After building the Simple tracks, I wondered if I could make a tumbler version, i.e. one that could also run up-side down. As you can see here, that was not that difficult. Below you find some images that show the design and the draft designs for the gear box. You can also download the design in Ldraw here. I made it on a Mac with Bricksmith, but the file should work with other Ldraw software for Lego designs.
As you can see in the video, the machine keeps trying to take the hurdle, even after it tumbled backwards. Could it switch direction while it tumbled, so that it would move away from the hurdle? Yep. Check out version 2
Views of the model
Because the 4.5V engine does not have a lot of torque, a gear box to gear down is necessary. I made a few attempts before I found something that worked.