unusual parts
 

My team needs a counterweight for our arm. We want to use a trailer hitch ball, but is that allowed? WHat about a small barbell?

Look At the Additional Materials List
 

Neither item you mentioned are in the kit or on the additional materials list, so they are not allowed. Springs and surgical (latex) tubing are on the list and could be used to supply a counterforce. I suppose a stack of 3/8" rod stock pieces (nicely duct taped together!) would accomplish the same thing. The important thing is to stick to the list.

extra battery?
 

how about using a second battery as a counterweight? It would not be used to power anything, just as a counterweight. The materials list isnt clear on this however i think i remember someone mentioning it last year.

Extra battery
 

This is not allowed. The implication would be that you have wired two batteries for extra juice.

If you need additional weight, use any material found on the additional materials list or in the kit of parts. Consider the density when choosing.

Look close there are many good choices. Styrofoam would not be a good choice.

Use Styrofoam as a Counterweight?
 

Martus is right: You should definitely not use styrofoam as a counterweight, unless of course, you were having trouble making weight.

Um.. even if you have trouble making weight, you still couldn't use styrofoam becasue a counterweight is not the size, but mass. For a counterweight, you need something heavy and dense, like metal.

Why does it seem that normally the bots need to go on a diet, but this year they can eat a feast? :)

Better than styrofoam:
 

Martus and I were joking about the styrofoam counterweights, of course, but there is another light thing you could counterforce with: a vacuum. Put your little cylinder close to the pivot, set the arm to the height you like, then seal the two ports of the cylinder. It will apply significant force trying to return to that original position, so your motors will not have to exert as much force. I have never actually tried this, but it seems like it should work.

latex tubing
 

Latex tubing would be a much better option than a counter-weight, unless by some miracle you are not even close to 130lbs

I believe springs are also on the additional hardware list. Our team used them effectively on last year's OCCRA machine. Ours were about 1.5" in diameter and approximately 4" long and we used 3 of them.

As a side note for future competitions, it would be good to have a material on the additional hardware list that made for good ballast. Anything steel or even some dumbell weights.

Springs
 

Yes springs are on the additional materials list. They must be "off the shelf" NOT custom made or fabricated.

Re: Springs
 

How about 'gas springs' They are the things on the doors that make a 'hisss' sound when closing. they also are used on the glass backs in cars and would be avaible in an auto store. I can't remember which one we could go to, but i know we could go to at least one. If we bought them right off the shelf, would they be legal? (They are a spring)

Thanks

Gas springs
 

This has been covered before. same answer. They are not what we consider the common definition of a spring. They fall more in line of Gas Struts - and they are not on the additional materials list.

While they may give the same results, they are not what we think of when we say spring.