I have about 3 rides on my M51, mounted on my 500AF. Knobs are starting to disappear. I don't ride rocks (just MX tracks with that thing...wish there was a desert nearby!). The knobs are broken off at their base. Anyone run in to similar issues with their M51? Any suggestions for a good, all-around rear meat for the 500AF? I ride Colorado tracks, so they'll range from blue groove, through light silt, to deep tilled kiddy highways (which is how so many MX'rs today like their tracks).