Help! I have a wheelchair with an 18 x 13/8 tire. The new tire is exactly the same make and spec as the old one and the tube is the correct size too. As soon as the tube is inflated it pushes the tire off the rim. I and several experienced others have spent hours trying to make the tire fit. We have tried three new tires and several tubes. Its an old wheelchair that fits a house too small to use a new chair in. Any ideas?