Reported as "the wheels are in the wrong place, it is floating", and it was
three separate faults in one wheel.
The one that showed: the part declares `origin` at the axle so `setRoll` turns
the wheels in place, and a part with an origin has its geometry written
RELATIVE to that point — the anchor supplies the rest. The geometry was ALSO
translated by the axle height and position, so both were applied twice. The
wheels sat 304 mm off the ground at twice the axle height, and the cart stood
on its two shaft tips.
The one hiding behind it: the felloe segments were turned into place and then
moved out to the rim. That looks equivalent to moving out and then turning and
it is not — `rotateZ(a)` sends local +X to (cos a, sin a) and local +Y to
(-sin a, cos a), so a piece placed at (cos a, sin a) has its ARC pointing
radially. Every rim segment was a stubby block sticking 89 mm past its own
felloe, straight through the iron tyre meant to bind it. `cart-wheel` has had
the correct order all along, with a comment saying why; this model did not
follow it.
And the hub: a lathe is built around Y, the wheel is drawn in the XY plane, so
the hub stood upright inside the wheel like a little drum instead of running
along the axle. Invisible behind the spokes, which is how it lasted.
Then a fourth, found while checking the fix. The ground meets a pole's SURFACE,
not its axis: the shafts ran their axis to y = 0, so the square-cut end of an
inclined pole buried its low corner 13.7 mm, and the cart rested on two buried
tips with its wheels still fractionally clear. The axis now stops one tip
radius short and wheels and shafts share one floor.
`restsOn` is the check that would have caught all of it. The support check asks
whether every piece is connected down to the ground, and a cart standing on its
shaft tips answers yes: bed hangs off shafts, wheels hang off bed, nothing
floats. It reported 0 failing cases throughout. Connectivity cannot tell a
wheel from a decoration, and the renderer cannot either — the contact shadow is
cast from the model's own lowest point, so the shadow looked right the whole
time.
What the geometry cannot know, the author does. Naming the parts that carry the
weight turns "it rests on its wheels" into arithmetic, at 1 mm, which is tight
enough to have caught the 13.7 mm as well as the 304 mm. Declared on the cart,
the trestle table, the bench and the market stall.
CRT
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