Potential mpg loss % with custom aero trailer.

[freebeard]

Somewhere on the site was a post or thread about a white aluminum box trailer with a split-gate boat tail, but I haven't been able to find it. I used the C-119 as a surrogate.

Here's the closest thread I did find. ecomodder.com/forum: Which Trailer shape is better?? Pay attention to posts by slowmover. He got himself banned but took towing seriously.

[ennored]

Originally Posted by freebeard

Somewhere on the site was a post or thread about a white aluminum box trailer with a split-gate boat tail, but I haven't been able to find it. I used the C-119 as a surrogate.

I posted about that. Found it.

https://ecomodder.com/forum/436708-post34.html

[freebeard]

Thanks. That would be the one.

edit: If it were me, I'd make the profile a Gothic arch (less lift, especially crosswind) with a gantry in the top. The track would telescope outward with a traveling winch on it.

For the loadin' and liftin'...

[Gschuld]

Thx. Yup I saw that too. Nicely done. Custom steel frame, air bagged suspension, it’s impressive.

I have a sizable file set aside for inspiration and reference.

The Dryden tests suggested that the last 3-4 foot of the tail didn’t effect CD much compared to a “chopped off” flat end about halfway from initiation of the taper to the point. But coming to a full point has a certain dramatic flair. That flat area from a chopped off tapered rear offers a convenient place to mount the tail lights and plate though.

I’ve read many of Slowmover’s posts. His towing experience and knowledge is extensive.

George

[freebeard]

Except for the weight, a lift gate could serve as a ramp, porch or work bench.

[Gschuld]

I’d think a lift gate would be an unnecessary burden. The trailer is low ground clearance anyway. Getting bikes/tools out will be easy.

George

[freebeard]

I was thinking that aside from loading and unloading the bikes, it would be useful for seating or if it would raise up to ~32" as a workspace. Manually operated it might add [as little as] ten pounds.

[Gschuld]

Any design that includes a lifting roof would favor the entry point for the trailer being from the rear face. A lift gate at table height would block entrance to the trailer.

I like the thinking, but I can’t see how to incorporate it logistically.

[euromodder]

Originally Posted by Gschuld

But I’m considering custom building an aerodynamic trailer

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.

Rear trailer entrance only, perfectly smooth surfaces all around.
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.

Looks like you have it already well thought out, and that it could be a very efficient trailer

The front of the rounded (in top view) trailer box would be 24 inches from theJeep bumper surface allowing 50 degrees of turning angle per side.

Closing that gap with a splitter plate or trailer front tapering to a rather narrow "point" would also help

The gap is a big cause of extra drag.

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.

Keeping the trailer inside the wake of the car will be very effective from an aero perspective

But there's no escaping the weight

[Gschuld]

Thanks for the thoughts. I’ve debated the front end shape of the trailer quite a bit and the all important gap regarding the turbulent flow.

One limitation for me is the logistical ability to open the rear hatch of the jeep SUV. With the trailer inline with the Jeep, the arc of the lifting hatch requires the trailer nose be a mere 6-8” behind the ball. I have an extra 12” or so as it’s currently set up(but that distance was set because of the turning clearance limit) I do though have more room to close that gap near the hitch further.

What gives me some pause is cross wind effects. From studies that I’ve seen detailing cross wind flow, a rounded front (in top view) was considered very effective at both reducing sway and drag. Sway control in particular has my attention as my as designed single axle distance to hitch is on the short side and I sure don’t want it to be dancing around back there at 75-78mph. I chose to have modest little round around the roof edge because the top front of the trailer is 5” below the projection of the flow coming off the roof of the Jeep(in a 3 degree downward slope FWIW).
So it should, at 75mph, make the short jump to the roof of the trailer without too much hassle. I may be wrong. I have been considering ONE easily removable add on aero aid to the Jeep for towing. Creating a new much larger trailing edge roof lip. The current SRT specific one is extending 12” deep from the natural side profile shape of the back end. I could make a much larger one (in clear coated carbon fiber maybe) that easily clips onto the current one, would be about 30” long rather than 12”, with sides even with the back window edges with a shape coming up vertical from the back surface of the profile of the Jeep closing off as much gap as possible while retaining the clearance needed for trailer articulation. It would help smooth the airflow onto the trailer further giving the flow even less reason to go turbulent.

I’ve considered a centerline cross flow blocker. It could be made with a foam sweep seal and really bisect the air on both sides (removable for hatch access). I could also as you mention use that full rounded profile but bring it to a point at the center reducing the gap 12” farther in the middle (which would not effect my trailer turning clearance).

Weight, yes, no way around that. That additional 2700-3000lbs will have a cost no doubt. My hope is the extra weight will primarily affect highway mileage more negatively the more hilly the terrain is and the more traffic there is on the road. Hills and traffic require more acceleration events, where the extra weight really shows itself.

On flat, steady, no traffic roads at 75mph, I’m hopeful the penalty will be minimal if overall aero efficiency of the Jeep/trailer combo is near equal to the jeep running alone (plausible IMHO). This is also where I suspect the overpowered 6.2 475hp Hemi comes in handy. It has generally been my experience that(in steady flat road open highway running) overpowered vehicles tend to take less of a mpg hit on the highway when adding around 1/2 the weight of the tow vehicle in extra trailer weight compared to a more modestly powered vehicle. The idea being a big engine has to work only a little harder, and a smaller engine has to work much harder than their respective trailerless effort.

The wildcard, for me, is the “ECO” 4cyl mode on this Hot Rod engine. It honestly doesn’t seem to make much difference going in and out of that mode, based on the rolling mpg figures not changing hardly at all when it’s on or off. But I doubt the engineers would have gone through the trouble if it didn’t have some benefit.

When pulling my open 24ft flatbed trailer recently, with a low but moderately draggy load, the jeep was happy running in ECO 4cyl mode for near the same amount than without the trailer at 75mph. That was encouraging since at perhaps 2400lbs trailer weight, it pulled like nothing was back there.

Perhaps tomorrow I’ll take that trailer, load on enough concrete blocks to make 3000lbs all up, and do some highway 75mph mpg runs, then drop off the trailer and repeat the same runs minus the trailer to compare back to back.

I’m confident I can build a trailer that is more aerodynamically efficient behind the Jeep SRT than that big open dual axle trailer. So therefore (hopefully) I should be able to best, if only slightly, the mpg results for that set of tests.

I should have more data soon.

George