I have a Specialized 2002 Expert; I just put a new rear cassette, chain and chainring, I think I have the chain at the right length (i.e. When in my lowest gear my rear deraillure is straight up and down), but when I ride and coasting over bumps all I hear is the chain banging against the bottom of the frame. Is there any way to get more tension out of the rear deraillure ?

I dont know about your deraileur but atleast Shimanos deraileurs are adjustable in the way that you dismantle it, open the part where the spring is (where it is attached to the frame) and voila, the end of the spring sit in a hole, and there is another hole next to it, just move the end of the spring to that hole and you got more tension.
But please, get my word verified before you start taking things apart. And as I said, this is for Shimano rear deraileurs.

people are probably going to yell at me for this one becuase i dont know if its *proper* but heck it works for me. When i had that issue I actually re adjusted the angle the actual derailleur is mounted at. Just look for spare bolts and washers. I did this untiul i got a fine piece of SRAM equiprment (long cage) and the problem is solved.

Sounds like the chain is too long. To get the correct chain length, put the chain on the biggest chainring and largest sprocket with it going around the derailleur. Pull the chain tight and then allow two extra links. Cut the chain there and re-join.

Assuming that the new cassette and chainring are the same size as the old one, Just count the number of links you had on the old chain and make sure the new chain has the same number of links..

I agree with wrench. I'd check the chain length first. I think that the arm on most Shimano derailleurs should backward when you are in the little/little combination.

I'd think twice before trying that spring adjustment trick too. Those springs are a whole lot easier to take apart than they are to put back together.

I agree with phantomcow. Not about the washers but the sram rocks !!!!

Yeah, sram ROCKS!!!!

Short of the chain, a tensioner might be looked at. If you do a lot of kicks, hops..they are nice to cut the noise, and keepthe chain on while jumping in higher gears.
I have seen 'simple' lower single units.

Probably just a long chain............