I am taking my rear cassette off to put on my new rims. My question is. The plastic spacer that goes between the cassette and the hub doesn't fit my new rims. Is this spacer necessary. What if I leave it off. Am I risking the chain jumping into the space? Should I try to get my old one to fit somehow? Can you get new spacers? What to do.
They stopped putting the little plastic disks on bikes years ago. But if your bike is badly tuned and the chain does go into the spokes it will chew the heck out of the spokes, and its hard to get it back out again. I think newer hubs are built with a smaller gap so the chain won't fit into the space.
A plastic or sheet-metal disc that fits between the cluster and the right-side spokes of a rear wheel. This is intended to prevent the derailer from getting caught in the spokes, which can cause very extensive/expensive damage/destruction to the wheel, the derailer, and the frame.
A spoke protector is not a necessity on a bike that is well treated, because the derailer can't go into the spokes if it's properly adjusted and if it is not bent. Bicycles which are subjected to rough handling, however, are prone to getting the rear derailer bashed in, and in such a case, the spoke protector can prevent very serious damage.
So - basically of your bike is properly tuned you shouldn't need it but not having one when you need it can be either a needless expense or an opportunity for much needed upgrades.