1.The rear tires are generally supposed to give the vehicle a good grip especially when breaking u might observe most of the performance cars andbikes have thicker tires in rear than front ones. the front ones are not supposed to that thick as rear becoz they need to be turned. 2.U might be thinking why not use thicker instead of two the answer is if we use thicker tyres and it blows thebus might go out of control and in case of two tyres if one goes out other one controls the weight.