Well, I have 20 X 10s with a 5.25" back space and had to use a .40" spacer which gives my tire about a quarter inch of space to the frame. Don't make sense. Why would the BS work differently with an 18" wheel? If my 20s had a 5" backspace, and I didn't use a spacer they would be at full contact with the frame on both the front and rear of the wheel.