okay, so I don’t have a picture because it’s not built out of Vex, but a long time ago I (following instructions in the box) managed to build a Lego robot that crawled across a string suspended in the air. It was really cool, because it didn’t roll like a ski lift, but actually had little arms that would lift up as it crawled. It actually looked like a monkey. 
If anyone can pull this off in Vex, please, let me see it. What the lego did was take the motor in the center, split the feed so that it was going at a 90 degree angle to the output, and then turn those into a large…um…I think cam shaft is the right word…but I may be wrong… Anyhow, and then these raised and lowered the arms in sequence so that it crawled.
I think that these would be cool to build an entire Vex competition game based off of, because not only would you have physical dimension limits, you would have WEIGHT limits as well. A whole new level of challenge. Plus, these work great for crossing “ravines”. New title: Ravine Rampage."