Box.Center was -Extent.Z*0.25 (below the root bone); the box then sat under the ground and the suspension pushed the chassis up. Raising it to +Extent.Z*0.35 cuts the float from ~33 m to 7.8 m, confirming the direction. Still not settled - the box centre and/or the root bone (grounded at Z 0 by normalise_scale) need to move to the car's centre of mass.