Potential mpg loss % with custom aero trailer.

[freebeard]

I’ve considered a centerline cross flow blocker.

Lower right, Vortex Traps. Passive Porosity in lower left should include perforated base plates.


http://i1271.photobucket.com/albums/...titled7_22.jpg

[Gschuld]

Yes, vortex traps. My terminology was not right. I’m still uncertain what would be the most beneficial overall when accounting for cross winds. It’s more straight forward (pun intended) aero wise when not considering cross winds.

And the lower picture in the post above gives an idea of what I could incorporate into the Jeep SRT back end. Nice example thanks.

Ok, dumb question. My computer skills are lacking here (I’m an old school mechanical drawing table guy rather than a CAD guy, unless we are talking about cardboard aided design)

In another forum or two I frequent, there is a section at the bottle of each post being written that directs me to images I pull from my photo collection(I’m doing his from my iPhone 8). It works great.

I’m not sure how to post images from my photos here. Again, I don’t use a laptop, or tablet, just my iphone8 and my thousands of images from my albums stored on the phone. I haven’t used photobucket or any of that.

I read through a thread on posting images here, but it all appeared to be PC based.

I’ve surpassed my minimum 15 posts, so I’m free to post images now. I’m just not sure how yet here.

George

[Gschuld]

OK some more data. I used my 24ft, dual axle, 7000lb capacity trailer, strapped down 20 standard cement blocks, and weighted it at my local recycling center. The trailer has a 6’ wide welded steel frame with 2x12 pressure treated deck boards with 1” gap between them all, 8’ width at the tires (205/75/15 Loadstars). Bottom surface of trailer is open steel channel right up to the underside of the deck planks(draggy). The concrete blocks were strapped down 1 block high in a 5 wide, 4 long pattern. Total trailer weight is 2735 lbs.

Jeep Grand Cherokee SRT is 5400 lbs with me in it. So 8135 lbs in total with trailer. The highway had near zero traffic, rolling modest grade of 75ft altitude difference tops. All tests were 10 minutes duration or longer.


MPH. W/Tr No/Tr % drop

55. 23.2. 31.1 25.4
60. 21.3. 27.1. 21.4
65. 20.3. 26.4. 23.1
70. 20.5. 24.9. 20.5
75. 19.6. 21.9. 10.5
80. 18.2. 22.5. 22.5
85. 16.2. 21.3 21.3

Drafting efforts with trailer attached(percentage was based on nearest no trailer numbers.

69. 20.7. 24.9. 16.9. ACC off, kept 50ft distance, semi
69. 19.5. 24.9. 21.7. ACC on, 75ft distance from semi
66. 20.5. 26.4. 22.3. ACC on, 75ft from truck with 10ft tall stuff
62. 21.6. 27.1. 20.3. ACC on, 75ft from LG box truck

So my take is the only clear anomaly is the trailerless number at 75mph. That is a bit off the trend and is likely an error on my part(possibly not resetting the MPG when changing speed from previous test).

Also, drafting behind a semi, big box trailer, etc had near zero effect when pulling this combo. I suspect the trailer was at the convergence of the wake of the semi/box truck and neutralized any benefit I was getting without a trailer in previous tests. There was a clear and repeatable improvement in mpg in previous test when drafting at 75ft (ACC on) with no trailer, from 60-75mph. It got slightly worse mpg actually with the trailer drafting at 75ft. Even taking the ACC (adaptive cruise control) off and carefully drafting to 50ft distance from 75ft, had little effect and improved mpg only marginally.

So, as a whole, assuming I can produce a trailer with a SUV/trailer combo with a bit lower drag than that big open trailer (I’d like to think so), I’d be pleased to improve on the numbers above by a small margin.

At 75mph, 19.6mpg currently with a 2735lb open trailer. If I could break 20mpg average at 75mph pulling 2700lbs of trailer I’d be thrilled. Assuming the no trailer mpg was really 23mpg, 20.5mpg would represent an 11% loss from running the same 75mph without the trailer. Tough, but plausible maybe?

So far it’s all pretty positive news from where I’m standing. And happily inline with what I was theorizing (guessing) was possible.

George

[freebeard]

You now know more than the rest of us.

How are you situated for space, tools and skills for fabrication? Do you think you could do better than an U-haul trailer with vortex traps, spatted and skirted pontoon fenders and a trailer tail?

I don't think I've posted my teardrop low-poly template trailer yet. Have you seen it?

edit:
Hey, you could rent different sized U-hauls (and do some Cardboard Aided Design) to see the difference that height and width makes.

[Gschuld]

Yes, I’m absolutely confident I can beat a box style UHaul trailer modified as you described if interior volumes matched(UHaul 6x12 would be closest). The small UHaul enclosed trailers are pretty good starting points aero wise. Nice rounded edges, etc. When modified even better for sure.

I haven’t seen your design. Please share….

Starting from scratch offers me a significant advantage.

Tools, space, skills. I have a sufficient amount of all 3. I’m a wooden boat builder/restorer with 25 years experience in the higher end of the market. Wood, metal, carbon, composites, etc. Plenty of experience with each.

My limits on what I can build are more constrained by justification of the cost/time to entertainment ratio. Far more than dollars and cents FWIW. For example, I’m not inclined to build a full production quality negative mold of a compound curved trailer and build a one off trailer with a vacuum resin infused carbon/honeycomb/carbon method. That would represent an outrageous expenditure of time and effort for such a one off project. And no I’m not inclined to go into the custom trailer business either.

Wood is my preferred medium visually as few things to me are more pleasant than a varnished interior of a boat (or enclosed trailer)

For a standing height trailer, I’d consider 1/4” birch aircraft ply with 1” Divinycell foam over it(rounding the corners) followed by 2 layers 7oz glass cloth set in epoxy. Prepped smooth, and sprayed with an automotive finish. Varnished inside, well insulated, lightweight. Well suited to one off construction.

An expanding height trailer gets a bit more complicated, and a bit heavier. But lower drag potential, and more challenging and unique.

George

[Gschuld]

https://www.uhaul.com/Trailers/6x12-...ler-Rental/RV/

Maximum recommended speed is 55mph. Ha. When I was testing today down to 55mph, guys were wizzing by doing 80. Most at 75mph. I felt like a rolling roadblock in the slow lane.

George

[Gschuld]

Ok, some numbers in practical terms for my intended usage.

Races next year that I’d like to attend(in distance order, one way): (I’m based in coastal NJ)

654 miles Kershaw SC
768 miles Roebling GA
777 miles Gingerman, South Haven Mi
860 miles Tally in AL
885 miles Barber, outside of Huntsville AL
910 miles Blackhawk, South Beloit IL
3006 miles Laguna Seca CA

Laguna is a long shot(long drive)

Examples of time/gas $ spent based on the info above assuming I could match the figures above with a custom trailer(I’m cautiously confident). Based on $3 a gallon fuel cost for convenience.

Let’s take Blackhawk at 910 miles.

At 60mph 15.1 hours each way, $128 in fuel, $256 round trip
At 75mph 12.1 hours each way, $139 in fuel, $278 round trip (+$22)
At 80mph 11.4 hours each way, $150 in fuel, $300 round trip (+$44)

So several factors stand out to me. I can comfortably stand 12.1 hours one way, but 15.1 hours is pushing my comfort zone to be alert and ready for race practice the following day. I’d likely need to stay over meaning the round trip would have me losing 2 more full days of work. That will cost me FAR more than the $22 extra fuel bill.
80 mph is quite nice too, but I don’t expect to average that speed. But it’s nice to know that there isn’t much of a penalty.

The potential BIG trip. Laguna Seca

At 60mph, 50 hours each way, $423 in fuel, $846 round trip
At 75mph, 40 hours each way, $460 in fuel, $920 round trip (+$74)
At 80mph, 37.5 hrs each way, $495 in fuel, $990 round trip (+$144)

At 60mph, 50 hours means 3 16.7hr legs or 4 12.5hr legs. So 4 days each way
At 75mph, 40 hours means 3 13.3hr legs. Maybe …3 long days each way.

Going faster(75mph) makes more sense in my situation IMHO.

Just for giggles, If I’m at 19.8mpg at 75mph, that’s $920 to do the 6012 mile round trip to Laguna Seca. My buddy Matt at 7.5mpg at 75mph(verified over multiple trips) would spend $2405 for fuel vs my $920 for the same round trip, same speed, carrying the same stuff!

So, if we both did the same events from the same starting point, it would be 15,720 miles for the whole schedule, 9708 miles if skipping Laguna Seca.

Matt would spend $6200 and $3883 respectively in fuel for the year
I would spend $2381 and $1470 respectively in fuel for the year.
I’d be spending $3907 less and $2413 less in fuel respectively.
Plus driving a trailer that I’d hardly feel back there making the trip far more relaxing. Matt would be fighting cross winds, semi wakes, and the extra motion with additional overall frontal area, height, and total trailer weight.

Agreed, it’s no hypermiling achievement.

But I’d be getting not far from 3 times the fuel mileage at the same speed, carrying a near identical live load weight and size.

That makes me smile a bit. And makes the project easier to justify.

George

[freebeard]

This is as far as it ever got. Photographed and crushed. It was an interpretation of the Roswell design I did post. And I did post the frame (based on a silver Streak Clipper i owned). Today I'd use a Gothic arch.

Stitch-and-glue plywood would be a good choice. I experimented with Polymetal.



The thing I like is it's prefinished. So construction could be really fast.

[Gschuld]

Oh right, you posted the model pics earlier. Neat concept. Stitch and glue, yes, well a composite variation on that method anyway. Bravo, not many know what the stitch and glue method is. My method can be done completely frameless on the inside of the trailer at that size. Like half an octagon in cross section at max width/height. 6’ wide at the walls, 4’ tall side walls, followed by 3’6” width of panels canted inward about 30 degrees, with a 4’ wide flat top. Tapering at the front, less so at rear. All with 1” structural closed cell foam bonded to the exterior with 2 layers glass cloth over it. With tail cone to match.

I’ve done something with that method before. It came out great. Frameless, light, and very stiff.

I could finish it in clear coated carbon fiber. That would sparkle nicely in the sunshine. Ha. That’s a little over the top for me…

George

[freebeard]

https://www.youtube.com/watch?v=oXz6BLoQsQI

Youtube videos don't load at this time in this location; maybe you can see this?

From the thumbnail, the inlaying/fingering is impressive. Turning that inside out and building out from there would be a nice approach.

Just for sake of completeness, here's a formless construction technique [I've only done in paper]:



Bigger pix in my album [humblebrag]. Even though it's laid out in triangles, pulling the gores together generates shallow cones.

This was done as an aeroform for the Arcimoto FUV.