You'll have to do your homework a bit better!!!
Imagine you need to unfold a cylinder. The only way too unfold it is by cutting it open and unroll it as a sheet of paper.
This principle applies to all cylindrical and conical shapes....
The best way to open a side is to detach a quad polygon from the object manually and attach it again. Then select only 2 of the 4 vertices that are along the same neighbouring polygon and weld those. Do
NOT weld the others, because if you do, you won’t have an open edge anymore and your unfolding will fail again.
As homework redo all steps of the truck tutorial on the chillipage:
http://www.chilliweb.co.uk/chilliski...ter_Truck1.htm
http://www.chilliweb.co.uk/chilliski...ter_Truck2.htm
at page 2 you'll see how the wheels are unfolded....