I have a brand new 60" Super Z HD with flex forks and can't go much past half stick/speed on hard surfaces without caster going crazy. I've owned a early standard deck SZ which it was never a problem. I own a 66" XR7 SZ with flex forks and it was a problem until they came out with the poly bushing/washer to be added to the top of the caster. I demoed a new HD SZ and the rep said flex fork should fix it. It didn't. This is a fast mower but if the casters a flying back and forth the speed is worthless. It may have gotten a little less sever since I have 40 hrs. of wear on the ribbed tires now but not much different. I did verify that the dealer installed everything correctly when they installed the flex forks. Wondering if there is a fix and if not it a second poly friction washer should be tried?