One of my setups is running 15 degrees of rake with 22" tires. I have not run into a a situation where I felt I needed more. With larger tires and small obstacles, I seems to me, you would not need as much rake. If part of the desire to have rake is for landing on your nose off a jump, I don't see tire size affecting the amount you would use. The same goes for running into the base of a hill.