I am looking to put on a new pair of MX tires, but I have a 9" rear wheel. I have noticed that most 18" tires are for 8" wheels. Should I get new 8" wheels, or are the 9" OK? The only difference that I can think of is that the 8" wheel may be a bit less harsh...and obviously my tire brand choice would be much more expanded for an 8" wheel. I only ride for fun, so if it's not a critical difference, then I'd like to stick with my 9" wheels. What do you think? Thanks in advance.