After making version 1, I wondered if it would be possible to make a version that would continue in the new direction after tumbling backwards. There was some empty space left in the design of version one for some extra mechanics. Then the question was how to make a switching mechanism. Early Technic Lego does not have that many options as the current-day Lego has. The basic idea quickly came to mind (I don’t know how, it was just there) to use gravity. It seemed the easiest solution, I guess. The video shows that it works as a proof of concept, but there is a big downside to the design, which I discuss below.
You can also use the photos below, be it that the design is slightly different from the Ldraw version.
Something had to tumble over to make a different connection between gears and wheels or gears and engine. I opted for the engine because it is heavier than the gear box in version 1. If the tumbling part is not heavy enough the power of the engine combined with the resistance in the gears may simply lift the mechanism back upwards instead rotating the gears. That would cause a lot of noise and wear on the gears. As you can see in the design, the weight of the engine itself was still not entirely enough. One option would be to increase the distance between the engine and its pivot point, but there was no space for that. Well, unless I would lengthen the wheel-base, but that was not an option because I lacked enough chain links to do that. So, the other option was to put more weight on the engine with extra plates.
One downside of the design concerns the gear box. It contains to sets of gears, both connected to the same axle that drives both tracks. The engine connects to one set when it is in the ‘down’ position (when the studs of the vehicle point upwards), and to the other set if it is in the ‘up’ position (i.e. when the vehicle is upside down and the studs point downwards). The gear box gears down to power the tracks but that means that while that happens the other part of the gear box is gearing up (powered by the drive axle). Even though it is not driving anything this other side still uses power from the engine for nothing. It could be solved by using freewheels but to my knowledge they do not exist in Lego. Perhaps one could make one or find another solution, but I didn’t have the time to search.
My impression is that this downside of the gear box has an effect on the machine: it does not easily start, it seems. In my build, that may also have been caused by two other factors. Firstly, the engine that I am using in the demo video is quite old and a bit hesitant. Secondly, there seemed to be something wrong with the on-off switch in the power box. I left the Lego at my mother’s place, so I cannot work on it for the time being.