For my crawler, I originally created a wheel with many “knees” so that only the forward option would bite into the ground, with the backward motion only causing each joint to close. However, it only went around in circles.
To get around that, I tried playing around with the design so that motion would be forward and backwards. I tried making a larger forward leg to see if the uneven legs would cause the crawler to move forward if a peg attached to the motor struck the forward and backward leg.
Then I tried placing a large rubber band across the center and have the motor tense and relax the tension of the rubber band. Unfortunately I did not succeed – the crawler stands motionless.