Speed (m/s) correlating with vibration pitch.

Additional traction loss telemetry visualization showing torque drop and vibration dynamics during wheel spin.

The Situation: You apply full throttle in a low gear. The rear tires break traction and spin significantly faster than the road speed.

The FFB Sensation: The steering wheel feels vague and floating, as if the rear of the car has detached from the road. Overlaid on this lightness is a smooth, high-frequency hum that rises in pitch as the wheels spin up.

How it Works: LMUFFB detects positive SlipRatio.

  1. Torque Drop: It multiplies the total force by a reduction factor (e.g., 0.6x), creating the "floating" sensation.
  2. Vibration: It generates a vibration based on Slip Speed (the difference in m/s between tire surface and road).

5. Slide Texture (Lateral Scrubbing)

Plot A (FFB Output): Slide Texture Signal with Sawtooth waveform. Plot B (Physics): Lateral Patch Velocity (m/s). Plot C (Physics): Tire Slip Angle (rad). Plot D (Physics): Vertical Tire Load modulating the amplitude.

Additional slide texture telemetry visualization showing lateral scrubbing dynamics and tire load effects.

The Situation: You are cornering hard. The car isn't spinning, but the tires are operating at a high slip angle, "crabbing" or scrubbing sideways across the asphalt.

The FFB Sensation: A granular, sandpaper-like texture through the rim. It feels like the tire is "tearing" at the road surface.

How it Works: When LateralPatchVel (the speed at which the contact patch slides sideways) is high, LMUFFB injects a Sawtooth wave. The sawtooth shape mimics the "stick-slip" physics of rubber friction better than a smooth sine wave.


Part 2: Complex Interactions & Dynamics

This section details how LMUFFB handles conflicting signals to create a cohesive, natural driving feel.

1. The Power Slide (Spin vs. SoP)

Plot A (FFB Output): Total Force showing vector direction (SoP) but chopped amplitude (Spin). Plot B (Physics): Lateral G High, driving the SoP. Plot C (Physics): Rear Slip Ratio High, driving the Torque Drop. Plot D (Physics): Yaw Rate showing the car rotation.

Additional power slide telemetry visualization showing the interaction between Spin effect and SoP effect.

The Scenario: Exiting a corner, you mash the throttle. The rear end steps out violently (Oversteer), but the wheels are also spinning wildly (Traction Loss).

The Interaction:

  • The Conflict:
    • SoP Effect wants to increase force to tell you the car is rotating and urge a counter-steer.
    • Spin Effect wants to decrease force to simulate the loss of rear friction and the "floating" rear axle.
  • The Result: A "Light Counter-Steer". The wheel pulls in the direction of the correction (SoP), but the resistance is lower than normal (Spin Drop).
  • Why it feels natural: This mimics reality. When rear tires are spinning, they have very little lateral grip. The steering should guide you into the slide, but it shouldn't feel heavy or "locked in" because the rear of the car is effectively floating on a layer of molten rubber.

2. The "Dive" (Load Transfer vs. Understeer)

Plot A (FFB Output): Total Force showing rising peak, then sudden drop-off. Plot B (Physics): Front Tire Load spiking due to weight transfer. Plot C (Physics): Front Grip Fraction dropping as tires saturate. Plot D (Input): Brake Pressure vs Steering Angle.

Additional trail braking telemetry visualization showing the interaction between load transfer and understeer effect.

The Scenario: You brake hard while turning into a corner (Trail Braking). The weight of the car transfers to the front tires.

The Interaction:

  • The Conflict:
    • Load Sensitivity sees massive weight on the front tires (3000N -> 6000N). This increases the amplitude of road textures and mechanical trail.
    • Understeer Effect watches the grip limit. If you brake too hard, you exceed the grip circle, and the effect tries to reduce force.
  • The Result: "Heavy to Light Transition". Initially, the wheel feels incredibly heavy and detailed (due to Load Sensitivity) as the nose dives. As you exceed the limit, the weight suddenly vanishes (Understeer Effect), giving you a clear tactile cliff edge: "You pushed too hard."

3. The "Tank Slapper" (Snap Oversteer Recovery)

Plot A (FFB Output): Total Force showing the violent reversal/spike. Plot B (Physics): Rear Slip Ratio dropping from High to Zero. Plot C (Physics): Lateral G crossing zero rapidly. Plot D (Physics): Steering Arm Force showing the raw game physics aligning torque returning.

Additional tank slapper telemetry visualization showing snap oversteer recovery dynamics.

The Scenario: You are counter-steering to catch a slide. Suddenly, the rear tires regain grip.

The Interaction:

  • The Sequence:
    1. Slide Phase: SoP is high (pushing wheel correction), Spin Drop might be active.
    2. Grip Regain: Rear Slip Ratio drops to 0. Lateral G snaps the other way.
  • The Result: "The Snap".
    • The "Spin Torque Drop" vanishes instantly (Force returns to 100%).
    • The SoP vector reverses direction violently as the car snaps back.
    • The FFB delivers a high-torque "whip" motion, simulating the stored energy in the chassis releasing. This cues the driver to center the wheel immediately to avoid over-correcting into a wall.

6. Dynamic Slope Detection (Advanced Understeer, v0.7.40)

The Situation: Instead of using fixed "optimal slip" numbers (which change based on car and track), the app actively watches the shape of the tire's grip curve.

The FFB Sensation: A much more precise and progressive lightening of the wheel. It correctly communicates the "peak" of grip regardless of whether you are driving a GT3 or a Prototype.

How it Works (v0.7.40):

  1. G-Slope: Monitors the relationship between Lateral G and Slip Angle. When G stops increasing despite more slip, the slope goes negative, and the wheel lightens.
  2. Torque-Slope: Monitors the drop in pneumatic trail (Steering Torque vs. Angle). This provides an even earlier "warning" cue before the car actually starts to wash wide.