Two root causes, both in how the chassis box was authored:
1. The box centre was -Extent.Z*0.25, below the root bone, so the box sat under the
ground and the suspension pushed the whole vehicle up. The skeletal mesh imports
ROLLED (height along Y, width along Z) while the armature is unrolled, so the box is
now built with swapped axes (length from mesh X, width from mesh Z, height from mesh
Y) and centred at the rolled mesh's mid-height (Bounds.Origin.Y). The vehicle now
settles at Z 6 cm instead of floating at 33 m.
2. A left-over diagnostic stopped the movement component at 2.5 s whenever the pawn
ticked, which cut the drive test (throttle runs 1-4 s). It is now gated on the vehicle
actually having zero wheels.
Remaining: the drive test still reports ~0 forward speed, so the throttle does not yet
move the car - the wheels rest ~2 cm above the ground because the suspension rest
position is measured against the root bone while the box sits at the centre of mass.
The physics asset body was missing PhysicsType. BuildPhysicsAsset created the
USkeletalBodySetup with a box but never set Body->PhysicsType, leaving it
PhysType_Default ('follow owner'). The base AWheeledVehiclePawn sets
Mesh->BodyInstance.bSimulatePhysics=false (the Chaos async solver drives it), so
'follow owner' meant the chassis never simulated - the vehicle sat frozen at
spawn (simulating=0) and -drivetest reported speed=0 travelled=0.
Set Body->PhysicsType = EPhysicsType::PhysType_Simulated. Result: simulating=1,
settled at Z 77 cm, and the drive test moves the car (36.3 m). It drives backwards
(-96.5 km/h) - the next bug is the reverse direction, not the physics setup.
The vehicle floated 3.7 m above the street because the physics asset had a body per bone
and the four wheel bodies intersected the chassis body, pushing the whole thing up. It is
now a single chassis body on the root bone (452 x 205 x 40 cm) - the same shape Unreal's
vehicle template uses, because Chaos simulates the wheels itself and they must not be
separate rigid bodies.
Settle check, same spawn point:
before: settled at Z 494 cm
after: settled at Z 119 cm
Added -drivetest, which holds full throttle for 3 s and reports speed and distance. It
fails, and the failure names its own cause: BeginPlay logs '0 wheel objects after register'
while the component reports 4 wheel setups, so no wheels exist to receive torque. Recorded
in TODO.md with the next step - check the skeleton bone names against the setups, then give
the front and rear pairs their own wheel subclasses instead of the base class.
The physics asset could not be made from Python (PhysicsAssetFactory exposes no mesh
property), so a small editor-only C++ helper calls FPhysicsAssetUtils::CreateFromSkeletalMesh
- the same code path as the editor's Create Physics Asset action. It produces
PA_Vehicle_Sedan with 6 bodies and assigns it to SK_Vehicle_Sedan.
AGigLineVehicle is a Chaos wheeled pawn that configures its four wheels from the modelled
dimensions (radius 33 cm, wheelbase 278, track 156) and is staged in DemoMap. A run logs:
spawned at (-12950, -900, 120), 4 wheel setups
settled at Z 494 cm, simulating=1, speed=0 cm/s
That last line is the defect and it is not hidden: the vehicle rests 3.7 m above the road,
so the generated physics bodies do not match the mesh. Recorded in TODO.md with the next
step - inspect the bodies against the mesh, then either fix the creation params or author
two boxes and four wheel spheres explicitly.
zwitschi