In the military, there are many names for things known only to the soldiers familiar with them. I expect, if it exists, there is a specific term for something like this on a tank (although it's not clear why you would put a protruding metal bar on a tank, as it's a potential weak point in the armor), but I have no clue what it might be.
Colloquially you might call such a thing a "strut" or a "spar", depending on its purpose. However, both of these are normally load-bearing, functional items, and not just extraneous pieces sticking out from a vehicle.
So "bar" or "rod" might be better.