Originally posted by Crazy Cyclist
I was just wondering what purpose it serves, and what is it called, and also do I really need it.
I concur with L8R, if you have good equipment, keep it in proper adjustment, and shift into the largest cog only when going slowly (which is the only time you need it, anyway!). On most rides, my largest cog, which I keep in reserve for that last big hill after I "bonk," is my spoke protector.
You do need a Dork Disc if you have an ancient bandspring Simplex Tour de France derailleur. Because of the reverse shift pattern, snapping the shift cable can will throw the cage and chain toward, and possibly into, the spokes. Most Campagnolo Cambio Corsas came with, and apparently needed, Dork Discs, as well.
The only disadvantage of Dork Disc-ectomy is that it moves your cogset a couple of mm toward the centerline of the bike, which will degrade your chainline in the larger cogs and chainrings, although it will improve your small - small combinations.
By the way, if you are converting from a standard-space 6-speed to a 7-speed freewheel, removing the disc will give you a welcome 2mm of additional cog-to-frame clearance.