Algodoo

Two-cylinder Opposed Piston Spawn Engine Mobile with Stanton bicycle suspension.
Controls in-scene. Course rightward to traverse.

The spawn event triggers off opposing pistons, so if this goes out of time too bad it simply stops working. The timing gears are proving robust enough however.

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Been experimenting for a while with even smaller scale engines, but finding that anything smaller than ~4ft scale has a significant dropoff in transmission & power. The size of this unit seems to be the smallest I can make them (at least in the method I use) that are still usable.