A method of assembling a tire having one or more reinforcements or supporting inserts in the sidewalls is provided. The reinforcements are positioned onto one or more air impermeable layers. One or more reinforcing plies are positioned over the reinforcing supports and suspended therebetween. A gas pressure is used to expand the one or more air impermeable layers away from the forming drum and against a reinforcing ply. As such, the creases or wrinkles in the one or more reinforcing plies can be avoided along with other non-uniformities such as deradialization and/or an uneven overlap where the ends of a ply are joined.