I have DA 7800 brifters, a 12-27 DA cassette, Ultegra compact crankset, and a DA chain with about 1000 miles on it. I live at Lake Tahoe and do lots of climbing so need the 27, but I was getting dropped on the downhills to buddies with 52-11 or 53-12 tall gears. So, I bought an 11t Shimano cog, put it on and removed the 16t cog. I can shift into the 11 fine but it makes an awful racket and skips almost continuously. Did I miss something? The 11t has an "HD" stamped on it while the others have "FD". Does that matter? Thanks for any help.