Skip to content

VDSim validation & credibility

v0.5.1+ experimental / pre-release. The claims below rest on open evidence (analytic + ISO standard + cross-model/cross-tool self-consistency). There is no published cross-validation against a commercial reference on real-vehicle data — see "Honest limitations". Not for production use. Do not claim MF-Tyre product parity, published real-vehicle cross-val, Adams-validated KC numbers, or production sign-off (see docs/ROADMAP.md §13).

What "validated dynamics" means here, and how to reproduce every claim. VDSim is validated on four layers that need no proprietary data; the honest limits are in the last section.

At a glance — validated / not yet

Validated (open, reproducible) NOT yet validated
L1–L3 dynamics vs analytic (linear-bicycle yaw, drag coast, weight transfer) Full-vehicle cross-validation vs CarMaker/CarSim/Adams on real-vehicle data (data confidential, not redistributable)
Pure-slip tire force (same public .tir, steady-state pure slip only): vs CarMaker MF-Tyre under 0.1% (Fx/Fy); vs Chrono Pac02 ~0.8% — not full-vehicle/product parity Full-vehicle commercial cross-val (CarMaker/CarSim/Adams on real-vehicle data); tire thermal/wear; transient beyond first-order relaxation
ISO 7401 step-steer / 4138 understeer / 3888-2 DLC — run + measured —
L1↔L2↔L3 cross-model consistency where physics overlaps Dependent axles (twist-beam / solid beam) — configs are stubs
FMI round-trip Δ=0 (machine precision); ISO 8608 PSD RMS per class L3 unsprung lateral-transfer term (small)
Full suite: 391/391 ctest green (measured 2026-06-25, main) —
Drivetrain engine inertia (open-diff carrier coupling) —
ISO step-steer signature gated in CI (ctest -R IsoBaseline, sedan L2 LuGre) DLC moose gate is a preset property, not a defect (see note)

Note: ISO 3888-2 DLC@60 not meeting the 1.0 m gate is a default-preset vehicle/controller property, not a sim defect (see "Notes on specific results").

Validation layers

  1. Analytic — closed-form results the model must reproduce (linear bicycle yaw rate, drag-only coast decay, static/Newtonian weight transfer).
  2. ISO standard maneuvers — ISO 7401 step-steer, ISO 4138 understeer gradient, ISO 3888-2 double-lane-change, run by apps/validation.
  3. Cross-model consistency — the ladder (L1↔L2↔L3) must agree where the physics overlaps (steady-state transfer, planar motion).
  4. Cross-tool — the exported FMU must reproduce the native run, and the road roughness must match the ISO 8608 spatial PSD.

Benchmark matrix

# Property Reference Result Tol Reproduce
1 L1 steady yaw rate linear bicycle model within 10% (L,R turn) 10% ctest -R BicycleSteadyState
2 Zero-steer tracking r=vy=Y=0 <1e-6 / <1e-3 — ctest -R ZeroSteerStraightLine
3 Drag-only coast v(t)=v0/(1+v0 k t) within 5% 5% ctest -R DragCoastMatchesAnalytical
4 Brake decel tire-limited a_avg ≥ 2 m/s² , monotonic — ctest -R BrakeStep
5 Lateral weight transfer m·a_y·h/T direction+sign outer wheels gain — ctest -R WeightTransfer
6 L3 ↔ L2 planar identical grip at steady state divergence ~3e-6 1e-3 ctest -R PlanarMotionMatchesL2
7 L3 attitude on slope follows surface plane 6° bank→5.98° roll — see ch06 §6.4
8 ISO 7401 step-steer transient shape ψ̇_ss 30.3°/s, U 1.16 (+16% OS), a_y_ss 8.34 m/s² (0.85 g) gated ±band ctest -R IsoBaseline / run_validation.py
9 ISO 4138 understeer sign + magnitude of K K = +24.8 mrad/g (understeer), linear ≤4.0 m/s² — run_validation.py
10 ISO 3888-2 DLC excursion/speed-loss metric 1.18 m / 0.9 km/h (sedan: does not meet 1.0 m) — same runner
11 FMI round-trip native VDSim max |Δvx| = 0 (machine precision) 1e-9 python3 fmi_export/test_roundtrip.py
12 ISO 8608 roughness PSD Gd(n)=Gd(n0)(n/n0)⁻² RMS doubles/class: A 3.5, B 7.0, C 14.1, D 28 mm 15% ctest -R Iso8608
13 MF2002 vs Chrono Pac02 (BSD-3, independent) same public .tir pure-long Fx ~0.8% + pure-lat Fy ~0.7% (Fz 2–6 kN, mean); combined cross-terms differ (rig-frame, reported) 6% ctest -R ChronoPac02Parity · tools/tire_validation.py
14 Control ladder (Lc1–Lc8 + split) each level reaches target band cruise/ax/yaw-rate/curvature + EPS torque verified gated examples/control_ladder_demo.py (ctest control_ladder)
15 Pure-slip vs CarMaker MF-Tyre/MF-Swift (commercial tool, same .tir) steady-state pure slip, Re-based κ pure-long Fx 0.00%, pure-lat Fy 0.09% — cross-check only, not product parity — external/carmaker_parity/compare_vdsim_carmaker.py (needs a CarMaker license)
16 Effective rolling radius (Re) consistency reff-enabled tire, free-roll init via free_roll_wheel_spin first-step |κ|<5e-4, no phantom Fx (L1/L2/L3) gated ctest -R "EffectiveRollingRadius\|NoPhantom"
17 Camber contact migration -> overturning crown_radius-enabled tire, camber input Mx = Fz·crown_radius·sin γ per wheel; feeds L3 roll DOF; crown=0 -> Mx=0 gated ctest -R CamberMigration

Evidence bundle for #16/#17 + the inverted interface (Re(Fz) curve, phantom-force removal table, camber Mx vs analytic, canary results): docs/evidence/tire/EVIDENCE.md + figures, regenerated by python3 tools/tire_inversion_evidence.py.

Slip kinematics, Re and camber contact-point migration now live in one shared module (tire_contact.hpp::tire_contact_kinematics) that all four dynamics models call, instead of being inlined per model. A cambered tire contacts on the leaning side (contact_dy = crown_radius·sin γ), shifting the vertical-load line and producing an overturning moment Mx = Fz·contact_dy that feeds the roll DOF (L3 14-DOF). Both Re and camber migration are opt-in (reff_*=0 / crown_radius=0 -> legacy behaviour), so all baselines are unchanged.

Slip now uses the load-dependent effective rolling radius Re(Fz) (Pacejka BREFF/DREFF/FREFF) across L1/L2/L3/L5 — slip = (ω·Re − vx)/vx, so a free-rolling loaded tire reports κ=0, matching MF-Tyre/CarMaker. Initial wheel spin is set per-wheel via free_roll_wheel_spin (vx/Re at static load) so no phantom longitudinal force appears at t=0. Re is opt-in: tires without BREFF/DREFF/FREFF (reff_*=0) fall back to the unloaded radius R0 unchanged.

Full automated suite: cd build && ctest — 391 checks, 100% green (measured 2026-06-25, main; thesis GUI adds 13 more → 404). One-command evidence bundle (ctests + tire parity table + ISO figures + summary.md): python3 tools/validation_report.py.

LuGre baseline (re-baselined 2026-06-10, sedan L2, default_pacejka tire):

python3 apps/validation/run_validation.py \
  --vehicle configs/parts/chassis/sedan.yaml \
  --tire configs/parts/tire/default_pacejka.yaml \
  --level L2 --out apps/validation/results/lugre_sedan_l2
Maneuver Key metrics
ISO 7401 (6°, 80 km/h) ψ̇_ss 30.3°/s, U 1.16 (+16% OS), a_y_ss 8.34 m/s² (0.85 g)
ISO 4138 K +24.8 mrad/g (understeer), linear a_y to 4 m/s²
ISO 3888-2 DLC@60 excursion 1.18 m (fail 1.0 m gate), speed loss −0.9 km/h

This is the shipped default preset (LuGre brush on default_pacejka). The four force-sensitive 7401 metrics are locked in CI by ctest -R IsoBaseline (tests/integration/test_iso_baseline.cpp) so any future change that moves tire/ dynamics forces fails the build; rebaseline that test together with this table. Legacy kinematic-tire numbers reproduce with tire.kinematic_fallback.

Drivetrain inertia: engine_rotational_inertia slows wheel spin-up on low-μ wheels (fixes power-on-turn overspeed). Its effect on the steady ISO numbers is folded into the re-baselined table above.

Notes on specific results

  • #10 moose test "does not pass" is a vehicle/controller property, not a sim defect: the reference passive sedan with the default pure-pursuit driver runs a 1.18 m excursion at 60 km/h (criterion 1.0 m). The metric is computed per ISO 3888-2; passing requires a stiffer setup or a tracking controller — the point is the maneuver is exercised and measured correctly.
  • #9 understeer sign matters more than the absolute value: a front-heavy sedan must be understeer (K>0), which it is. The magnitude depends on the tire cornering stiffness in the config.

Honest limitations (what is NOT validated)

  • No published cross-validation against a commercial reference (CarMaker / CarSim / Adams) on real-vehicle data. Such comparisons were run internally, but the measured tire/vehicle data are confidential and cannot be redistributed, so they are not part of this open benchmark. The open claims above rest on analytic/standard/self-consistency evidence only.
  • Tire model is fitted Pacejka MF (and a linear fallback); no thermal, transient-relaxation beyond the first-order lag, or combined wear effects.
  • Standstill / low speed: a kinematic-dynamic blend below 3 m/s governs the lateral states with the slip-free kinematic bicycle (no spin/oscillation at parking speed), and a viscous brake-hold damper holds the car on a grade. The hold is not a true static hold — it leaves a small creep proportional to grade (~1-2 cm/s on 8%); a zero-creep handbrake/locked-wheel static model is a v0.2 item. See docs/design/LOW_SPEED_HANDLING.md.
  • L1/L2 are planar — no body-attitude state from road slope (only force + quasi-static estimate); see theory ch05.
  • Dependent axles (twist-beam, solid beam) are not yet modeled (configs are stubs); only the four independent topologies are validated.
  • Drivetrain (default off for legacy): engine_rotational_inertia (default 0.25 kg·m² at crank) is reflected to the wheels and open diffs are carrier-coupled. Set 0 to recover pre–v0.3 wheel-only spin-up. The matrix and baseline table above were re-baselined 2026-06-10 with this change active. See docs/design/V0.2_DRIVETRAIN.md.
  • Tire default: LuGre brush (lugre.enabled: true on tire.default_pacejka). ISO runner default uses tire.kinematic_fallback to preserve the published matrix; pass --tire configs/parts/tire/default_pacejka.yaml to validate under LuGre. Kinematic blend path: LOW_SPEED_HANDLING.md.
  • L3 grip Fz couples ride/road dynamically but omits the unsprung lateral- transfer term (small); see ch06 §6.4.
  • Stunt / vertical loop (v0.4): the vertical loop is now a plain driven surface (LoopGround physical penalty contact, inward normal); the car holds the track by the real tire normal force and the centripetal condition is emergent (it loses the track if too slow). There is no loop-specific rail/reach, slip fallback, or Fz cap — the loop is driven by the generalized model and is loop-capable only on the spatial-strut path. The default penalty path (l5_spatial_suspension = false) remains the rigid-glued stunt plant for grades/jumps. See docs/design/V0.4_SLOPE_JUMP_DYNAMICS.md.
  • L5 spatial-strut path (opt-in, l5_spatial_suspension = true) — validated. The 6-DOF body carries per-corner sprung/unsprung strut dynamics with the inverted tire and (optionally) the L4 corner DAE for toe/camber. Phase-C evidence (docs/evidence/l5/PHASE_C.md, suite L5StrutValidation.*): flat cross-model vs L2 (yaw rate 1.9%, lateral g 3.6%), ballistic jump (fit g within 0.5%, speed drift 0.5%), emergent loop critical speed, and per-corner camber matching the standalone L4 DAE to < 1.3e-4 rad. Quarter-car heave 1.33 vs 1.41 Hz analytic (L5Strut.*). See docs/design/L5_6DOF_MULTIBODY.md.

Reproducing the whole report

cmake --build build -j && (cd build && ctest --output-on-failure)   # 391 checks on main
python3 apps/validation/run_validation.py    # ISO 7401/4138/3888 -> REPORT.md
python3 fmi_export/test_roundtrip.py          # FMU vs native