You guys hold a slavish devotion to this less than 15 degree thing, Just curious, do you ever consider the fact that when the airflow that is leaping over the roof of the truck, reaches the rear of the cab, it is already headed downwards at a 5 - 7 degree angle?
I think the thing to do is to put a 2 foot long piece of yarn on top of your tailgate in the middle go fast. If it drags behind your truck, then the air wants to be there naturally, drop the cap to the top of the tailgate. If the tuft whips wildly to and fro, or ends up remaining inside the bed of your rig....you should strongly consider raising the separation point up.
I have just posted two videos to Youtube of flow illustrator movies showing my truck with a 82” bed and a shortened version, I just guessed at the shortened version length. Flow Illustrator is not an exact program, so it isn’t critical that I be spot on. To that end, the real world air flow could be 8-12 inches above the tailgate, which is why a long tuft on the trailing edge of the top of the tailgate will tell us a lot.
So I ask, is this “12 degree roof line rule” based on a super streamlined shape? Or is it based on air which has just been violated by the front half of one of our Great American forms of transportation?
Take a look at the Vids. Tell me what you think.
Pax