Potential mpg loss % with custom aero trailer.

[Gschuld]

My name is George. I’m a consistent lurker, and a first time poster. I’ve thoroughly enjoyed the many threads I’ve poured over on this forum and it’s clear that many of you have much to be proud of in your own aero accomplishments. I'm starting to dive deeper into the Aerodynamics of Road Vehicles textbook(4th edition) and enjoying it as much as my old Aerodynamic Drag book from Hoerner.

I’ve been doing background research into a potential project that may interest some of you. It was certainly partially inspired by some of you. I apologize in advance an email for what is sure to be a long post in an effort to give a good picture of what I’m after and why.

I’m looking to embark on roughly 9000 miles worth of 6 round trips from NJ to vintage motorcycle road race events (AHRMA) spread around the US next year.

My tow vehicle is not what one would generally consider a fuel friendly choice.

A very low mileage garage kept 2016 Jeep Grand Cherokee SRT. Yes, the 475hp 6.2 liter, 8 speed trans, AWD Hemi muscle SUV with towing package(7200lb capacity). It’s fun to drive and is IMHO and delightful road trip machine.

This vehicle sports a nifty list of systems and performance data displays. Such as an ECO mode(don’t laugh��) that turns the 6.2 into essentially a 3.1 liter in “4cyl” mode. And the adaptive cruise control is fantastic, along with the on the fly ability to reset the average MPG and reset the cruise control at different speeds for x amount of time so I can get a decent idea of mpg in different situations and speeds on the same trip.

I’ll have to verify the accuracy of these tools of course but here are some general results (ACC is Adaptive Cruise control below). Everything done on the NJ Garden State Parkway with near zero total elevation change but fairly constant gradual hill variations of about 100-150ft elevation fluctuations.

65mph ACC, clear ahead, mild traffic, 14 minute duration, 25.6mpg
65mpg ACC, 75ft behind semi with open deck and a few 5ft tall pallets of whatever, no traffic, 10 minutes duration, 29.3mpg
65mph ACC, 75ft behind box semi, 10 minute duration, 10 minutes duration, 32.5 mpg
70mph ACC, clear ahead, moderate traffic, 10 minutes duration, 26.1mpg
75mph ACC, clear ahead, no traffic, 20 minutes duration, 24.3mpg
75mph ACC, clear ahead, moderate traffic, 10 minutes duration, 22.3mpg
75mph ACC, 75ft behind big dump truck, mild traffic, 20 minutes duration, 27.9mpg

So I can easily temporarily trade enclosed trailers with a friend for next season. I have a 24ft Haulmark 8.5 x 24ft dual axle race car trailer and he has a Pace 7x14 dual axle cargo trailer(still bigger than I need for 2 motorcycles and some tools and gear). But I’m considering custom building an aerodynamic trailer to test the possibilities, and satisfy my thirst for a challenge to see how much better a custom solution can be over the status quo.

I’m currently looking at a theoretical trailer 16ft long overall, 5’6” wide up front with a full round nose (in top view like the Aerovault) widening to 6’6” at the single axle wheels(tucked inside), then back to 5’4” wide at the tail end in a smooth curve front to back. 5’3” tall from ground flat top with a 3” radius from top to sides. The bottom skinned in aluminum or fiberglass smooth. 8”-9” ground clearance on 3500lb torsion axles and a 3ft curved box tail approximating the truncated Dryden tail that hinges away as a unit.

Rear trailer entrance only, perfectly smooth surfaces all around. No protrusions, bumps, rivets, etc. Even the wheels are within the body with covers at 6’4” wheel track outside.

The trailer height would be 4” below the trailing edge of the back of the roof on the SRT for good flow transition. The front of the rounded (in top view) trailer box would be 24 inches from the Jeep bumper surface allowing 50 degrees of turning angle per side. Total trailer weight with race bikes gear and is currently calculated to be 2700lbs all up(Jeep SRT is 5200lbs).

I suspect the combination of max frontal area being near equal between TV and trailer, and what I think would be a rather low drag combined tow vehicle/trailer combo would yield a fairly minimal loss of mpg compared to running the SRT without trailer.

It seems that a 30% drop in fuel economy for pulling a reasonably aero considered mass produced standing height cargo trailer pulled behind a mid sized SUV is pretty good.

I’m aiming to be able to travel at 75mph(with proper speed rated trailer tires) where possible rather than the better overall mpg I’d achieve at 60-65mph. Most of the trips are at a distance where 75mph allows a reasonable one day run to events, where 60-65mph likely pushes me into an overnight stay each time. That adds 2 more days to each trip, 2 hotel stays, etc. So I’m gearing for 75mph knowing full well that aero efficiency becomes more and more important as speed increases.

My pressing question is what potential fuel efficiency loss could I achieve with a custom setup like described above. 20%? 15%? Less?

My rough target(assuming my Jeep SRT is feeding me accurate data, yet to be determined) is 20-22mpg at 75mph on flat terrain pulling that hopefully very efficient 2700lb trailer and up to 24–25mph at 75mph if I can get 75ft behind a big semi traveling at that speed. It seems theoretically possible, but a tall order.

FWIW, as a side note I restore, build, and customize wooden boats for a living(25 years experience). Metal, wood, fiberglass and carbon fiber composites are materials I have a lot of experience with. I have built many custom projects far more complicated than an aero efficient trailer. It’s more about justifying my time to build it rather than whether I can build it. Dollars and sense would have me using the free to me 7x14 dual axle Pace cargo trailer and just eat the likely significantly greater fuel consumption cost. Sometimes I just enjoy the project and challenge.

I’ll be away on a road trip to Michigan (going to support a friend who is racing) until Monday night. I’ll keep an eye on the thread when I can. I’ll be doing some
hand calculations on fuel used on the trip to compare to Jeep calculated numbers.

Any and all thoughts are welcome. It’s a pleasure to be here.

George

[Isaac Zachary]

I wonder if you take your vehicle and plug it's numbers into the Aerodynamic drag and rolling resistance calculator to see if you can get an idea of what's all going on with your current aero, rolling and efficiency.

Then take the same aero numbers and just add more weight to the equation, the weight of the trailer. That would give you a guess at how much more fuel you'd be using with just the weight increase alone.

Maybe someone else could chime in but there could be the possibility of improving the coefficient of drag, or you could hurt it. You're right for trying to keep the frontal area the same and then tapering back.

[freebeard]

Welcome.

Any and all thoughts are welcome.

Sounds like a nice tow vehicle. Does it have Mercedes parts, similar to my son's 2010 SRT8?

Have some sketches ready for when you hit five posts.

Light weight suggested a monocoque shell. I'll repost this in case you haven't seen it before.


Like an Airstream end cap laid on it's back. By varying the curve of the cuts you could make it boxier.



If you're into precise calculation there're superellipses:



A question in my mind is whether all the rear taper in plan with little or none in elevation would effectuate a rudder-like function.

[Vwbeamer]

my thoughts are that if the trailer is done correctly, there maybe very little loss in gas miliage and possibly a small gain at a steady speed. There will of course be fuel used to accelerate the trailer to speed.

[Gschuld]

Thank for the thoughts so far.

Yes, the Grand Cherokee SRT shares the platform architecture with the Mercedes equivalent.

And yes, I’m quite pleased with it. It is on the indulgent end of things for me, as I’ve grown used to older and more modest vehicles.

I took my 12.5hr trip from the NJ coast to South Haven Michigan yesterday. I was able to manage 21.8 mpg average on the first 482 mile leg heading out running from 65-78 mph depending on speed limits. There were not many truly flat sections, mostly slowly rolling up and downs. I spent the bulk of the time in the 78mph setting under a 70mpg speed limit. It matched the measured mileage/fuel used mpg I calculated by hand compare to the dash data display when I fueled up within a very small margin. That is encouraging.

While running the flat sections at 78mph, 24mpg was about the average. The rest was a bit over 21mpg.

I did find that most of the semi trucks were sticking with 65mph or so, not helpful for me to draft behind. I had better luck with the occasional lead foot 5th wheel RVs, Big motor home RVs, and trucks pulling big cargo trailers running 70-75mph. I was able to set the ACC (adaptive cruise control) and shadow behind them for a stretch at about 75ft behind. The ACC did a great job keeping a constant matching distance behind. The mpg numbers jump 3-4 mpg consistently for the rare times I was able to locate one going fast enough to draft behind for a bit.

I put what info I knew into the Aerodynamic and rolling resistance calculator, adjusted to get for 24mpg at 5200lbs (bare SRT, flat road, 75mph) and added the 2700 all up trailer weight only and and up with 21.2 mpg.

21.5mpg at 5200lbs (bare SRT, rolling elevation roads, 75mph) when adding 2700 lbs came to 19.1 mpg.

So assuming the aero of the SRT/trailer combo is equivalent to the bare SRT(big if), real world roads would bring me from about 21.5mpg down to about 19mpg(probably a little less when accounting for the minor extra rolling resistance of the two trailer tires. That would represent about a 12% drop in overall fuel efficiency.

That would be great if true.

There is certainly greater aero potential than what I specced out for the trailer. The design was the result of my needs logistically(two race bikes loaded from the rear, equipment, tools, spare race engine, small generator, spare race wheels/tires, small air compressor, 10x10 EZ UP tent, camping gear, race fuel, etc.

Still, I think it may be possible to satisfy my logistical needs and create an SUV/trailer package capable of hitting my goal of 20-22 mpg on flat roads at 75mph, and giving me 18-20mpg on more typical roads(modest rolling elevation changes).

That’s really encouraging.

My friend has a new setup this year with a Ford F-150 super crew with the 3.5 Ecotec engine, pulling a standing headroom 7x16 v nosed dual axle cargo trailer(sharp edges all around) with about the same cargo inside (two race bikes and gear) as me with a 3600lb all up weight (his trailer is much heavier, plus two axles) and he also favors 75-78 mph on his trips. He reports roughly 21mpg truck only at that speed but a rather shocking 7.5mpg with the loaded trailer. He runs a tonneau cover, so the bulk of that frontal area of the trailer is fully exposed and draggy.

That’s a horrendous drop in FE and what inspired this whole thing. His first three trips came up with nearly identical numbers each, both from the dash instruments and the hand calcs.😳

9000 miles at 7.5 mpg at $3 gallon fuel is $3600 per year going to and from the races.

9000 miles at 20 mpg at $3 gallon fuel is $1350 per year. That $2,250 savings potential isn’t my the driving force behind my interest, but rather the challenge it represents and the potential uniqueness of the end result.

Anyway, I look forward to exploring this further.

George

[Hersbird]

Frontal aera is the frontal aera doesn't matter the shape. So an F150 has more frontal aera and therefore the trailer is better protected and causes less of an effect than it you were towing it with say a Sonic.
That said the 3.5 EcoBoost seems to be terrible under load. Like they are running it extra rich. TFL truck did some testing with the 3.5 hybrid vs a 5.7 Ram on the Ike pass towing 7500 pounds and the Ram got 30% better MPG. So making the same HP (because they both climbed the hill in exactly the same time towing the exact same trailer) the Hemi is much more efficient.

Now my Hemi doesn't do that great even going just a teardrop which is no bigger than the Jeep itself, 12.5 mpg, but it doesn't get any better than 16 not towing. Towing a big 8' wide 10' high camper it gets more like 8.5 mpg and that's keeping it under 65.

[freebeard]

Anyway, I look forward to exploring this further.

Plow the tow vehicle wake:



In another thread the subject of Peter Brock's 1971 VW panel van came up. Incidental to that in a Jay Leno interview, Peter Brock (he designed the Cobra Daytona coupe) let drop that he has a trailer company.

www.bre2.net/aerovault.info/breakthrough-construction/


Check out the construction photos. It has aluminum bellypan and side walls with fiberglass wheelwells and cap.

These are prolly $$$ but worth it. But they're 80" wide. A less expensive alternate might be the fiberglass trailer they were making in Oregon City a few six years back.



Here's one made from a real bus with the correct details (down to the jail bars). It would be 68" wide.


______________

I owned one of these for about 25 years:



The construction is really simple, a 5" tube backbone, 6" C-channel crossmembers each pinned to the plywood deck with a carriage bolt. Aluminum from there. Here it is laid out for a teardrop like the Roswell.

[Gschuld]

Originally Posted by Hersbird

Frontal aera is the frontal aera doesn't matter the shape. So an F150 has more frontal aera and therefore the trailer is better protected and causes less of an effect than it you were towing it with say a Sonic.
.

I understand what you are saying overall but I’m not sure if I fully agree with your first statement. If a cube and teardrop shape have equal frontal area, the teardrop shape will have significantly less drag.

Granted, the CD difference between a modern full size truck and a Jeep Grand Cherokee SRT is pretty minimal I’d think. I assume that is what you were getting at.

The airflow over a modern truck with a tonneau cover appears to leave the vehicle mostly just over the tailgate. Pulling a standing height 7’ wide cargo trailer, the airflow over his truck is running into about 30 sq ft of “new” frontal area. That has to hurt.

George

[Hersbird]

Originally Posted by Gschuld

I understand what you are saying overall but I’m not sure if I fully agree with your first statement. If a cube and teardrop shape have equal frontal area, the teardrop shape will have significantly less drag.

Granted, the CD difference between a modern full size truck and a Jeep Grand Cherokee SRT is pretty minimal I’d think. I assume that is what you were getting at.

The airflow over a modern truck with a tonneau cover appears to leave the vehicle mostly just over the tailgate. Pulling a standing height 7’ wide cargo trailer, the airflow over his truck is running into about 30 sq ft of “new” frontal area. That has to hurt.

George

I didn't say anything about drag, I just said frontal aera. Right though you do take the frontal aera times the Cd to get overall drag. So even a small poorly shaped object may have better economy than a large optimal shaped object. The Ford and the Jeep are pretty poor on both Cd and frontal aera so that's out the window.

Anyway, when towing, it's all going to be the frontal aera of the trailer and the Cd of the trailer. I'm just saying something big and high drag in front of the trailer is going to seem less effected by the big high drag trailer.

[Gschuld]

Oh for sure, if he had a raised cap on his truck taller than his cab, there would be far less “exposure” of the frontal area of his trailer.

The Jeep Grand Cherokee SRT is far from an ideal shape for aero by itself, but it doesn’t seems too bad overall if paired with a trailer that essentially matches it’s cross section with blended curves, and presumably a more efficient tail end of the trailer compared to the larger tail end of the Jeep.


George