On tractor-trailer rigs, does the gap between the cab and the trailer affect air resistance?
On tractor-trailer configurations, the gap between the tractor cab and trailer plays an important role in the aerodynamic efficiency of the vehicle. The interaction between these two bodies is the same as two race cars drafting or slip-streaming. As the gap between the two bodies is increased, the tractor and trailer begin to act independently, each having a positive and negative pressure. They essentially become two separate vehicles (with the air resistance of two vehicles) powered by a single engine. An air shield in combination with a large gap may compound air resistance further. If the gap is large enough, the overall drag may be less without the air shield.
