Silverado aerocap build thread

[Big Dave]

I do all my testing between mid-May and mid-September. The day-to-day variations around here are smaller in summer than in winter.

[bondo]

I get my best numbers here in Arkansas from about the first of April till about mid November. When air temperatures drop below 40 degrees, so does my fuel efficiency. Below freezing, it is terrible.

Since all vehicles are subjected to the same conditions, with or without aerodynamic enhancements, those with superior aerodynamics will still get better fuel efficiency than a vehicle with out any aerodynamic modifications. In cold weather, this percentage will be lower.

Aerodynamic add on devices like boattails and aerocaps do produce a larger reduction in aerodynamic drag because of their greater surface areas over that of a front air damn, wheels skirts and other smaller aerodynamic add ons. All of these devices will see a drop in effectiveness in cold air.

The thing to always try to design into any product is functionality and utility. With the areocap I constructed, this was a driving force in the design of the article. My plans were then, as they still are now, to get the product to market. There have been some excellent aerocaps built by others on this site which are done very well along with boattails and other devices which show remarkable reductions in aerodynamic drag. However, in the real world, you have be able to get into your truck bed or be able to park your car in a parking garage and also adhere to 5 mph bumper regs , third brake light and other DOT specs.

My product is intended to replace the old square camper shell, which has poorer aerodynamics than a baseline truck with nothing over the bed. A truck running a squared off camper shell in very cold weather will get worse gas mileage than a truck running an aerocap in the same conditions, the percentage of difference between the two will be less. In warmer air, it is a different case all together.

Bondo

[KamperBob]

I found this group recently and am slowly catching up on various threads. Before going any further I take my hat off to everyone who is sharing. This is awesome.

It seems like many members have some domain expertise. It also seems like most of these eco caps focus on side view profile. My engineering classes did not include aerodynamics but I don't consider myself completely ignorant of practical experience and would like to think my intuition is pretty good. My observation is that trucks (all vehicles really) have two sides and one top. How much air is displaced sideways versus over the top? I know, I know, it depends. Of course. But, air has three main places to go -- left, right, over. I'm dismissing under as the smallest path due to resistance from the road boundary versus free space on the other three sides. So does more air get pushed over the one top than the two sides combined? Doesn't at least some of that air try to close back in over the tapered wedge as it goes by? Isn't that what sets up the corkscrews typical when following pickups in light snow for example?

On a unrelated note, does anyone happen to do CFD work using MATLAB? How about building a small scale wind tunnel to do your own validations on models that a guy on a hobby budget can afford?

Cheers
KB

[bondo]

Welcome to ecomodder KamperBob. This is the best site on the web on the subject and you will learn alot here as I have.

On your questions, I will defer to others to maybe explain about the air flow over the top opposed to the sides of a pickup. Also, on the computational fluid dynamics, even though I have sculpted models which were scanned for CFD, I have never ran a simulation myself. There are others here also who may be able to answer your question about the CFD program.

There is some good information on the web on how to build a wind tunnel but they can get expensive. The following link is from a couple of students at Western New England College who built a water tunnel and tested a pickup truck model in it. Their resulting Cd numbers are real close to the real world in scale so decent data can be gained from a simple wind/water tunnel.

Did you Know

Bondo

[KamperBob]

Thanks, Bondo. You caps are sharp. The lid is great for utility. I've read some of your struggle going to market. I don't know where you are today but I wish you every success!

Cheers
KB

[KamperBob]

Originally Posted by bondo

Bondo, that wind tunnel pic is excellent. It's not lost on me that it probably cost some cash and/or favors to acquire. Much appreciated!

If I read it right, it seems to show air from beside the cab rolling up over the side and down the top of the cap as the wedge decreases height down wind. That part seems perfectly intuitive. I guess your rounded edges and gentle taper are allowing that air to mix from side to top in a controlled way. You made an excellent design decision to trade off a little aerodynamics for a lot of utility. Otherwise, the ability to lift the lid and access the entire bed of the truck would be greatly hampered. In engineering, perfect is the enemy of good enough, and your design nailed that. Keep up the great work.

Fubeca, apologies for bending your thread. This is supposed to be about your cap I realize. And I don't mean my compliments Bondo's design to be any kind of dig at yours. Rather, I suspect your design goals differ, and perhaps both succeed. Your bang:buck ratio may be the highest I've ever seen. So, do I gather correctly that your bed is not used much? To haul something in the back you would just unclamp and removed the cap, right?

Thanks
KB

[bondo]

Accept my apology for filling your thread with selfish defenses. Agreed you got the best cost effective aerocap yet and your surfaces are spot on.

Keep up the good work!!!!!!!!!

Bondo

[MetroMPG]

Originally Posted by Fubeca

I'm curious as to whether my mpg difference will be consistent as the weather warms up or if when the weather is warmer I might see something more like 10% increase with the topper.

I think your relative efficiency increase will remain close to 4% regardless of ambient temperature.

Phil can correct me if I'm wrong, but when he said his cold weather aero enhancements didn't really "show up" at the pump until the return of good weather, I believe he was referring to his "previous best" (warm weather) MPG performance, not strictly a before/after comparison in cold conditions.

All else being equal, your absolute MPG will also be higher when the warmer weather returns, but the relative improvement of aerocap vs. open bed should be about the same (at the same testing speed).

[MetroMPG]

KB: maybe you've arleady seen this.... but bondo's is the Swiss army knife of truck caps! It opens like a hatchback, can be converted into a conventional square camper cap with the addition of side & rear curtains, and the upper hatch can be removed entirely for transporting tall objects. See this thread for photos: http://ecomodder.com/forum/showthrea...d-cap-583.html

Also, regarding flow effects, you may be interested to view on-road tuft testing of a tapered bed cap: http://ecomodder.com/forum/showthrea...per-11522.html . That thread somewhat addresses the issue of the flow transition from the side to upper surfaces (in that case a sharp transition, more likely to trip flow, yet it still appears to work well - though the upper angle is shallower than most).

[Fubeca]

I mostly use the bed for carrying 4 x 8 sheet goods or other lumber. The Chevy bed has accommodations for placing a board across the bed near the back at about half the bed height. I can open the tailgate and side the wood into the bed resting on the board at the back and the bed at the front. Then I just wrap a strap around it to keep it tight.

For moving taller items, I would have to remove the cap or I use a small utility trailer. If I end up satisfied with the topper, I may do something a little more permanent and with more utility like Bondo's masterful design. This is a temporary design and build that sacrifices some utility for quick and cheap.