MPG hit? More or less Aero designs, how do they compare? (Science and data inside)

Getting curious with simulations again I decided to take a look at camper styles and how they affect driving efficiency. So far I have done a baseline and a couple styles to explore.

All simulations are done at 100km/h. While percentage differences look small they do not account for drivetrain losses (possibly linear) and changes in engine efficiency (probably not linear) at different loads.

For a sanity check of the sim and my simplified model I back calculated the baseline fuel consumption (assumed engine eff to be 25%) to be ~8.4L/100km. This is to overcome the wind force only. This seems reasonable as I can get 10l/100km and this number does not include drivetrain losses. All data and calcs can be viewed in this spreadsheet.

If you would like a design tested, please let me know and I will run it.

 

... while accounting for both of these would be necessary for extrapolating the mpg hit?

 

In theory Yes, but without some real world data to create the model that would allow for extrapolating actual MPG just doesnt exist.


I added a more accurate model to the spreadsheet this morning with the Four wheel camper dimensions. its getting more real but still under a bit from numbers ive seen around TW.

 

so... we should just find out what mpg people with caps are getting... by asking those people?

so we can extrapolate. in theory.

 

That is yes, but also a data collection endeavor that makes my head spin (100% open to pulling in collab data). There are a lot of variables that will for sure skew trends as the data set would be small. To overcome this knowing what peoples deltas are, would be a better gauge (providing the cap is the only change). There will still be some variability but would probably be not too bad. I dont think getting within 10% estimate would be unrealistic.

 

A shell level with the roof line at the front, then tapering down to 6 inches above the tailgate. The top/slope the width of the top of the cab would be interesting.

I'd also love to see what a kammback extension on a GFC style shell does.

 

Without controlling for almost everything that would be variable between the trucks, locations, weather conditions, roads, tires.....asking people what they get for mileage with different caps is almost worthless unless you get a large enough sample size that other factors average out.

 

so would that be more or less worthless than modeling, in theory?

 

Summary



The Kammtail does have some reduction in forward drag. however not having this extend to the bottom of the bumper does not help. I should also note that the simplified model of the truck has a very smooth bottom and is not a good reference for aero below the beltline of the truck. Looking at the cutplot of this, adding some kind of spoiler at the top window edge may be more significant in gains.




We have, the spoiler


less than 1% gain. it appears that the low pressure under the overhang doesnt let the air get detached from teh top of the spoiler. more extreme measures now.



it did worse than no spoiler, ha! The air recycling in the roof gap is still a major problem along with this design.



now for the fastback cap initially requested. We got 5% BETTER!!!

 

For validating the model...of very little use without strictly controlling for variables. For prognosticating, depends on if the people you ask have similar drive cycles and set up.

 

Sweet. The lack of improvement for a front deflector/spoiler matches my understanding. Many try to shape a box "aerodynamically" by putting a bandaid on a finger booboo by smoothing the front and leaving the sucking chest wound of drag in the back.

The kammback on the gfs style (more cab profile than the square of the 4wc when viewed from the front) is of interest as I've got a ripcord wedge camper on my rig and I've been considering doing a kammback to recover some of the losses. How it all goes together would be a challenge as it has to work with the wedge lift and back liftgate.

The "fastback" gains are pretty remarkable. I've seen some very similar set ups on ecomodder.

 

I was equally surprised by the gains of the 'fastback' cap. "more cab profile than the square of the 4wc when viewed from the front" -> do you mean have it all sit within teh profile of the truck in the XY plane (referenced to sim models)? I think doing this and using the overhang at the rear with some vertical tails might be an awkward "optimal" config.

I dont think any of the commercial deflectors are anything but someone's random guess/ doing what looks visually correct. I've always wondered on how much improvement features like vortex generators could provide without needing large or bulky mechanisms like a Kamm tail shroud (potentially makes for a nice "porch").

This is a good aero reference that has helped with forming a bit of seat of the pants feel for how things are affected.
VEHICLE BODY ENGINEERING: COMMERCIAL VEHICLE AERODYNAMICS (itsmyblogvbe.blogspot.com)
(cant find the og paper this information is from but this guy has plagiarized extracted the meaningful charts, etc..)

 

Yeah. The GFC shell is about the width of the top of the truck cab. Tapering out to the width of the bed when viewed from the front.

The 4WC shell is the width of the bed and square sided when viewed from the front...

Viewed from the side they look similar your model seems accurate, viewed from the front GFC is a fair bit less frontal area (narrower at the top). Though the Cd is likely similar.

 

Just drove 980 miles each way to Moab, UT with my 02 extra cab, 3.4L, 5 speed with a "AT Habitat" (340lbs) on the back. Was carrying maybe 500-600lbs worth of camping gear, cooler/ ice, recovery gear and such.... We drove an average of 65-75mph on the highway (2,400-3,000 rpms) and I was running 91 octane were available. After Vegas it's mostly uphill all the way to a 7k foot pass and on the way back mostly downhill. Got 15.3mpg average both ways. Came back thinking about adding a roof spoiler but after seeing this thinking I should work on taking less gear/ less weight. Interesting topic!

 

@somebody you seem to know a lot about CFD and aerodynamics, and I’m pretty ignorant, but just a thought. Decades ago I worked in the engineering department of a heavy duty truck manufacturer and some colleagues were doing a lot of wind tunnel testing. Something they told me, that has stuck with me for 40 years, was about was the effect of crosswinds on drag. Not the kind of crosswind perpendicular to direction of travel that also affects stability, but just the fact that when you’re driving 100 kph down the road, you’re not in still weather, there’s likely to be a some wind velocity relative to the ground and it’s likely to be blowing at some angle relative to the vehicle heading.

It could be mostly headwind creating a higher airspeed vs ground speed, or tailwind decreasing airspeed, but in any case there is always some component of the overall drag force which is perpendicular to the length of the vehicle, and can have a significant effect on drag. Not only is this acting on a larger effective frontal area, but the cd is affected as the shape presented to the air is different. I hope my explanation makes sense, might be worth “steering” the vehicle model off axis a few degrees and seeing the effect.

 

https://gaiacampersusa.com/products...85054&pr_ref_pid=7834618921150&pr_seq=uniform

I wish they had these for the long bed. I'd like to test the results in the real world.

 

This is interesting.

Anecdotally, I can add that I just removed my LEER 100XR Camper Shell from my 2021 TRD Sport 4x4 Automatic V6 and saw an approximate 10% increase in average gas mileage when I took it off.

16 MPG w/ camper shell.
18 MPG w/o camper shell.

My daily commute is about 80% highway and these are consistently my general averages, +/— 0.5 MPG (usually minus) depending on conditions.

 

What are your thoughts on a tonneau cover vs open bed?

Intuitively I always felt like the tailgate would act as an "air brake" and create drag, but according to a Google search removing the tailgate entirely is worse.

So I wonder how a flat bed cover compares...

 

I was thinking about getting a RTT but leaving it in a fastback configuration when driving. Mostly like a tonneau cover. Having it on hinges so I can lift it up and level it when in use. They all hang over the back a few inches and would look dorky but it may help it aerodynamically.

 

Although I know this is an old thread that was recently bumped, I'll contribute to the revival.

2020 Pro
265/75r16 AT4W
No lift
OTT
LEER 100XR
80% highway @70mph

Before cap with open bed I saw an average of 17 mpg
After cap I see an average of 16 mpg