[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