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

[MetroMPG]

To add to the discussion:

1) http://ecomodder.com/forum/showthrea...p-off-601.html

Relatively well done on-road testing shows that specific cap increased drag.

2) http://ecomodder.com/forum/showthrea...ruck-1228.html



Obviously "traditional" cap styles vary a lot. Some increase frontal area, and have insufficient radii on the leading edges. Others are truck-specific and are nicely integrated into the OEM shape at the back of the cab.

[MetroMPG]

Here's the Pop Mech Google Books link:

Popular Mechanics - Google Books



Their testing methodology is questionable. It's just "A-B-C", and doesn't say anything about how much effort they made to be as-controlled-as-possible.

I'm skeptical, and suspect they may be wrong.

[winkosmosis]

Wait a minute... So lowering the tailgate does reduce drag, presumably by creating a lengthened surface for flow to reattach to.

[MetroMPG]

One cap vendor posts this on their web site:

Documented tests confirm that the aerodynamic enhancements gained over open-bed pickups reduce the vehicle’s coefficient of drag and boost gas mileage.

source: Snugtop - Custom Fiberglass Manufacturing, Truck Cap & Tonneau Top Covers, Camper Shells Tops, Customized Truck Toppers, Pick Up Truck Canopy

... and of course provides no additional information about the tests. (Wonder if they're referring to the Pop Mech article....)

[MetroMPG]

Let's keep the discussion about caps please.

[bgd73]

I learned something odd with this subject and a lower compression 1979 chevy pickup. the engine got bigger to weight, and it reminded me of a 1960s gas 5ton truck...also with strangely factory low compression.

if you have high compression, performance engine, it is a tinny versus a sledge hammer. the tinny smackers, need to play aero games, the workers do not. the cap can do two different things on the same truck, depending on drivetrain. My own experience with the older low compression was sqaure cap, lack of aerodynamics, just to keep the engine chugging a bit more..INTO efficient.. Most noticable starting with the larger v8s (5.7 liter and onward) the smaller the engine, the more it is straining through any weight, any compression, making it less noticable.

It is almost like a tractor trailer- they are the same loaded sqaure, unloaded, winged looking like a ricer...it is all the engine, the engine, the engine....

the more I see wings and aero, the more I know the engine sucks.

[MetroMPG]

This (dead) link is supposedly to an SAE study using a pre-2004 model F-150:

Technology Enhanced Learning with Moodle

Which showed that a tonneau was the most efficient of all options studied, including traditional cap. (Not very useful unless we know details of the cap though.)

[Frank Lee]

Yes, the PM article's testing leaves something to be desired. Back in '82 they knew more than I did.

The Dodge article was done by students who did water tank testing on an itsy bitsy scale model that lacked who knows how much detail. I'd be loathe to read too much into the "values" given. I think they had no business going to that many decimal points.

The caps thing is fraught with variables as Metro noted; caps come in zillions of designs, many of them not fitting to the cab very well.

A quick Google shows many truck cap makers and retailers claiming better aero. Not much in the way of legit proof, except for the tonneau cover claim at the Leer site, which isn't germane to this.

[MetroMPG]

Bottom line is I don't think we've yet found quality evidence that shows a traditional cap outperforming some other traditional configuration (open bed, tonneau).

I wrote to the Snugtop people asking for more info about the studies they reference. I doubt they'll reply, but I'm willing to be proven wrong in my skepticism.

[Fubeca]

The PM article also uses a long-bed Isuzu which has a relatively short cab height and a very long bed.

With all the talk of rotating bubbles helping the air get to the tailgate - I'm sure the height of the cab and the length and height of the bed play a role. On a full-size short-bed truck, topper vs. tonneau vs. aerocap might be totally different than a small, long-bed truck.

PS - I used to own a 1984 Isuzu diesel 2wd longbed (with a square camper top) and consistently got better than 35 MPG. Unfortunately I never did any ABA testing with the topper. I wish someone made a truck today that would get that mileage.