A New Aeroshell- and Lessons Learned

[rrhatbruce]

First, thank you all. I’ve been lurking for about 6 months, learning everything I can, and appreciating all the knowledge I can draft off of. Getting sucked along, knowledge wise, is a good thing and has helped me a lot.

I’ve wanted an aeroshell since at least the mid-eighties, when I realized that a shell could be shaped to dramatically improve mileage. Stumbling onto Eco-modder has increased that conviction and after gathering all the info I could, dove in.

I started with a used shell from Craigslist for $45. Someone suggested cutting one down and I believed that was a reasonable plan. Lesson #1 was that I should have pilfered it for the hardware and thrown the shell away. It has turned out well, I think, but I could have made it much lighter, with a faster build time by making a wood frame and covering it with FRP bath panel sheets instead of trying to repurpose an old one that really didn’t fit in the first place. But that’s for the guy behind me to profit from. A big reason I modified an existing shell is my wife had once expressed extreme hatred of the smell of Bondo/polyester resin permeating the house (I’m no fan either). The wood and FRP version would have needed extensive work in that vein.

There has been discussion about the ideal angle for airflow to remain attached on aeroshells boattails, kammbacks, etc. with angles from 12-22 degrees being mentioned as workable. The 12 degree number seems preferred but if I ran a straight line from the cab roof to the tailgate it was already 16 degrees. I wanted to wait until past the side window before dropping down (Lesson #2 by the way, and another reason to do the wood and FRP version, I wouldn’t have had the window frames to dodge that way.) The chosen solution was to incorporate a large radius and hopefully gently persuade the airflow to remain attached and head down at a 19 degree slope. I also cheated by extending the back of the aeroshell and raising it up, reducing the slope angle. As built it is 18 degrees and while the data is still incomplete and inconclusive it does seem to be working; now.

This week, after over a month of work, it was finished enough to mount on the truck and follow me on my daily commute. A quick test drive didn’t seem as smooth and easy as I had hoped but I reserved judgment until going to work. Arriving I checked the trip mileage on the scangage expecting an improvement only to find my average had actually dropped! AAuughhh! Much wailing and gnashing of teeth! What have I done? The cursed thing seemed a parachute behind me instead of coasting down gently.

It could be better than that, I had proved it could. I had it on earlier and with all openings paved with cardboard and taped shut I ran a test route and got 24.5 mpg, up from 20.2 with windows and cab-shell gap open. Now I’m note even breaking 20! Two days of sulking and inconclusive testing led me back to the side window frames and hatchback edges. Lesson #3, watch those edges! I hadn’t realized just how much a ¼” high plexiglass edge, running the full length of the hatch would trip and confuse the airflow. Taping those and the side window frames up put me back up to 23.7 mpg on the test route. Even better it would actually coast now, instead of throwing me forward against the shoulder harness. So last night I glued some ¼” moulding on either side of the hatch and caulked the window frames much better. This morning’s trip actually showed a decent improvement (I’m waiting awhile before posting numbers. The morning commute is tough to get good mileage on, especially since the school down the street restarted Wednesday.)

Overall I’m encouraged. There are more improvements to be made. I need serious help on my cab to shell gap seal for instance, anything short of duct tape just lets it flop madly. Follow my fuel log for awhile and we’ll both see if I know what I’m doing!

PS. It seems I'm unable to link to photos from my photo album on this site. Go there for a selection of photos showing what I'm talking about. Thanks,
Bruce

[rrhatbruce]

I just realized I had my laptop, with pictures, out in the truck. Here they are.

[MetroMPG]

Hi Bruce -

Welcome to the site, and congrats on some nice looking work!

Any plans to do a cruise control A-B-A test on your new shell? I'd be curious to see a "controlled as possible" comparison.

Darin

[robertwb70]

Nice, I have a couple of trucks too but they generally only leave the yard when they are loaded.

Looking at this website I think you might be able to implement some of their research into your design- check out the pictures near the bottom and the chart quantifying their improvements- Aerodynamic Teardrop Trailer

I found it especially interesting that the hump showed a reduced wake behind the trailer even though in the flat top version the air was re-attached long before it gets to the back already.

I've seen some people here that are absolutely opposed increasing frontal are but this data clearly shows that increasing the frontal area by 12.5% reduced the force of drag at the same speed by 36% so I'd say the shape in more important than the size (at least in this case).

[winkosmosis]

That is really nice. Matches the lines of the Tacoma perfectly.



robertwb70, that teardrop trailer is like the teardrop roofs on boxy vehicles I was asking about the other day

[rrhatbruce]

Any plans to do a cruise control A-B-A test on your new shell? I'd be curious to see a "controlled as possible" comparison.

MetroMPG: Yes I've got lots of plans. I've also got plans for Cd coastdowns and tuft testing to confirm or deny airflow attachment. It will be mid-September before I have time though. I have a class next week, then vacation after that. I will get lots and lots of miles but no ability for those 'controlled comparisons'.

robertwb70 Those trailers are... different. I like them but I'm not sure how they would fit with American trucking styles.

[bondo]

Very nice work rrhatbruce and welcome to ecomodder. I know how many hours you put into your aerocap and the results of your hard work show. Excellent job! I really like the big side windows and the clear roof takes care of any blockage of rearward visibility.

Those fuel figures will come. It is in the average you see the true gains which takes time. There are so many variables which cause the mpg numbers to change from day to day, wind direction and speed, air temperature, etc.

Again, congratulations on a fine job. Do you need any part time work?

Bondo

[Big Dave]

Let me echo what Bondo said.

A few observations.

1) You'll probably want to do a beta version reducing weight and improving details. I haven't yet but that is due to good results and my own laziness.

2) Insetting the lid maybe a quarter inch or so does not foul up the air flow and eases fabrication.

3) As you know, windows are a complication, and I question just what you can see through rear windows laid over that far.

4) Good MPG tests have to be fairly long (I used at least 1,000 miles) to average out variations in weather, traffic, driving style, and filling inconsistencies. Like Bondo said, as time goes on the result will improve.

[aerohead]

rrhatbruce,nice work! She's a beaut.With respect to "angles" you might want to check out the "aerodynamic streamlining template".You can see it being used at MetroMPG's "permanent Kammback for the Metro" thread.------- It's really more about degree of curvature which will sustain the final exit angles.Your Tacoma's bed is so short,it may not be able to support such a "steep" angle at the back as 16-degrees,and you may have separated flow,which will frustrate your mpg improvement.

[skyl4rk]

That looks great! Now build up a tail that comes to a point. Use the trailer hitch receiver and a few pieces of steel to form a base of plywood, and use coroplast to match up against the tailgate. Look at a trailing edge of an airplane wing for design ideas.