Airflow around a car-top cargo box.

Factual Questions
1

I’m about to go on vacation and I bought one of those car-top cargo containers to go on the roof. It comes with soft pads to sit directly on the metal surface or it can be fastened to the roof rack.

I did the latter, since I don’t want to mar my paint, but from an airflow point of view, is it better to have the gap under the box or have it sit directly on the roof?

My best guess would be that overall wind resistence would be better with it raised off the roof, on the rack, since the air has four sides to go around rather than three. However, the air under the cargo box has to compress to fit underneath so there’s energy wasted there.

Anybody want to hazard a guess?

-B (with just enough aerodynamic smarts to be dangerous)

2

It’s gonna suck either way. My WAG (and this is just a WAG) would be that it would suck marginally less if it were mounted directly on the roof.

3

A bump in the form of an agreement…

Yah, I know it’s going to be a drag snerk.

Hey - where’s our Aero Engineers?

4

The turbulence of funneling air down into a two-inch slot makes it, in terms of drag, about the same as solid matter. I wouldn’t risk your paint for the tiny difference it would make.

5

My experience with a cartop carrier attached to the roof rack on my Volvo wagon was that the weight of the stuff I put in the carrier bent the bottom of it down so that it was in contact with the roof of the car anyway… lay down a towel underneath it, even if there are pads at the corners, just in case.

6

I agree with Asknott; the air “piles up” in the slot and oncoming air sees it as a solid wall.

7

I would hesitate to give a ‘common sense’ answer, because the airflow around a car can be hideously complex and sometimes counter-intuitive. There ‘could’ be intersection drag from the exposed legs and such that is bad. On the other hand, the slot could actually improve the drag situation. Hard to know without a wind tunnel.

It’s like the old question of whether a truck gets better mileage with the tailgate down. The ‘coomon sense’ answer is that it would, because the tailgate acts as a big wind brake. But in fact, on most trucks having the tailgate up actually improves the drag situation because it helps create a ‘bubble’ of air in the truck bed that has the effect of making the truck more streamlined. So all those people that spent good money on slotted aftermarket tailgates actually paid to make their truck get worse mileage.