Truck Cap Aerodynamics: Wedge vs Standard

[rruff]

Yeah, kept it general intending to make the thread relevant to a wider group of folks, but this would be for a 2016 Toyota Tundra regular cab long bed 4x4. I expect to be living out of it most of the year, occasionally in urban areas but mostly on BLM and forest service land out west. I'm almost 6'3", hence the desire for the high cap.

Advice from someone who lived in their truck for 13 years... make the interior height tall enough to comfortably sit up straight in. That would be around 58" +- a couple. The Tundra bed is 20" as I recall (too lazy to walk outside and measure), so you need ~38" on top of that.

If you want to cut down on aero drag with a square box, I'd make a rounded leading edge piece out of XPS and fiberglass. Glue a couple pieces of 2" XPS together, put a 4" radius on each edge, cover with fiberglass, paint, and glue it in place. Actually, if you are handy at making things you can do the whole shell yourself. Not hard to make a box that size.

Oh, another consideration for Tundras is that the frames flex a good amount... which means your shell needs to twist.

 

[olshaggy]

Bed is 22" deep, so with a 36" cap, minus two inches for insulation and flooring, I'll have about 56" of interior height. I've tested it out and that leaves me with a couple headroom inches to spare seated on an 18" high seat, and just enough room to kneel without stooping too.

Just found this thread on DCU flex / twist. Hoping to revive it to get more feedback on that issue. Any suggestions for alternatives that would hold up better and sit about 36" tall for under $5k or so? If not, I'm open to learning more about how I might build a better cap than I can buy. Found you in this XPS cap build thread @rruff. Do you have one of your own?

 

[rruff]

I made a sit-up height shell to go on a '84 Toyota PU, and later a stand up camper that mounted to the frame. Both were made with 3/4" core of foam and wood slats, with 2.7mm luan skins. The 2nd had an outer layer of fiberglass which I'd highly recommend. Currently working on a big camper to mount on the Tundra frame (with "pivots" using PU body mounts to account for the twist), that is PVC foam with hand laid fiberglass and carbon skins. I'd not recommend that method...

A simple shell would be easy. I made that one in someone's garage with minimal tools in 2 weeks. The larger camper took 10 weeks. The current one? Right around 3 years so far since I changed my mind and started over...

Pretty certain the shell I built for the '84 would have flexed enough to be ok on a Tundra. If you make the whole back a door, then the shell can flex the same as a truck bed. It will be harder to seal the door when the shell twists, but it shouldn't break. A rear panel with a door inserted is going to cause a lot of stress somewhere.

Happy to discuss details if you wish.

I did go out and measure it, and yep it is 22" deep! So the ARE 36" might be tall enough. If they have an option that is completely open in the back, I bet that would survive twisting ok too. One issue with aluminum-framed shells is that aluminum is a really good thermal conductor. Were you planning to insulate at all?

 

[olshaggy]

Yes, definitely planning to insulate.

If they have an option that is completely open in the back, I bet that would survive twisting ok too.

These are the options. I've had my eye on the double full rear doors, which, though they take up most of the rear, I guess still fit the category you describe as "a rear panel with a door".

 

[rruff]

The flip up door that retains the tailgate looks the closest to what I was thinking... and the idea is that the rear of the shell will be more easily able to distort. On my truck when I go into a sharp ditch at an angle so that I'm teetering on 2 wheels, the rear of the bed is tilted ~2.5" over it's width, compared to the front. The shell will need to deal with that amount of distortion.

Would be good to ask the manufacturer and see what they say, and what their warranty is like.

If you are handy and have a place to do it, making your own seems like a good way to go.

 

[Shawn686]

I have had both types and mileage and handling is really no difference. Where there is a huge difference is wind noise. The wedges are significantly less loud YMMV

As for the door, double full, with or without window, or bust. Once you use it you will never go back

Shawn

 

[dstefan]

If you want to cut down on aero drag with a square box, I'd make a rounded leading edge piece out of XPS and fiberglass. Glue a couple pieces of 2" XPS together, put a 4" radius on each edge, cover with fiberglass, paint, and glue it in place. Actually, if you are handy at making things you can do the whole shell yourself. Not hard to make a box that size.

This ⬆️.

This makes interesting reading:

https://ntrs.nasa.gov/api/citations/20120000497/downloads/20120000497.pdf

It’s NASA’s report on their research into truck aerodynamics …. I kid you not.

Also this: scroll down to post 248 for some real interesting graphics illustrating drag coefficients for various shapes and vehicle combinations. Searching this forum brings up some more real interesting stuff.

https://ecomodder.co...-t-7839-25.html

 

[rruff]

Drag reduction from .88 to .36 with rounded leading edges isn't too shabby!

 

[olshaggy]

Those resources are great, thanks @dstefan! Only briefly skimmed the NASA publication so far, but looks like both that and the ecomodder graphics confirm the significant reduction in drag provided by not only a tapered rear but also tapered or rounded leading edges (as @rruff pointed out).

They also have me thinking it might make sense to line the perimeter of the gap between the truck cab and cap with a flexible material along the lines of the rubber boots people use as a dust seal around the open windows connecting the two spaces. Looks like it would cut down on drag as well as wind noise.

 

[olshaggy]

I'm sold on the wedge shape now, but concerned about the flex / twist issue that rruff has pointed out:

On my truck when I go into a sharp ditch at an angle so that I'm teetering on 2 wheels, the rear of the bed is tilted ~2.5" over it's width, compared to the front. The shell will need to deal with that amount of distortion.

I'll definitely ask the commercial cap manufacturers and dealers about the extent to which their warranties would cover damage to the cap caused by this twisting and also whether they'd cover re-mounting a cap that's shifted or had gaps open up between it and the bed. Regardless of their answers though, seems like the best possible solution is a strong, rigid cap that's mounted to the truck with a flexible barrier that can stretch as the bed twists. How do-able is a flexible mounting solution? Any other ideas?

Also still interested to hear from everyone about the extent to which flex / twist has caused damage or mounting issues, regardless of cap type.

 

[rruff]

Ya... what the heck are those boot/seals called?

On the camper I built 22 years ago I used M-D Building Products garage door seal on both surfaces (overlapping). It worked great. I glued it on with the gnarly 3m weatherstrip adhesive which certainly won't come loose, but if you goof or want to redo you'll have a very difficult and annoying job. https://www.walmart.com/ip/Md-Products-03749-16-Rubber-Garage-Door-Bottom-Seal/19869745

I think you'll have a tough time with a flex mount between the bed rails and shell... easier to make the shell so it's able to twist.

 

[billiebob]

SPEED. below 55mph.... the Nixon Double Nickel of the 1970s..... aerodynamics have little effect of efficiency. 75mph Interstate speeds tho have a dramatic effect. So on box aerodynamics if you want to "overland" the rural backroads of the world, the shape of the box should be irrelevant. But if you want to Grand Tour the USA Interstates definitely buy a Grand Touring Vehicle.

 

[calicamper]

The big slab /square rear side of all caps and other squared off back side vehicles and trailers share similar drag issues. Its a little of the frontal and a whole lot of the back side.

Note the defusers truckers deploy on the back of their box trailers.
You want a measurable improvement at the lowest cost rig up defusers you flip out to disrupt the really poor air flow off the back of the vehicle. ?

 

[olshaggy]

I think you'll have a tough time with a flex mount between the bed rails and shell... easier to make the shell so it's able to twist.

To maximize the twist-ability of the shell, you've suggested keeping the tailgate and designing the back door so it takes up the full area back there, which makes sense to me even though I'd be bummed to give up the double barn doors. You've also suggested building it myself out of foam, wood and fiberglass. Was that mostly about being able to customize the shape or also about maximizing sturdiness or flexibility?

 

[rruff]

The big slab /square rear side of all caps and other squared off back side vehicles and trailers share similar drag issues. Its a little of the frontal and a whole lot of the back side.

The backside problem is simply area. Tapering down is the only good solution... but you lose space and functionality when you do that. Deflectors will help a little in the back but not many big rigs even bother with them. If you are accessing the camper frequently they'd be a pain.

Lots to gain though by just rounding leading edges, and front tapering. At least you get the airflow to run smoothly along the camper, instead of separating completely at the leading edges.

To maximize the twist-ability of the shell, you've suggested keeping the tailgate and designing the back door so it takes up the full area back there, which makes sense to me even though I'd be bummed to give up the double barn doors. You've also suggested building it myself out of foam, wood and fiberglass. Was that mostly about being able to customize the shape or also about maximizing sturdiness or flexibility?

You can make it cheap, custom, flexible, strong, light and insulated. Since you want insulation, it seems like the way to go... to make it part of the structure. And since you are asking I'm guessing you have a garage and a few tools? It's a pretty simple project. You could still do barn doors I think (or any full size door), but if you go all the way from top to floor with any door it increases the amount of relative motion when twisting happens. I favor the hatch style which serves as a small awning. Depends on how you live though. When I had that sort of shell, I'd point my truck south and have the hatch open most of the time, especially in winter... so I could sit or cook back there. It's hard to find shade in the desert.