This is a pie plate. It's only purpose is to keep the chain from shifting past the largest cog in the rear and into the spokes. This is never a problem if your derailleur is adjusted properly, and most bikes don't even come with them. This is the one I took off my old Fuji several years ago. The Roubaix Pro never had one. Never will, but I could have used one last week.
I didn't mention that in my attempt to make the Wipperman chain shift I managed to get things so badly adjusted that the chain shifted past the last gear and into the spokes. Since that locked the rear wheel, I came to a screeching halt, and it took a while to free the chain. I didn't really want to say anything. I'm a better mechanic than that - most of the time. I lapse once in a while. Anyway, I didn't think much about it.
This morning I went for a short ride, only 20 miles. At about mile 18 a spoke broke on the rear wheel. I opened up the brake, wove the spoke between the others so it wouldn't flop around and proceeded on home. I still didn't think much about breaking a spoke since I had broken a few on the old bike. they just fail once in a while. So, it was another trip to visit a bike shop in Greensboro. This time I went to Cycles de ORO, the bike shop I used to use all the time before Performance opened their store right up the block. Since the broken spoke was on the drive train side I knew I'd have to get a tool to remove the cassette, and to be honest, I just felt like going in and knowing whoever I dealt with would know exactly what I needed, even if it might cost just a little more. I bought the tools I needed and I got 10 spokes so I'd have some spares.
It wasn't until I took the cassette of that I noticed this:
The remaining six spokes that exited the hub between the cassette and hub flange were all badly bent and gouged from having the chain jammed into them. No wonder one had broken. I was glad I'd gotten ten of them, since that let me go ahead and replace all of the damaged ones, and I still have four spares, just in case. The wheel is all fixed and true.
I mentioned the Wipperman chain to the mechanic at Cycles de ORO, and he said he'd had exactly the same experience with one. He also told me to take it back to Performance, they'd give me my money back. Next time I go up there, I'll bring it back and see. I'll let you know. Actually, I'd be fine with a store credit, since I know I'll be buying more bike stuff.