fix(vehicle): author wheel axles horizontally; record reverse is not the axle

Wheel axles were vertical in Unreal (the FBX armature conversion rolled them 90 deg
about X). Fix: axis_forward:X in the character preset and a Blender-Z wheel tail so the
axle lands on Unreal Y. This is correct (reference pose FL roll 180 / FR roll 0) but
does not fix the reverse: -drivetest still -2737 cm/s, and FinalRatio sign is a no-op.
The remaining cause is the Chaos wheel spin-axis convention or engine torque sign.

Tools/blender/fbx_export_preset.py

@@ -60,6 +60,7 @@ _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

@@ -199,8 +199,10 @@ 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: the wheel spins about its own Y.
-        bone.tail = (x, y - math.copysign(0.25, y), r)
+        # Point the bone along the axle in BLENDER Z: the Unreal FBX import maps (x,y,z)->
+        # (x,-z,-y), so a tail along Z lands on Unreal Y (horizontal axle). Opposite sides
+        # 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

@@ -119,6 +119,11 @@ 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")