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Old 03-01-2019, 11:59 AM   #1
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Truck Box Caps: Do they help with drag

After fighting headwinds I am thinking of putting a cap on my Silverado to hopefully direct the wind a little higher up the front of the toy hauler.

Has anyone gone capless to capped and seen a measureable difference?
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Old 03-01-2019, 12:05 PM   #2
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Ahhhh the cap question. Asked and answered numerous times, just pop it into the search window. Short answer: a 'cap' may result in a negligible mileage increase. Tonneau covers seems to decrease mileage a bit. Those studies are truck alone, not towing.
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Old 03-01-2019, 12:27 PM   #3
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This is just my opinion, but having been in the aviation field for the past 30 years, I might know just enough to be dangerous...


But I would say that I feel that it should make some level of benefit aerodynamically. If you notice, most of the 18 wheeler big rig trucks have a big deflector on top of the tractor cab to cut the air for the top of the trailer... and those guys are all about improving aerodynamics to save fuel. The topper would serve the same purpose aerodynamically in my opinion.


The sticky area would be the distance from the trailing edge of the topper to the front of the camper... if the distance is too great, then too much air can spill into the area between the truck and the camper, and you would see no benefit... the closer the better.


Without wind tunnel data, it would take a LOT of road data collection to definitively say it does or doesn't help and by how much because there are so many variables... but I don't think that it will hurt aerodynamics compared to no topper.
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Old 03-01-2019, 12:40 PM   #4
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Don't forget that a truck topper will add 250-300 pounds to the trucks payload.
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Old 03-01-2019, 01:13 PM   #5
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Don't forget that a truck topper will add 250-300 pounds to the trucks payload.
That's a hell of a topper. I use to take mine on and off by myself. 8' Fiberglass.
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Old 03-01-2019, 01:23 PM   #6
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I'd say that 250-300 lbs is in the ballpark for a good quality topper like Leer, Sportsman, etc. Fiberglass isn't lightweight.


But I think it is a small penalty to pay for all the advantages... especially if you have enough truck and aren't operating on the edge of the envelope.
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Old 03-01-2019, 01:25 PM   #7
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If the topper was built to match the trailer in tow, being as high and very close, might reduce the drag some but enough to gain mileage or fight headwinds? Doubtful.

Might be more effective to add a "bubble cap" to the front of the trailer that helps move the air around that huge "barn door".

If you take a look at Semi's moving down the road today, yes they do have the aerodynamic aids on the cab but they are also adding skirting to the trailers to move air smoothly under the trailer and many even have "tail cones" that can be extended or collapsed as necessary.

Most toppers I've seen aren't high enough to do anything but move the bug splatter line a little higher on the nose of the trailer. I suppose one could have a topper made that would be high enough to help some but then would you want to drive that truck around and not be able to go through a bank or fast food drive-thru because you are now 10' tall instead of just under 7'?
No more parking garages either.

For a while there were a lot of "Midget Racing" air foils being mounted on truck cabs but after people found that they really didn't help all that much I haven't seen one for years.
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Old 03-01-2019, 01:29 PM   #8
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I'd say that 250-300 lbs is in the ballpark for a good quality topper like Leer, Sportsman, etc. Fiberglass isn't lightweight.


But I think it is a small penalty to pay for all the advantages... especially if you have enough truck and aren't operating on the edge of the envelope.
Mine was a Leer. I'd get it off the truck, crawl underneath, lift over my head and walk it to the side of my garage. Balancing was the hard part. Once I found the centers, I marked them inside so I knew where to lift. I'm no body builder...or remotely close, but I don't think my topper was anywhere close to 300 lbs. Maybe today their stronger and heavier, had mine back in the 80's early 90's.
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Old 03-01-2019, 01:30 PM   #9
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You may gain some aerodynamics, but the added weight to your rig will counter any benefit in terms of MPG. Probably for the worst.

I think a better option is to beef up your suspension so that you experience less sway. Think air bags, sway bar, RAS, etc.
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Old 03-01-2019, 01:32 PM   #10
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Whatever you think of the appearance factors and whatever the aerodynamic factors, toppers have some disadvantages.

They decrease payload
They do not work with 5th wheel trailers at all
With most, if not all, security is merely an illusion
Getting to stuff stored up against the cab is difficult and uncomfortable
Down here in the desert, contents can be badly overheated.

For me, having had 2 trucks with paint-to-match caps, the negatives outweigh the positives by a considerable margin
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