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

[bryan11]

I'd also question the PM results.

Assuming tailgate down initially without any topper or cover, there is turbulence, really a low pressure area, right behind the cab. Adding a cover splits the low pressure area into two, one behind the cab and another behind the tailgate. Adding a topper moves the lower pressure area completely to the back of the vehicle.

As PM mentions, this lower pressure area is effectively a parachute being dragged behind the vehicle. With a topper, it's a larger area and thus a larger parachute. With an aerocap, the low pressure area behind the cab is much less, as the air behind the window is all directed without turbulence as Bondo's tests demonstrated.

[mully]

Camper shells - increase or decrease MPG? - Diesel Forum - TheDieselStop.com

Here's the same disscusion on a different forum.

[saddlsor]

I've done many a trip over long distances in my ford ranger, both with a traditional cap and without. if there was any difference in mileage I couldn't tell it, and I'm kind of nutty about keeping track. one experiment I did was to add a piece of aluminum across the back of the shell to deflect wind. it's low profile and has a curve in it to shoot the wind down across the back window. after many more thousands of mile on trips (15k last year ) it has consistantly gotten 2mpg better. I'm figuring it's breaking up the wind on the back and creating less vacuum on the backside. now the problem is, I've been reading too much on this site and I'm thinking about doing a kammback design to see if that does any better. you people are causing me a lot of work.

[MetroMPG]

Colour me surprised. I heard back from the company whose "documented tests" I referred to in post 14. The results were posted on the vendor's site after all. (In my defense, it would have been useful if the page I originally found that mentioned the tests had a hyperlink to the relevant page!)

Have a read - their experiment is documented here. Their test methodology beats the heck out of Pop Mech's. But it still could have been improved - easily.

They did A-B testing of a cab-high cap vs. open bed on a 2004 Dodge Ram 1500 SLT Quad Cab with a 5.7L Hemi V8. They used a closed course, and cruise control at 45 mph for 50 miles of each condition.

Here's an example of their cab-high cap:



Their claims:

19.38 mpg (US) - cap on
18.06 mpg (US) - cap off

... suggesting a 7.3% increase in MPG with the cap.

(They also document testing two tonneaus on two other trucks.)

The two most obvious pitfalls to their testing were:

1) it wasn't A-B-A, and,

2) they relied on the AAA's official "3-click" method of filling of the tank to determine fuel use (I don't care whose filling methods they were following to the letter, that approach still opens things up to potentially significant errors).

EDIT: the other important omission from the test is that it doesn't say whether the cap is better or worse than a tonneau.

[Frank Lee]

Originally Posted by saddlsor

I've done many a trip over long distances in my ford ranger, both with a traditional cap and without. if there was any difference in mileage I couldn't tell it, and I'm kind of nutty about keeping track. one experiment I did was to add a piece of aluminum across the back of the shell to deflect wind. it's low profile and has a curve in it to shoot the wind down across the back window. after many more thousands of mile on trips (15k last year ) it has consistantly gotten 2mpg better. I'm figuring it's breaking up the wind on the back and creating less vacuum on the backside. now the problem is, I've been reading too much on this site and I'm thinking about doing a kammback design to see if that does any better. you people are causing me a lot of work.

Your window cleaning airfoil might have negated any aero benefit.

[bondo]

I remember when Snugtop did this test. I am confused as to how the cap got switched to the Dodge, which actually had a tonneau cover on it and showed the best mileage gains of the three trucks.

http://www.youtube.com/watch?v=oYO21Frl9Vc

Snugtop Fuel Economy Test Results - Chevy, Ford & Dodge Comparison - Sport Truck Magazine



The Chevy had the cap on it and showed a decent .5 mile per gallon increase.

I guess I am confused as to why Snug top would switch the trucks in the text describing the test on their web page? Maybe they have run another test since the one I provide two links to. You can clearly see the Chevy having the cap on it, at least in this test it does.

Just fighting em fair,

Bondo

[MetroMPG]

That is confusing. Excellent point, Brett.

Something else I noticed: at 1:26 the Dodge is shown following close enough behind the Ford to benefit from drafting. It happens again later in the video, and suspiciously, it's the truck with the cap on it that's drafting.

Surely the drivers (racing people) would have known better than to follow that closely.

[MetroMPG]

Wrote back to the Snugtop rep asking for more info re: the video vs. the web page.

[aerohead]

Brett,my data is all dated now and since pickup design has evolved over time,the goal-posts have moved.
Here are some random thoughts from a failing memory:
A pickup, according to the Big-Three,is no more than a body-on-frame two-door sedan without a trunklid,with separate trunk,and rear bulkhead cut away to allow the tailgate.Sounds scary!
Back when pickup Cds were on the order of 0.50,adding a traditional topper would cut the drag of a pickup to that of a station wagon or Carry-all,Suburban,etc.,at around Cd 0.45.With this you might expect a 5% mpg improvement( HWY).
Full tonneau covers were also figured at about a 5% mpg improvement.They basically added back the 'trunklid.'
Ford discovered that a 2-foot wide section of plywood laid across the frame rails ahead of the tailgate would cut drag enough to show at the gas pump.
Later GM noodled around with this theme and arrived at their 1/2-tonneau,for which they received a US Utility Patent,with the claim that it cut drag even better than a full tonneau.The Gale Banks turbo-powered GMC S-15 used this 1/2-tonneau in part,to set a LSR at Bonneville at over 210-mph.
When the Ford guys at Texas Tech published their results of model and full-scale testing of covers in 1988,they had concentrated on the wing-1/2 cover because of "utility" issues for truck owners.
But from the table of results that they published,which did eventually make it into Sport Truck Magazine,the "aeroshell" was clearly the winner as far as drag reduction,with a 20% reduction which would translate to a 10% mpg increase at 55-mpg,even higher percentage at faster velocity.
Modern pickups have higher rails now which changes the camber line over the top and the relative "drop" from the cab roof to the rail/tailgate top is less pronounced.So from everything learned about passenger car aero since Hucho was working on the first VW Rabbit,is working it's way into trucks.
I don't know what kind of numbers the new trucks would generate with mods.Their weight and frontal areas have grown to negate much of the improved aero they've seen,and mpg benefits they might have realized.
It's a tougher call.
But clearly,the aeroshell design appears to be the undisputed heavyweight champion for low drag in pickups.
There is nothing published in the public domain to suggest an architecture which exists,that will do more for reducing drag in a pickup.
And there is nothing in Fluid Dynamics which suggests an architecture which could.

[Frank Lee]

Are the rails really higher relative to the cab? Looks like all of it is soaring into the sky now.

IMHO they've DESTROYED the utility of new pickups. For crying out loud, I'm 6'4" with long monkey arms and I can't reach into the box for anything on the new trucks- gotta climb up in there for everything- where on the older lower ones I could reach into the box from standing besides the truck for a lot of things.

Oh- people are stupid.