Hi im new to this so bear with me. Im looking to buy a bike and it seem that most adventurers use the 21-18 knobbies. why not use the 17-17 knobbies, more area equal better traction or im missing something. I really dont know how much time I be spending off-road but I do have plenty of sand,dirt,rock - south texas desert here. I might end up buying a set of warp 9 wheels but not completely sure if to get a bike with 17 or the 21/19 and then buy the other wheel set.