I have a variable speed fan,which I run @980rpm, as fast as it goes. My problem is on hillsides with a 8rw head,200bpa corn I overload my sieves, unless I'm at 2mph. By cutting and putting expanded metal in my shields I was able to increase from 1.5mph. Unless you guys think that there is somthing else going on. I can 5mph no problem on the flat.