Quote:
Originally Posted by Ecky
I'm not certain. A tonneau cover can simply cover a void, isolating the air under it, and cut drag. I suppose there is force transfer going on there, but it doesn't seem proportional and mostly would be preventing the venturi effect by separating high and low pressure air. I haven't fully wrapped my head around the physics of it though.
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As far as automakers are concerned, pickup trucks are just large sedans, without a trunklid.
Flow separates off the rear of the roof, and there's nothing to reattach to.
There's no pressure recovery.
High drag and high lift.
A tonneau cover is essentially a 'trunklid.'
If it's sufficiently close enough, vertically, and sufficiently 'far' longitudinally, it provides a surface for reattachment.
A vortex is captured on top of it, and the outer free stream will skim over the 'locked-vortex' as if it were a solid structure, plus 'touch' the back of the cover before separating.
If 'smoked', you'd see streamline filaments diverging as they decelerated down the 'contour' of the vortex, picking up pressure as they lost velocity.
This is the drag reduction.The higher pressure is communicated to the base, behind the tailgate, raising the base pressure, which lowers the pressure drag, the largest component of aerodynamic drag; and why we streamline.
And since slower, higher pressure air impacts the rear of the tonneau, it also kills most of the rear lift.
A 'half-tonneau' works better due to reattachment, plus the low pressure of the vortex core telegraphing under the cover, to the forward face of the tailgate, increasing the pressure differential between the front and rear face.
GM has the US Patent on it.
If you watch a vinyl cover, you'll notice a dip at the rear where air is attacking it.
At the front, behind the cab, it will look like Yoda's under there trying to get out, creating an upwards bulge in the fabric. Highs and lows.