Nice hangar, can stack a few gliders in there...!
If the top channel section is what I think it is, it will contain sets of 4 wheel trolleys, which roll along the inside of the channel. The metalwork is quite expensive, for 18ft of it I was quoted 250 pounds, but it does last a long time.
If you put the door in the most stiff position, there may be lock-nut adjustments that can be made to ease the binding. Hopefully that will not make the door too loose at the other end of its travel.
Best of luck..