physics: fit dyn_bike_si_of to Jolt (tires, direct drive); P8b training now helps on Jolt

docs/roadmap/NAV-VEHICLE-PHYSICS-ROADMAP.md

@@ -96,6 +96,41 @@ The plan, in order:
    trained on the dynamic model beats the point-mass one over paired seeds.
 4. **P8c:** a differentiable power model and energy in the loss, calibrated to the P3 ledger.
 
+### P8a's vehicle fitted to Jolt (2026-10-20)
+
+`nav_plant::dyn_bike_si_of` now gives the dynamic model the values the Jolt plant's one-tick
+residual asks for. Jolt runs of the Humvee and the RoboRacer were fitted on held-out seeds, which
+made the twist residual about 11x lower.
+
+- **Tires.** Jolt's lateral friction ramps linearly to its peak at `tire_peak_slip_angle`: a
+  cornering stiffness of 1 / alpha_peak per unit load. A Pacejka tire peaking at the same angle
+  starts 4.5x steeper. B is now set so that B C = 1.12 / alpha_peak (`kJoltTireStiffness`).
+- **Front and rear.** They are alike now (`front_stiffness` 1, was 0.85), since Jolt's tires have no
+  load sensitivity.
+- **A direct drive** (one gear, no torque converter: the RoboRacer) gets no drive lag and no drag or
+  rolling resistance. The driver model's integral trim tracks the commanded acceleration, so the
+  command is the achieved acceleration.
+- **A geared drive through a converter** (the Humvee) keeps its 0.1 s lag and its resistances.
+
+The P8b gate rerun on Jolt RoboRacers (city, 20 held-out seeds, paired bootstrap; Linux), before
+and after:
+
+| training | dynamic-trained, before / after | dynamic - untrained seed, before | after |
+|---|---|---|---|
+| lr 5e-5, 150 steps | 50.8% / 51.6% | +0.2 [-1.8, +2.2] | +0.9 [-1.0, +3.0] |
+| lr 1e-4, 300 steps | 48.5% / 52.1% | -2.1 [-4.2, +0.1] | +1.5 [-1.4, +4.1] |
+| lr 2e-4, 300 steps | 43.1% / 53.8% | -7.5 [-10.0, -5.3] | +3.1 [+0.7, +5.5] |
+| checkpoints selected on 32 validation seeds | 52.2% / 53.2% | +1.6 [-0.4, +3.6] | +2.6 [+0.7, +4.6] |
+
+- **The point-mass arm and the untrained seed are unchanged:** 51.4%, 49.9%, 22.2%, 52.2% for the
+  point mass, and 50.6% for the seed.
+- **Training through the dynamic model now helps on Jolt** rather than only hurting less:
+  - its validation reach rises steadily over training (0.497 -> 0.526 at step 210), where before it
+    fell to 0.41;
+  - the trained policy beats the untrained one outside the CI at lr 2e-4 and with checkpoint
+    selection.
+- **At the default learning rate the change is within the noise:** +0.7 [-1.4, +2.9].
+
 How the physics channels enter training:
 
 - **Per unit mass.** Each channel is defined per unit mass, for example grade as g·∇z converted with

inc/cvc/physics/nav_plant.h

@@ -121,15 +121,26 @@ class nav_plant final : public cvc::nav::vehicle_plant {
   //   - mass; yaw inertia of the chassis box, m (hx^2 + hy^2) / 3;
   //   - the centre of gravity's distances to the front (steered) and rear axles, and its height
   //     as terrain_config_of has it;
-  //   - Pacejka B and C from the tires' peak slip angle and sliding fraction: sin(C pi / 2) is
-  //     tire_sliding, and the force peaks at tire_peak_slip_angle. vehicle_desc has one tire,
-  //     so `front_stiffness` scales the front's B: a road vehicle's understeer comes from the
-  //     front's load sensitivity and steering compliance, which the single track leaves out;
+  //   - Pacejka C from the tires' sliding fraction (sin(C pi / 2) is tire_sliding) and B fitted to
+  //     how Jolt's tires corner. Jolt's lateral friction ramps linearly to its peak at
+  //     tire_peak_slip_angle, a cornering stiffness of 1 / alpha_peak per unit load, and a Pacejka
+  //     tire peaking at the same angle starts 4.5x steeper (B C = C tan(pi / 2C) / alpha_peak).
+  //     So B C = k / alpha_peak with k = kJoltTireStiffness (1.12), fitted to the one-tick
+  //     residual of Jolt runs of the Humvee and the RoboRacer (libcvc #749's fit_dyn_bike;
+  //     held-out twist residual about 11x lower than with the 4.5x tire). `front_stiffness`
+  //     scales the front's B; Jolt's tires have no load sensitivity or steering compliance, and
+  //     the fit puts front and rear alike (1);
   //   - the drive split from which axles are driven, the brake split from the axles' brake
   //     torques; drag 1/2 rho C_d A and the rolling resistance;
-  //   - the steering's rate; `drive_lag` for the force (nav_plant_options::latency's default).
+  //   - the steering's rate; `drive_lag` for the force, < 0 for the vehicle's own: a direct drive
+  //     (one gear, no torque converter) answers within the tick (0), a geared drive through a
+  //     converter takes 0.1 s (nav_plant_options::latency's default);
+  //   - for a direct drive, no drag or rolling resistance: the driver model's integral trim tracks
+  //     the commanded acceleration and makes up for them, so the commanded acceleration is the
+  //     achieved one (the RoboRacer's fit: lag 0, resistance x0; the Humvee's keeps both).
+  static constexpr double kJoltTireStiffness = 1.12;
   static cvc::nav::dyn_bike_si dyn_bike_si_of(const vehicle_desc &d, double mu = 0.9,
-                                              double front_stiffness = 0.85, double drive_lag = 0.1,
+                                              double front_stiffness = 1.0, double drive_lag = -1.0,
                                               double gravity = 9.80665);
 
   // The same vehicle in a normalized frame where its wheelbase is `wheelbase`: S =

src/cvc/physics/nav_plant.cpp

@@ -314,19 +314,24 @@ cvc::nav::dyn_bike_si nav_plant::dyn_bike_si_of(const vehicle_desc &d, double mu
   si.cg_to_rear = std::max(1e-3, d.center_of_mass.x - rear);
   si.cg_height = std::max(0.0, -z + travel + radius + d.center_of_mass.z);
   si.mu = mu;
-  // Pacejka: sin(C pi / 2) = tire_sliding, C in (1, 2); the peak at B alpha = tan(pi / (2 C)).
+  // Pacejka: sin(C pi / 2) = tire_sliding, C in (1, 2). B makes the cornering stiffness per unit
+  // load, B C, kJoltTireStiffness times Jolt's: its lateral friction ramps linearly to 1 at
+  // tire_peak_slip_angle (jolt_backend.cpp).
   const double kPi = 3.14159265358979323846;
   const double sliding = std::min(std::max(d.tire_sliding, 0.05), 0.999);
   si.tire_C = 2.0 - 2.0 * std::asin(sliding) / kPi;
-  const double B = std::tan(kPi / (2.0 * si.tire_C)) / std::max(d.tire_peak_slip_angle, 1e-3);
+  const double B = kJoltTireStiffness / (si.tire_C * std::max(d.tire_peak_slip_angle, 1e-3));
   si.tire_B_front = B * front_stiffness;
   si.tire_B_rear = B;
   si.front_drive = front_driven && rear_driven ? 0.5 : (front_driven ? 1.0 : 0.0);
   si.front_brake = brake_front + brake_rear > 0.0 ? brake_front / (brake_front + brake_rear) : 0.5;
-  si.drag = 0.5 * d.air_density * d.drag_coefficient * d.frontal_area;
-  si.rolling_resistance = d.rolling_resistance;
+  // A direct drive: one gear, no torque converter. The driver model's trim makes its commanded
+  // acceleration the achieved one within the tick, drag and rolling resistance included.
+  const bool direct = d.gear_ratios.size() <= 1 && !(d.converter_k > 0.0);
+  si.drag = direct ? 0.0 : 0.5 * d.air_density * d.drag_coefficient * d.frontal_area;
+  si.rolling_resistance = direct ? 0.0 : d.rolling_resistance;
   si.steer_rate = d.max_steer_rate;
-  si.drive_lag = drive_lag;
+  si.drive_lag = drive_lag >= 0.0 ? drive_lag : (direct ? 0.0 : 0.1);
   si.gravity = gravity;
   return si;
 }

src/cvc/tests/data/dyn_bike_golden.json

@@ -1,43 +1,43 @@
 {
  "format": "cvc-dyn-bike-golden-1",
  "order": ["x", "y", "psi", "vx", "vy", "r", "delta", "a"],
- "params": {"m": 2676, "Iz": 6283.69384765625, "lf": 1.5369999408721924, "lr": 1.7630000114440918, "h": 0.74000000953674316, "g": 9.8066501617431641, "mu": 0.89999997615814209, "B_f": 36.121696472167969, "B_r": 42.496116638183594, "C": 1.2021652460098267, "front_drive": 0.5, "front_brake": 0.70588237047195435, "drag": 1.5006250143051147, "c_rr": 0.014999999664723873, "tau_steer": 0.05000000074505806, "steer_rate": 0.34999999403953552, "tau_drive": 0.10000000149011612, "v_blend": 1, "v_width": 0.30000001192092896, "eps": 0.5, "fz_min": 0.05000000074505806, "iters": 3},
+ "params": {"m": 2676, "Iz": 6283.69384765625, "lf": 1.5369999408721924, "lr": 1.7630000114440918, "h": 0.74000000953674316, "g": 9.8066501617431641, "mu": 0.89999997615814209, "B_f": 10.708646774291992, "B_r": 10.708646774291992, "C": 1.2021652460098267, "front_drive": 0.5, "front_brake": 0.70588237047195435, "drag": 1.5006250143051147, "c_rr": 0.014999999664723873, "tau_steer": 0.05000000074505806, "steer_rate": 0.34999999403953552, "tau_drive": 0.10000000149011612, "v_blend": 1, "v_width": 0.30000001192092896, "eps": 0.5, "fz_min": 0.05000000074505806, "iters": 3},
  "g_out": [0.69999999999999996, -0.29999999999999999, 1.1000000000000001, 0.40000000000000002, -0.90000000000000002, 0.25, 0.59999999999999998, -0.14999999999999999],
  "cases": [
   {"state": [0, 0, 0, 12, 0.10000000000000001, 0.050000000000000003, 0.02, 0.5], "delta_cmd": 0.050000000000000003, "a_cmd": 1, "grade": 0,
    "limits": {"L": 3.2999999523162842, "vmax": 40, "hdt": 0.059999998658895493, "allow_reverse": 0},
-   "next": [0.72161232476023529, 0.0030915364619978677, 0.006158445274966143, 12.026961596240618, 0.15841392030776888, 0.10264075687696875, 0.035978411112368727, 0.72559417581953878],
-   "g_state": [0.69999999999999996, -0.29999999999999999, 0.881352227048531, 0.43908416096250991, -0.30332956174135622, 0.11778155321430109, -1.2584661505628023, -0.067429376024266241],
-   "g_delta_cmd": -0.52467945254116677, "g_a_cmd": -0.05543495498191852},
+   "next": [0.72170075978362558, 0.0014655898551522932, 0.0048612338851567365, 12.028322883477824, 0.10879309514548365, 0.08102056656356306, 0.035978411112368727, 0.72559417581953878],
+   "g_state": [0.69999999999999996, -0.29999999999999999, 0.88246385916630588, 0.44344941191751436, -0.58216522580427366, 0.44787693050917154, -0.31518187358286542, -0.067550279387271248],
+   "g_delta_cmd": -0.13140556288176811, "g_a_cmd": -0.055534351904751283},
   {"state": [1, -2, 0.29999999999999999, 15, -1.2, 0.59999999999999998, 0.25, 0], "delta_cmd": 0.29999999999999999, "a_cmd": 0, "grade": 0.02,
    "limits": {"L": 3.2999999523162842, "vmax": 40, "hdt": 0.059999998658895493, "allow_reverse": 0},
-   "next": [1.8862789801906317, -1.8347158945971369, 0.33547355793444861, 14.854786313864151, -1.2194566888495753, 0.59122597878907412, 0.26954533644854789, 0],
-   "g_state": [0.69999999999999996, -0.29999999999999999, 0.71841743216080634, 0.46493637589482706, -0.92038287845088973, 1.1426390353048035, 0.4563570614107727, -0.077760090566516074],
-   "g_delta_cmd": 0.04704770737140862, "g_a_cmd": -0.063928029207825282},
+   "next": [1.8867829073011033, -1.8363951317071947, 0.3362258629537182, 14.851775313286971, -1.2377173130869106, 0.60376439605715648, 0.26954533644854789, 0],
+   "g_state": [0.69999999999999996, -0.29999999999999999, 0.71944172000470541, 0.4699842462121625, -0.88320019268164862, 1.0507783479334352, 0.44490310874960692, -0.076361021377209612],
+   "g_delta_cmd": 0.045866872760495392, "g_a_cmd": -0.062777828181229955},
   {"state": [-3, 4, -1.1000000000000001, 14, 0.80000000000000004, 0.40000000000000002, 0.14999999999999999, -4], "delta_cmd": 0.14999999999999999, "a_cmd": -6, "grade": -0.029999999999999999,
    "limits": {"L": 3.2999999523162842, "vmax": 40, "hdt": 0.059999998658895493, "allow_reverse": 0},
-   "next": [-2.6139870110986414, 3.2746823456478453, -1.070979653140653, 13.693204346336563, 0.7055992128641444, 0.48367245846670809, 0.14999999999999999, -4.9023767032781551],
-   "g_state": [0.69999999999999996, -0.29999999999999999, 1.4919184613761005, 0.44997106717466218, -0.51421353207102205, 0.22798404545675632, 0.037739398844601396, -0.076678056992082619],
-   "g_delta_cmd": 0.087559812311952701, "g_a_cmd": -0.063038469107698336},
+   "next": [-2.6188025127373895, 3.2714360882174414, -1.0723766257472249, 13.702551861063213, 0.58732269798076042, 0.4603895811700956, 0.14999999999999999, -4.9023767032781551],
+   "g_state": [0.69999999999999996, -0.29999999999999999, 1.495635492069008, 0.45176658307929768, -0.64916167547811943, 0.61889519194170228, -0.0099817306380819981, -0.073112649574912106],
+   "g_delta_cmd": -0.023158780690115677, "g_a_cmd": -0.060107280784200925},
   {"state": [2, 2, 2.5, 10, 2.5, 1.1000000000000001, 0.40000000000000002, 1], "delta_cmd": 0.45000000000000001, "a_cmd": 2, "grade": 0,
    "limits": {"L": 3.2999999523162842, "vmax": 40, "hdt": 0.059999998658895493, "allow_reverse": 0},
-   "next": [1.4901741447892105, 2.3297160167472559, 2.5727797258014662, 10.119081136118659, 2.0853344678711072, 1.2129954571369974, 0.41954533644854791, 1.4511883516390776],
-   "g_state": [0.69999999999999996, -0.29999999999999999, 1.0221465448401579, 0.3680655773155822, -0.4105335944971768, -0.071966286436913274, 0.0064365975211407855, -0.068908097463021359],
-   "g_delta_cmd": 0.00066357504298502484, "g_a_cmd": -0.056650639617034933},
+   "next": [1.4909879252982987, 2.3339382319443089, 2.5710190927434646, 10.145752585238649, 1.9848686819797339, 1.1836515721810814, 0.41954533644854791, 1.4511883516390776],
+   "g_state": [0.69999999999999996, -0.29999999999999999, 1.0189468600494942, 0.36877586997421102, -0.5469687755509326, 0.34522820509560315, -0.2583148462334196, -0.07068503726719233],
+   "g_delta_cmd": -0.026630729143777142, "g_a_cmd": -0.058111495164835755},
   {"state": [0.5, 0.5, 0.20000000000000001, -2, 0.050000000000000003, -0.10000000000000001, 0.20000000000000001, -0.5], "delta_cmd": 0.25, "a_cmd": -0.80000000000000004, "grade": 0,
    "limits": {"L": 3.2999999523162842, "vmax": 40, "hdt": 0.059999998658895493, "allow_reverse": 1},
-   "next": [0.38234157507615946, 0.47820894213237564, 0.19215389327768181, -1.9942410241949737, -0.21255053807775098, -0.13076844829487239, 0.2195453364485479, -0.63535650549172329],
-   "g_state": [0.69999999999999996, -0.29999999999999999, 1.1505512679844894, 0.37186240076506216, -0.12784613682710022, 0.017428116952459605, 1.2345138787580188, -0.07080295209586783],
-   "g_delta_cmd": 0.12727106168621422, "g_a_cmd": -0.058208435157532337},
+   "next": [0.38163390582407375, 0.4801677888170704, 0.19227964555052293, -1.9995770449242993, -0.17430505462442358, -0.12867257703400767, 0.2195453364485479, -0.63535650549172329],
+   "g_state": [0.69999999999999996, -0.29999999999999999, 1.1493923760808287, 0.39264945863371031, -0.21968909271960391, 0.10685582785702351, 1.1849225895596516, -0.070144184766086387],
+   "g_delta_cmd": 0.12215849378781682, "g_a_cmd": -0.057666850177465923},
   {"state": [0, 0, 0, 0.59999999999999998, 0.02, 0.050000000000000003, 0.29999999999999999, 0.29999999999999999], "delta_cmd": 0.34999999999999998, "a_cmd": 0.5, "grade": 0,
    "limits": {"L": 3.2999999523162842, "vmax": 40, "hdt": 0.059999998658895493, "allow_reverse": 0},
-   "next": [0.035991441415742745, 8.9957242432581056e-05, 0.003613523021362957, 0.5998588717055453, 0.10550903842574866, 0.060225385035524874, 0.31954533644854788, 0.39023767032781553],
-   "g_state": [0.69999999999999996, -0.29999999999999999, 1.0891395975055744, 0.29781652332996111, 0.050398345344285118, 0.081666464100695529, 0.26493307713883441, -0.072170287614980372],
-   "g_delta_cmd": 0.027313029511808661, "g_a_cmd": -0.059332547338548272},
+   "next": [0.036071137283442252, -1.6430282470171074e-05, 0.0036051455458450143, 0.60118074078138972, 0.10349000305763768, 0.060085760440438306, 0.31954533644854788, 0.39023767032781553],
+   "g_state": [0.69999999999999996, -0.29999999999999999, 1.0891901600126965, 0.30814654633137256, 0.022983162019829553, 0.083479081063051172, 0.27490937611823435, -0.071961486030844291],
+   "g_delta_cmd": 0.028341526788878316, "g_a_cmd": -0.059160887639182654},
   {"state": [0, 0, 0, 0, 0, 0, 0, 0], "delta_cmd": 0.10000000000000001, "a_cmd": 2, "grade": 0,
    "limits": {"L": 3.2999999523162842, "vmax": 40, "hdt": 0.059999998658895493, "allow_reverse": 0},
-   "next": [0.0032481983257981982, 5.3123536200254369e-08, 2.062374045329597e-05, 0.054136639980098399, 0.0006057631500759013, 0.00034372901523787501, 0.020946006673345115, 0.90237670327815533],
-   "g_state": [0.69999999999999996, -0.29999999999999999, 1.0990255033157852, 0.42620268045264031, 0.0018837905264509067, 0.0068176134156984324, 0.57682703919426437, -0.068034399515581628],
-   "g_delta_cmd": 0.0020775379242710104, "g_a_cmd": -0.055932356144165696}
+   "next": [0.0032481985805457771, 1.4550464025321322e-08, 2.063503148721843e-05, 0.054136644212622924, 0.00060545142240215992, 0.00034391719914078894, 0.020946006673345115, 0.90237670327815533],
+   "g_state": [0.69999999999999996, -0.29999999999999999, 1.0990255302405116, 0.42627209229010737, -0.0055533005664103159, 0.0088728813432996916, 0.57684392487435543, -0.068034179542302314],
+   "g_delta_cmd": 0.0020775987408388598, "g_a_cmd": -0.055932175300006146}
  ]
 }