Does a traditional square pickup camper shell topper help or hurt drag & mileage?

[bondo]

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ADMIN NOTE:

There was some debate in this thread (http://ecomodder.com/forum/showthrea...ead-11611.html ) about the effect of square "camper" style pickup caps on drag.

I decided to split off the discussion here so the various bits of evidence can be kept in their own thread.

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My product is intended to replace the old square camper shell. A truck running a squared off camper shell will get worse gas mileage than a truck running an aerocap in the same conditions.

Bondo

[Frank Lee]

My product is intended to replace the old square camper shell, which has poorer aerodynamics than a baseline truck with nothing over the bed.

I was under the impression the opposite was true...

[bondo]

Originally Posted by Frank Lee

I was under the impression the opposite was true...

From what I have read in studies on the subject, for I am not an aerodynamic expert by any means, the larger area of the back hatch of the old type squared off camper shell in addition to the area of the tailgate produces a larger area of low pressure behind the pickup than the drag produced at the rear of a pickup with an open bed.

[Frank Lee]

IIRC the articles I've read said a snug fitting cap was best; better than a tonneau and better than nothing.

[bondo]

Frank,

I would like to read these articles. There is alot of information out there and you can never know it all.

[Frank Lee]

One that I recall from back in the day:

How To Increase The Fuel Economy Of Your Truck With Common Aftermarket Products by Gregg Hall at Article Archive (aarch122519)

In 1982 a test was conducted by Popular Mechanics without any variables to test the aerodynamics of a truck and whether taking simple measures to improve the aerodynamics of a truck affected the fuel economy or not. They had put on a tonneau cover on an open bed truck and drove it for a distance. Then they drove the same truck without any tonneau cover. The mileage increased by a staggering 9% with the cover. The drag created by the open bed of the truck as the experiment showed was eliminated by the tonneau cover. Popular Mechanics further insisted on installing a camper shell. A camper shell, they said, would take care of any turbulence at the back of the vehicle created by the cover and the air caught in the gap and would eventually increase the mileage by 13%.

Google books Popular Mechanics March 1982; didn't immediately see how to link it.

[bondo]

I had not seen that article and promptly read it. Thank you again.

In the article Popular Mechanics stated, in 1982, that the installation of a camper shell and the closing of the gap between the cab and the camper would increase the fuel efficiency of the truck by 13%. In the article Popular Mechanics states the installation of the camper shell would, " take care of any turbulence at the back of the vehicle created by the cover and the air caught in the gap.....".

In the two wind tunnel tests on the Aerolid, the smoke (air flow) would never get caught between the back of the cab and the gap between the camper.

I include a picture I took in the A2 wind tunnel in Mooresville, North Carolina (NASCAR) as the smoke was applied to the drivers side of my pickup. You can see the smoke passing right down the side and not getting sucked into the gap. At the wind tunnel in Allen Park, Michigan, in another test, I was told by the nice folks at that tunnel the air never gets sucked into the gap with a camper shell installed on the truck and the smoke in that tunnel showed that to be fact.

I have to concur with the Popular Mechanics article about a camper shell eliminating the air getting sucked into the gap between the cab and the camper. It is Popular Mechanics stating that the installation of a camper shell will take care of any turbulence at the back of the vehicle created by the cover is what I have to question.

Bondo

[Frank Lee]

I looked at the article again and did not see any such references???

[Christ]

Bondo -

To clarify a bit on the cap/no cap/tonneau thing:

When you look at a vehicles frontal area, you have to look at the largest cross section of the vehicle. For best aerodynamics from a frontal view, you want the largest cross section to be as close to all in the same area as possible. Refer to tear-drop profile - The largest cross section of the frontal area is all in one place, close to the front of the shape, with a heavy curve reaching it from a low stagnation point.

The same idea applies at the rear. When considering the cross-section of a trailing wake, the wake occurs at every place that the flow isn't attached. This includes the rear window of the pickup truck. The tonneau reduces the effect slightly by giving the flow a place to smooth out before running off the back of the truck into a new wake area, and the camper top helps the most because it moves the entire wake to one area.

If you imagine that each step takes X energy:

Open bed = detach at the roof, circulate in the bed, flow over the tailgate, detach at the tailgate edge

Tonneau = Detach at the roof, reattach (maybe) at the bed cover, detach at the tailgate edge

Camper = Detach after the camper.

It's not a perfect representation of the way things work, but it should give you some idea as to why a camper top is better (but not universally, of course.)

[bondo]

Christ, in cross section these things do happen. Also the proverbial bubble of air is produced in an open truck bed to trick the air in the boundary layer into not entering the bed.

Tonneau covers do improve fuel efficiency. Camper shells do decrease fuel efficiency, as proven in wind tunnels.

The following link has argument sustaining both sides of the camper shell issue. There is a link in the link to a study done in a wind tunnel on how a camper shell is worse aerodynamically but I could not get it to open.

How much does putting a camper shell on a pickup truck impact gas mileage? | Answerbag

Bondo