Advice on aero-cap shape
 

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

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.

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?

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

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

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.

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.

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

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.

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

Thanks to everyone.

Chaz...the pics I used aren't of my truck. Just to show the shortbed and have something to use with a paint program. All the colored lines are just guess work and to show a concept of 1,2,or3 angles used to make up the shape. I'm not sure what you mean by "lift gate". If it's the height above the tailgate then I haven't decided on anything really. It will be wherever it comes out. 12* straight from the cab would seem to cut way into the "template", where using 3 angles seems to more closely approximate it. Using the "something is better than nothing" philosophy, I think I am going to go for 2 angles of 5* each. The difference in the 10* slope is minimal from the 12* slope (thanks yugomodder) and only using an angle finder and my trusty EMT bender I might be off a bit. If I go over 12* then it's not so good, but if I shoot for 10* and miss then at least I'm in the ballpark for some decent results no matter if I miss +/-.

Skyking...I'm just a lowly electrician not a body and fender guy lmao. I am thinking about some rear skirts though. If I change the rear end to something with the 3:45's, I think some of the earlier model Rangers had rearends that were 1" narrower on each side. That's the ones I am looking for. That coupled with changing from 225 width tires to 195's and lowering the rear 2" should give me plenty of room for full skirts. If not I am thinking of gap fillers like on Cons Ranger. The stepside bed is only 44" wide so no plywood or sheetrock in there lol. I don't haul anything really anyhow. It was what was available when the old truck died. I didn't know anything about modding then. I'll try and post a pick of what I am thinking of for the side shape soon.

OK, refined version of what you're trying to do with self explaining captions.

http://i41.tinypic.com/krezc.jpg

Bigger Version of Pic

Hey, I just wanted to be first in line to cheer you on if you went that direction :)

I See what you are saying now, but I still have a question (silly as it may sound). Take a look at the pic and see if we are talking about the same thing. I was thinking it was the first 4* ( I had just quick guessed at 5*) and then 6* more to total 10*. Is that right?

It looks like it's between 18" and 24" back from the cab to where the angle changes.

Thanks for the excellent pics and overlays

These are my ideas for the frame shape.

The orange bed shows the tapered bottom frame. Wider next to the cab and narrower at the rear.

The purple truck shows the side idea. The numbers on the right are in degrees from horizontal. The ones on the left are the degrees of downward slope. The only reason for the 6*/10* thing is I think Chaz and I are talking about the same angle but I'm not sure how to write it.

At this point it's all just guess work. Except for the angles from Chaz. I am pretty sure that's what I am going to shoot for. I am open to all suggestions though.

Thanks to everyone

I'd leave the frame out in the back to try and smooth that flow there. The rest is great.

I think you've got the right idea. If you shoot for around 10 degrees and miss +or-, you're still looking at a huge improvement in fuel economy. I think you've got the right idea on the sides too. If what you're getting at is to first mark the profile for slope on the top and sides, and then go from that intersection down to the side of the bed for a transition, then that should work really well. It sounds to me like you've got blueprints in your head for how you're going to build it. You should be able to get the shape right, or at least close enough to get the benefit. The next question is, what are you going to use to build it? I'd suggest using some sort of transparent material at least for the vertical piece on the back to allow for some rear view.

Please keep us updated on your progress.

I had thought about that Skyking but was afraid that the air might get caught up in the space directly behind the cab, over the wheel well, and at the rear (somewhat). The cab is 63" wide (outside to outside) and the bed at the rear is 53" (outside to outside). If I keep the frame rectangle shaped as opposed to tapered this makes a 5" overhang above the red line in this pic. Wouldn't that be a problem? The tapered idea at least only gives me an overhang at the space directly behind the cab. That and I thought the tapered (boat tail?) effect was better than straight 90* lines. I am just guessing here but I thought I had read that here somewhere. Not saying you're wrong, just trying to remember what I thought I think I maybe read sometime in the distant past ago in some thread some guy posted in some forum here. LOL

I am anxious to get started but I know it will be a slow going process. Lots of trial and error with the emphasis on the error part for sure. But hey, that's what it's all about isn't it?

Thanks again for all the advice.

Thanks yugomodder. I am thinking about 1/2" or 3/4" EMT for the frame. It's a material I am very familiar with. 20+ years as an electrician and probably 20 more to go if the construction economy doesn't turn around soon. For starters I was going to use coroplast screwed to the EMT in panels so I don't have to fight the angles and transitions. I was thinking of a cheap pexiglass piece part way down the final (larger) slope with extra ribs added to give me a place to screw it to. I have an idea on how to make the bed accessible but I can't give away all my ideas, can I? LOL. I have to leave a teaser hanging out there to keep the interest up, eh?

Thanks for the help.

I've done some of the stuff you describe.

I general make you drive ratio as low as you can. I have 3.08 rear gears with a 0.8:1 Gear Vendor overdrive.

I lowered my 4x2 F-350 4" in front and 6" in the rear.

Both work.

Your truck presents an unusual opportunity. Having been recently whanged, you could rebuild your bed to taper in (in the plan view) about 5 degrees on each side. You'd have to shorten your tailgate and redo the side sheet metal but it would result in a true bullet-tail

Having overdone the taper I can tell you to slat your cap (elevations view down by no more than twelve degrees. To make the layout easy, use a 1-in-5.5 taper and chop it off at the back.

The distance you drive justifies almost anything.

Thanks Big Dave :p you've got me thining again. Hmmm.... use the hurt bed for the aerocap template to get a working model and make adjustments, then......hmmmm. Now my head hurts! Need to get out the sketch book again.

But on a serious note...does the 10* transistion go for the sides as well as the top?

It was 4° (Press and hold Alt while you type 0176 then let alt go for a °) plus 6° for 10° total. The break should be be cheated forward a bit to more closely follow the template, I didn't figure exactly how far back it was in inches but it looks to be about 40% of the way.


Yes. The sides should not taper in faster than the template


It all looks like you're on the right track except for the sides, you probably want to go from 60° to 30° front to back instead of ending up at 15° at the gate. I think that would optimize it just based on my "Spidey Senses".

Here's a true side view with the template laid over it. This will be more useful in actually constructing the structure than looking at photos with perspective.

http://i40.tinypic.com/u4p6u.jpg

Sorry I couldn't get a plan view- it appears all the other blueprints on the net are the Euro-spec Ranger which is very different from ours.

FYI I have an SAE paper here from Texas Tech (#881874) that says an Aero Shell should net you 10-20% drag reduction. This will probably translate to 5-10% MPG improvement. With that and a nice smooth coroplast belly pan you could probably get 10-15% MPG over the stock body.

I agree with skyking though. If you could build a metal framework off of the frame it would use similar techniques as you'd use for the topper, but on a larger scale. Skin it with coroplast, sheet plastic or sheet metal. Having a smooth side will help you a lot and custom building the entire rear half could REALLY help you out!

I've been re-thinking this idea. It's counterproductive to have a significant downward angle on the cap, without having similar angles on the sides of the bed. The downward angle is going to cause the top air to also move down at an angle, and it's going to react with the air moving on either side of the truck, and it's going to cause power-robbing vortices to form on either side of the truck's rear end. I've seen this happen with my own aerocap.

I'm going to design version 3 of my aerocap pretty soon, based off the master's Naval Post-Grad School paper that LT Williams wrote...

http://www.dtic.mil/cgi-bin/GetTRDoc...c=GetTRDoc.pdf

I am going to do this because, using LT Williams' numbers, he would be able to get a 13% increase in fuel economy using only his aerocap. This is better than the 10% I am currently seeing with my version 2 aerocap.

http://www.tom-viki.com/spgm/gal/Car...ewBedCover.png

Will there be strange vortexes coming off the rear edges of the bed? I've thought about this and refrained from it because the bed will protrude from the aero shape. With a generous radius on a normal aero cap it shouldn't be too bad. Can't be worse than an open bed!

T vago...That is the shape I had envisioned to start and then changed to increase the slope on the sides. I know nothing about aerodynamics and generally fly by the seat of my pants. The image you posted looks like the vertical angle at the top (nearest the cab) reamins fairly constant all the way down the sides. I "erased" some of your image and came up with this. With my bed being 10" wider at the cab than at the tailgate I'm not sure it exactly matches our "combined" picture. But if I made the rear match the tailgate width and the front match the cab width and kept the vertical side angle (still looks like 60* to me) the same for the lenght of the bed, and then the 4* slope for approx. 30" (in bed length) and then 6* more for the remaining 42", [whew, gotta catch my breath] how does that sound?

I'm not sure what the green area is, but it sure would make a nice cut out to be able to see the 3rd brake light.

Chaz and T vago...just to open up some discussion...I thought gradually sloping the sides inward (from 60* to 15*) would allow the air to flow better off the back. I remind you both that I know absolutely nothing about aerodynamics. It just looked "swoopier" to me. (You can both laugh now.) What about maybe like Chaz says and transition from 60* at the cab and graduating to 30* at the tailgate? The graduation being at the point that the top slope changes from 4* to 10*. I'll mark it with white spots on the pic. Is the problem with the air from the top (moving downwards) and the air from the sides (moving rearward) meeting up and causing problems? Does keeping the side angles at approx 60* for the full length of the bed negate this? I don't have a clue.

It sounds like I have the top slope pretty well worked out. Now the question is on the sides. I'd love to hear what you two think or anyone else that is interested. The more the merrier. Maybe someone else will be watching this thread and beat me to the punch and finish before me. LOL

And are there any thoughts on the other pic? If I keep the cover square and leave the 5" overhangs over the bedrails will this cause problems with trapped air or anything else? Or is my tapered idea better?

Thanks again to all.

p.s. Chaz...I tried the degree symbol thing and as soon as I hit the 1 it jumps to the top of the page. ??? Maybe the * will have to work for now.

p.p.s OK got it now. Grrrrr. Gotta use the number pad and not the numbered keys above the letters.

Ok so I goofed...try this pic instead.

If both the "ribs" on the white dots are at 60* and the "rib" on the yellow dots is 30* then it remains constant at 60* between the white ones and gradually transitions to 30* along the reamining length of the bed.

Does that help?

From an aerodynamics standpoint? Transitioning from 60* sides to 30* sides should be at least somewhat beneficial, as long as the transition is gradual and there are no sharp edges. From a fabrication standpoint? Could be a challenge.

straight vs plan-taper
 

The Ford/Texas Tech 'aeroshell' with straight sides was good for 20% drag reduction and 10% mpg @ 55 mph.
My shell,with plan-taper gave 13% mpg,so reverse-engineering yields the 26% drag reduction.
So the numbers are pretty consistent.Members just need to decide how much money and human capital they're willing to invest in a project.

vortices
 

I tufted mine and it showed nothing weird.The velocities must have been similar enough,top and sides,to prevent them.

As I am finally going back to work for a short time (re-fueling outage at a nuclear plant) I have decided to start with the mounting system for the cap. If it works like I had envisioned I will use it on the new bed and attach a flat cover for the time being. In my head it works well and I have started taking pics of the process so I can share them when I finish. I have not bailed on the aerocap idea in the slightest. The mounting system is much easier to work on and can be used (hopefully) for many different types of bed coverings. Working 12 hour nights sort of puts a crimp in my construction time and if all goes well a flat cover will help until I get the full cover finished. The material I use for the flat cover will probably go to making a belly pan of sorts or the wheel arch fillers that I want. I hope to order my sheet of aluma-lite for the temporary cover on tuesday. I think the mounting system will alleviate some of the problems with being able to access the bed once the full cap is on. It should work for anyone that is interested in doing a cheap and easy "tonneau" or "half-tonneau" also. Wish me luck. I'll keep you all updated.

why wouldn't the back top corner of the cap meet the back top corner of the box

http://i40.tinypic.com/u4p6u.jpg

You mean, like this?

You can see the aerodynamic template overlaid on the image of the Ford Ranger. Note how the proposed Ranger aerocap follows the roof of the aerodynamic template.

That template was devised by a hard-working fellow here who has done a lot of research and performed a lot of testing and modification. The template he devised has been shown to be the best general shape without having to do extensive computer modelling.

If we were to make the proposed aerocap to actually meet the tailgate, instead of following the aero template as it does, then aerodynamic drag would increase compared to the aero template cap.

thanks, where would i find a template for an 8' box or is it the same template and how do i get it over laid a 99 f250

Hm... Well, I could try my hand at photoshopping, or somebody else here could.

Hold on a few minutes.

Here... Try this:

http://www.tom-viki.com/spgm/gal/Car...99F250aero.png

images
 

t vago,you ARE aerodynamic kitteh!
Thanks for all these followup images you provide.great consultancy work!:thumbup:

what is the distance from the cab to the different degree changes or is there a web site that i can go to. thanks bud

Well, you could guesstimate by printing out that image, and using a ruler. Guesstimation will not by itself really affect the shape, as long as the general shape is followed.

thanks for all your help