I am in the process of ordering rear tires for my 85 Yamaha Tri-Z. I want the Maxxis Razr tire in 20*11*9 but do not know if I should get 4 or 6 ply. Which one should I get? What are the disadvantages of getting 6 ply and not 4 ply. Would they make my bike slower by the added weight? Please let me know your comments.