[smokescreen]

You can use as much extruded aluminum as you wish.

So you can make your robot structure out of round or square tubing. So there's no need to make square tubing out of angle iron. However, it is conceivable to make the entire structure out of 1/4" aluminum angle iron I suppose.
Although, because there is no limit on extruded aluminum, you could use something more extravagant than tubing for your chassis. You could use 80/20 for example.

Also, timing belt is simply a flat rubber belt with raised rubber sections. When the timing belt is used with timing pulleys, there is essentially no belt slip. Very useful if you wish to have one shaft driven by another shaft and want to keep the rotational alignment the same.

Some teams have used timing belt to give them extra traction by wrapping it around their wheels.

-smokescreen