Truck Cap Aerodynamics: Wedge vs Standard

olshaggy

olshaggy

Member
I'm trying to decide which shell to put on a regular cab long bed truck, and I'm curious about the extent to which the choice will affect drag at highway speeds. Pictured here is my mock up of two options: a 36" commercial cap, and a 30-36" wedge.

20221121_002137_2.jpg

My assumption is that the wedge will slightly improve fuel efficiency, as pushing a vertical wall that rises 7" above the cab should require a little less force than one that rises 13". Truckers have wind deflectors for a reason after all, but I know this case is different, so I spent a couple hours last night searching through these forums and the web more broadly and failed to find any direct comparisons of the aerodynamics of wedges and rectangular caps. All else being equal, I'd rather have the full interior height for the length of the cap and have a larger flat space up top for mounting solar and strapping down gear, but at what cost? A 3% decrease in fuel efficiency? 12%? I'm well aware that there are a lot of other variables (speed, weight, racks, etc.) at play when considering gas mileage, but let's assume all of those are held constant for this comparison. Please spare me your aesthetic judgments too ;)

Does anyone have concrete evidence, whether personal experience tracking mileage in both types of caps on the same truck, sufficient knowledge of fluid dynamics, or simply useful data found online, to speak to the extent to which the wedge would improve gas mileage and handling at high speeds?

23-36-cap-3.jpg

To complicate things a bit, what I might actually prefer is a topper that rises from cab height up to 36" (as pictured above). I've been planning to get an ARE DCU, but I heard back directly from ARE's engineers today that they can't make one this steep, so I might consider using a different manufacturer if I decide to go this route. Now that Tradesman is out of business, Century (which built this one) looks like the leading contender, though their build quality is reportedly lacking. Anyone else have experience with them? I assume this design would perform substantially better on the highway than either option above, but again, does anyone have a sense of the extent to which that's true?
 
olshaggy

olshaggy

Member
Yep, the only variable affecting drag force would be the drag coefficient, which is tough to determine without wind tunnel testing (or good computer modeling of it). Growing up in Miami, I built a simple wind tunnel as a science project in elementary school to determine optimal house roof slopes in hurricane country. You'd think the companies selling these wedges would have some data to support their design choices.

Would still like to have a clearer sense of how much of a difference it makes to slope up from 23" vs 30" vs starting at 36".
 
Buddha.

Buddha.

Finally in expo white.
olshaggy said:
I'm trying to decide which shell to put on a regular cab long bed truck, and I'm curious about the extent to which the choice will affect drag at highway speeds. Pictured here is my mock up of two options: a 36" commercial cap, and a 30-36" wedge.

View attachment 753216

My assumption is that the wedge will slightly improve fuel efficiency, as pushing a vertical wall that rises 7" above the cab should require a little less force than one that rises 13". Truckers have wind deflectors for a reason after all, but I know this case is different, so I spent a couple hours last night searching through these forums and the web more broadly and failed to find any direct comparisons of the aerodynamics of wedges and rectangular caps. All else being equal, I'd rather have the full interior height for the length of the cap and have a larger flat space up top for mounting solar and strapping down gear, but at what cost? A 3% decrease in fuel efficiency? 12%? I'm well aware that there are a lot of other variables (speed, weight, racks, etc.) at play when considering gas mileage, but let's assume all of those are held constant for this comparison. Please spare me your aesthetic judgments too ;)

Does anyone have concrete evidence, whether personal experience tracking mileage in both types of caps on the same truck, sufficient knowledge of fluid dynamics, or simply useful data found online, to speak to the extent to which the wedge would improve gas mileage and handling at high speeds?

View attachment 753217

To complicate things a bit, what I might actually prefer is a topper that rises from cab height up to 36" (as pictured above). I've been planning to get an ARE DCU, but I heard back directly from ARE's engineers today that they can't make one this steep, so I might consider using a different manufacturer if I decide to go this route. Now that Tradesman is out of business, Century (which built this one) looks like the leading contender, though their build quality is reportedly lacking. Anyone else have experience with them? I assume this design would perform substantially better on the highway than either option above, but again, does anyone have a sense of the extent to which that's true?
Click to expand...
This doesn't answer your question but its related.

Aerocaps for pick-up trucks

When people think about fuel economy, they usually think about small cars, perhaps a mid-size hybrid. If they think about trucks, its usually to contrast them with a more efficient vehicle (and perhaps chastise truck owners for their wasteful choice). […]
ecomodder.com
 
olshaggy

olshaggy

Member
Yeah, stumbled on aerocaps when I was researching last night. Honestly surprised they're not more common given the significant boost in mileage and increased capacity versus tonneau covers.
 
olshaggy

olshaggy

Member
That chart shows back windscreen angle, which I don't think is the relevant one here. The front windscreen angle chart appears to be behind the paywall, but a quick search pulled it up in this PDF. If the rectangular cap has a front angle of 90 degrees, then the sloped portion of the wedge would be 10 degrees, which is off the chart (it stops at 30) but it looks like the respective drag coefficients would be about .305 and .280. Looks like a big difference on the chart, but a) as the author notes, the drag coefficient only has a linear relationship with the drag force and, far more significantly, b) we're only looking at a 6" portion of the total vertical cross section of the roughly 90" tall truck plus cap.

I image the much larger factor here is the number of inches of cap that present a 90 degree angle to the wind. That's about 13" on the rectangular cap, and 7" on the 30"-36" wedge, but would be zero inches on a wedge that rises from cab height. What all this translates to in terms of fuel efficiency remains a mystery, but thanks for digging up this research. Regardless of that wedge angle, it's clear that the huge squared off back end causes massive amount of drag. ?
 
S

simple

Adventurer
The internets say pick up trucks drag coefficient is 0.5-0.65 where as cars are 0.3 and a euro style van is around 0.4. The graph above relates drag coefficient to fuel economy. It looks to me like drag coefficient is a fairly small factor in the calculation for fuel economy. The cross sectional area might be a higher factor for calculating drag force and fuel economy.
 
Last edited:
chet6.7

chet6.7

Explorer
DSCN0410.JPG
FWIW, there are 2 horizontal AL bars on the front of the camper ladder rack, all of the bugs are on the top bar.The bug splatter was about the same pre and post roof rack. I suspect I could have gone with a higher camper and not optioned the lower front.
 
olshaggy

olshaggy

Member
Oh that's interesting @chet6.7! If I'm understanding you correctly, you're concluding based on the bug splatter pattern that regardless of the roof rack, the shape of the cab is driving most of the air up toward the highest point of your ladder rack, in which case it may not have made much of a difference in aerodynamics whether you had a 26"-32" wedge (what I'm guessing I'm looking at) or one that started at 32" high. Is that right?
 
olshaggy

olshaggy

Member
simple said:
The internets say pick up trucks drag coefficient is 0.5-0.65 where as cars are 0.3 and a euro style van is around 0.4. The graph above relates drag coefficient to fuel economy. It looks to me like drag coefficient is a fairly small factor in the calculation for fuel economy. The cross sectional area might be a higher factor for calculating drag force and fuel economy.
Click to expand...
So based on the graph above, at 65mph gallons per mile goes down about 0.018 for every 0.1 increase in drag coefficient. That translates into the use of an additional 18 gallons of gas per 1000 miles driven, or one gallon every 56 miles driven. So if my truck got 14mpg with the most streamlined cap available, it would burn 4 gallons of gas driving 56 miles. If I added a rectangular cap that increased the drag coefficient by 0.1, it would now burn 5 gallons of gas to drive the same distance, driving my fuel efficiency down to just over 11mpg, a 3mpg drop.

If the difference between the drag coefficients of a euro style van and a pickup or a sedan are about 0.1, then we might estimate that the difference in drag coefficients between a truck with a 36" cap and one with a 30"-36" wedge is significantly less than 0.1, but how much less is still quite unclear. Chet's anecdote above suggests the difference could actually be quite small because of the way air flows over the cab.

Bottom line is we won't know how big a difference these cap variations really make until someone does some controlled testing, whether using actual trucks or 3D modeling.
 
S

simple

Adventurer
Thanks. I wasn't interpreting the graph right. I was thinking the left column was miles per gallon when in fact it is gallons per mile.
 
olshaggy

olshaggy

Member
Thanks. I see he notes that it "hurts MPG a little." The height certainly could be a factor but I suspect those extended sides are part of the explanation too.
 
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