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hat_man 03-12-2012 02:10 PM

Advice on aero-cap shape
 
4 Attachment(s)
Hello everyone,

I think I am going to jump in with both feet and try and build an aero cap for my little Ranger. I have a plan for materials but am wondering about the shape.

Trying to get the most out of this little gem of a truck has become a hobby now. I love telling people I am getting between 28-30MPG with only a few little mods. I am having to replace the bed after a slight mishap (got rear ended) and am going to use the old bed (it's still pretty close to its original box shape) as a template. Any final tweaking would be done on the new bed to get a good fit.

I hope these pics will explain my thoughts. These are not pics of MY truck but ones found on the web. Mine is a shortbed stepside though, but currently has the damaged bed removed. Which would be better? I am pretty sure they are all possible, just trial and error. The dimensions (line lengths) are only approximate. It's more to show the concept of the angles.

The other 2 things I am looking at doing is flipping the rear axle and getting some drop springs for the front. This would lower the rear about 4" and the front about 2". It rides 2" high in the rear now so this would level it out and lower it at the same time. And possibly swapping the stock 3:73 gears with 3:45's. I spend 80% of my drive on the highway at 55-58mph for 120 miles round trip to work. It wouldn't be very good in town and would require changing the speedometer gear on the tailshaft and the odometer gear behind the guage cluster, but I think the MPG's would be worth the extra effort.

Any advice or input would be greatly appreciated.

Thanks

yugomodder 03-12-2012 03:04 PM

Hi, this is a really good study on pick up truck aerodynamics.
http://csus-dspace.calstate.edu/xmlu...pdf?sequence=1

They have different modifications modeled in CFD to test how they affect drag and lift. Info on aero-caps starts on page 68

In summary they found this out about the drag.

Baseline 0.3453
Traditional canopy 0.3157
5degrees 0.2957
10degrees 0.2894
12degrees 0.2892
15degrees 0.2987
18.7degrees 0.3091

Now remember these aren't nice compound curves, but just a straight cut.
Now as you can see 12degrees yields the largest drag reduction, however it is barely any more than at 10 degrees, and a large amount of benefit can be seen at 5degrees. After 12 degrees however, the drag starts to once again increase but not to the point of the original configuration.


They also tested a 3D curved aero-cap which gave a drag coefficient of 0.2768. This is definitely the lowest drag option with a drag reduction of almost 20%. It also reduced the lift coefficient by over 40%.

My recommendation based on this info is to not exceed "the template" at any point, and if needed just taper less. Even at 5degrees you get a large portion of the drag reduction at 12degrees, as well as an even larger lift reduction. Try to get as close to "template" as you can for the largest drag reduction, and then do the same thing on the sides if it is possible.

Good Luck! Please keep us updated on your decisions/progress.

hat_man 03-12-2012 03:33 PM

Thanks for the info. I have seen the template but I don't really know how to overlay it onto another picture. I had heard that 12* (approx.) was optimal and was trying to find a way to evenly break it up. Maybe something like the 6*+6* picture but 5*+5*. Would be simpler to build and still be very good. I thought I had remember reading (somewhere on here) that air flow would detach (sp?) if the angles were to great between planes. (I think that's how I want to say that.) My thoughts were if the top of the cab were considered "level" or 0* then is a 12* deflection (?) like the first picture too much and not effective? I guess that's why I was thinking about multiple angles.

I haven't found the template for side angles/top view. Is there one?

BackroadBomber 03-12-2012 03:43 PM

I'm looking foreward to seeing your progress! I have a 94 ranger with 3.45 and believe me they're not that bad in town. Instead of a gear swap and lowering, I would look into finding an explorer rear axle with 3.45s. If I remember correctly they're axle-over-spring and the springs and all will bolt right in so you'd. be killing 2 birds with one stone, without the pain of tearing a rear end apart plus they were available with rear disc brakes, if that's your thing. As far as the aerocap, I can't really help you there I just take my gate off (easier) I would also love to see your grill blocks and future mods as you go. Good luck

yugomodder 03-12-2012 03:43 PM

I'll overlay some images for you. It'll just take me a sec to post them.

hat_man 03-12-2012 03:52 PM

Thanks. I'll ahve to get back to this thread later. Have to go have stitches taken out.

BackRoad...I was going to swap the entire axle. Just a gear swap is a nightmare. I had heard the 8.8 rear from an Exploder (haha) took some re-working and the disc brake swap was quite a bit of work. Trying for quick cheap and easy.

yugomodder 03-12-2012 04:30 PM

http://i1155.photobucket.com/albums/...template-1.png

Here is an overlayed image. It's a bit small so you might have to zoom in to see it better.

You can use measurements and angles on a piece of cardboard to see what it'll look like in person. You can take that same piece of cardboard to mark out the width profile. You'll still want to round the transition between the sides and top though. I wouldn't recommend going any farther down than the template as you'll see more drag than at template or slightly above, as well as less rear visibility. If you just use a slight curve you could build it a lot more like a conventional topper with a glass back. That will allow you to maintain rear visibility, as well as reduce drag and lift. That report has tons of good info on different mods to pick up trucks.

I hope your stitches go well.

hat_man 03-12-2012 07:52 PM

Thanks alot Yugomodder. After printing the pic out and scratching at it with a pencil it looks like maybe 5*from the cab back to about 24" down the bedrail and then another 5* for the rest of the length is pretty close. I'll post a pic of my ideas for the sides later tonight or tommorow. Bad paint drawings again but I'm no computer genius. I have to fight with 2 teenagers for computer time.

Thanks again

ChazInMT 03-12-2012 10:21 PM

http://i39.tinypic.com/i41st5.jpg


Bigger Pic

Here you go, I put the bottom of the template lower on this truck than what it should due to your pickup appearing to have a lift kit making it taller. By placing the template lower and making it bigger it compensates for this.

Notice you are shooting for a 10 or 11 inch "Lift Gate" above your bed. This makes for a 8 "slope" over all, with a 12 slope at the top of the tailgate/back of cap.

This will yield your best result.

skyking 03-13-2012 12:36 AM

Hat man, you have the perfect opportunity to make a custom bed/cap combo :)
The stepside looks cool and is not horrible aerodynamically, but using what you can get here, you can make it way better. Skirted rear tires, slightly longer bed, plan taper to a minimum 4' width ( a truck is not a truck unless it can haul a 4' wide sheet of whatever ;) )
Frenched in tail lights, or maybe modular ones that will easily move from the normal position to the boat tail you will design for it, along with the lighted license plate mount and stub bumpers.
Maybe a slide out stub bumper on each side with tail light built in?
Sorry, just dreaming out loud. Back to your regularly programmed show. :D


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