Thanks for all the replies, guys. You're right, DAVE, I could just make up my own formula and find an rpm range that it doesn't vibrate at, like 500 or so. With all the different methods, I think you have to consider the application. I don't know enough about it to be able to follow even my own advice! I guess find advice that closely follows what I'm trying to do, I'm trying to get a .40 to run smooth at 24,000+.