1888a123_Beginner_Dirt_Bike___GICL_Wiki_Dirt_Bike_Parts_List_Function
[Part and Quantity] Rear Frame (1) [Part Number] DR70-107 [Materials] Metal, most likely aluminum , because of its light weight. [Forces] There would be two forces applied to it. The first would be horizontal, in keeping the wheel vertical. The other would be a portion of the bike’s and rider’s weight. [Manufacturing Process] Square metal tubing is heated and bent into the needed shape. The cross bar is welded onto these parts. Holes and slots are then drilled into the part. [Shape] Specific Shape is needed because of the forces applied to it. Must be able to distribute weight and not break. [Part Complexity] 4 [Function] 
Functional: connects back tire to the rest of the bike.
Group 11 - Beginner Dirt Bike - GICL Wiki