I have a Trek 7100 I ordered online. I use it pretty much as is (nothing upgraded). There is a plastic round piece approximately 6" in diameter between the rear wheel and the rear gear set. It has come loose (broken?) and is slipping all around as I ride. It doesn't seem to be a problem, other than the noise as I ride. Can I take this off? Is it some sort of guard to keep the rear gear set clean?
Yes, this may be uninstalled, but if you do not have the tools, find a bike store that can do it for you. If your lucky, and they are not busy, they may do it for free, if you remove the wheel from the bike first. I think it is called a spoke protector.
Yep. It's a spoke protector. Mandatory safety item on certain bikes to prevent you from suing the manufacturer if the deraileur cage goes into moving spokes (which never happens, exept maybe with kids who abuse the bike, but continue riding it anyway). If you can get the bike shop to remove the cassette, they can take it off for you.
Since it's already broken and you don't want it anymore you can get the spoke protector off without removing anything else. You'll have to cut/break it into pieces - make sure the cracks and cuts go all the way to the hole in the center - and pull it all out. Will take more time but if you don't have a chance to get it to a bike shop it can work.
specialized roubaix, dawes sst ( steel single speed)
You don't want to take this off. It stops your chain from shifting into your spokes if your derailleur is out of adjustment or the hanger is bent.
They usually clip to the flange of the hub with "hooks". Look at your wheel from the non-drive side and see if you see anything like that on the inside of your disk. If you do, take your wheel off and try to get a putty knife between the disk and the big gear and then push the disk toward the hub until you feel it snap on. If it doesn't want to go rotate the disk a little and try another position.
If still no luck, take it to a bike shop. It is very quick and simple to remove the cassette with the right tools and they can either fix what you have or put another on.
The stop screws are called limit screws. They limit the travel of the derailleur from the little gear to the big gear. If you need more advice, visit the park tool website, or visit bicycletutor.com for a video of the adjustment.