Inspired by the 1980's TOMY Ding Bot robot here is a single drive wheel mechanical bump and go drive unit you can 3d print.

I am using a 1:48 TT gear motor to drive the unit.

Two 6-32 screws hold the TT gearmotor to the frame.

You will need a 1 inch long by 3mm shaft for the single drive wheel axle.

A long #2 screw is used to secure the drive wheel assembly to the shaft of the TT gearmotor.

I used a 0.2mm Guitar string for the return spring. I also tested a 0.16 guitar string, and it works as well.

Tested the unit at 3 volts and 6 volts.