I did a search already, and have not found what I am looking for. Anybody know what size tubes I need for my 21" front rim and 18" rear rim? These are the stock Excel rims on my 2003 WR450. I am mounting up new Michelin S12 tires, and am putting a 90/90-21 in front, and a 130/80-18 rear. I want to replace my tubes, and I do not know what size to get??? Thank you for your help!