docs(vehicle): revert the axis fix; the reverse is in the Chaos solver

The axis_forward:X / Blender-Z axle fix changed the reference pose but not the drive
(-2737 cm/s), and axis_forward:X also alters the shared character preset. Revert it.
The Chaos solver drives the wheel about the vehicle's Y (not the bone), and the
drive force is positive yet the car moves backward - the inversion is in the engine
torque / drive-force sign. Next: GetEngineTorque() sign or a SetupVehicle override.

MEMORY.md

@@ -1105,3 +1105,15 @@ Historical task context from 2026-11-09/10; later vehicle findings above may sup
   velocity axis) or the engine torque sign, not the bone authoring. The horizontal-axle fix
   is kept (it is correct even if insufficient): `axis_forward: X` in the character preset and
   a Blender-Z wheel tail (`r - copysign(0.25, y)`) so the axle lands on Unreal Y.
+
+## 2026-11-10 — Vehicle reverse is in the Chaos solver, not the assets (revert the axis fix)
+
+- Reverted the `axis_forward: X` / Blender-Z axle fix (`ecdd820`): the reference pose changed
+  but `-drivetest` still read −2737 cm/s, and `axis_forward: X` lives in the shared character
+  preset, so it would also alter every character export.
+- Traced the Chaos solver: the wheel's local frame is `GroundXVector = CrossProduct(VehicleRightAxis,
+  GroundZVector)` and the drive force is `FinalLongitudinalForce = DriveTorque / Re`, applied forward —
+  yet the car moves backward. The wheel spins about the **vehicle's** Y, not the bone's, so the bone
+  authoring is irrelevant. The inversion is in the **engine-torque / drive-force sign** (`FSimpleWheel`),
+  not the assets. Next: check `GetEngineTorque()` sign, or override `SetupVehicle`/`ProcessMechanicalSimulation`
+  in `UGigLineVehicleMovement` to negate the drive torque.

Tools/blender/fbx_export_preset.py

@@ -60,7 +60,6 @@ _METRIC = {
 _CHARACTER = dict(
     _METRIC,
     object_types={"ARMATURE", "MESH"},
-    axis_forward="X",
     add_leaf_bones=False,
     path_mode="COPY",
     embed_textures=False,

Tools/blender/vehicle_build.py

@@ -199,10 +199,8 @@ def build_skeletal(joined, wheels, out_fbx, exp_dir):
     for name, (x, y) in wheel_positions.items():
         bone = edit_bones.new(name)
         bone.head = (x, y, r)
-        # Point the bone along the axle in BLENDER Z: the Unreal FBX import maps (x,y,z)->
+        # Point the bone along the axle: the wheel spins about its own Y.
-        # (x,-z,-y), so a tail along Z lands on Unreal Y (horizontal axle). Opposite sides
+        bone.tail = (x, y - math.copysign(0.25, y), r)
-        # get opposite signs (left down, right up) so the axles point outward.
-        bone.tail = (x, y, r - math.copysign(0.25, y))
         bone.parent = root
     bpy.ops.object.mode_set(mode="OBJECT")
 

Tools/unreal/ue_vehicle_import.py

@@ -119,11 +119,6 @@ def main():
         ui.import_animations = False
         ui.import_materials = True
         ui.skeletal_mesh_import_data.import_uniform_scale = 1.0
-        # The FBX is already authored in the Unreal frame (nose +X, axle along Y); the
-        # importer's default axis conversion otherwise rolls the armature 90 deg about X and
-        # the wheel axles land on Z, which drives the Chaos vehicle backward.
-        ui.skeletal_mesh_import_data.import_rotation = unreal.Rotator(0.0, 0.0, 0.0)
-        ui.skeletal_mesh_import_data.import_translation = unreal.Vector(0.0, 0.0, 0.0)
         task.options = ui
         unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
         skel = unreal.EditorAssetLibrary.load_asset(f"{DEST}/SK_Vehicle_Sedan")