So after our earlier discovery that rice is indeed heavy, we re-thought our strategy for delivery. We now use 3 bins that rotate on a turntable. This way the mass is concentrated over the wheels instead of in front of the wheels.
Each bin is filled with enough rice to fill a bin half-way, plus 20%. Each bin has a servo-operated trap door at the bottom that allows the rice to be delivered all in one big “gush”. The turntable is driven by a small stepper motor, geared down by 4x so it can handle the necessary torque (did i mention that rice is heavy???).
Code and OpenSCAD files will be posted soon-ish.