The black things appear to be simulated weight, as was said before...
The only problem I see with the mecanum setup (other than what Eric mentioned) is that they appear to be on cantilevered banebots kit transmissions, and from the reaction I've seen... that is not a very good Idea. All it would take is four pillow blocks mounted on the extruded to remedy it though...
Other than that, though, It looks great! I like the weight distribution (assuming the battery will be opposite of the compressor).
One of the most Aesthetically pleasing extruded frames yet...
