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Old 01-11-2013, 03:10 PM   #11 (permalink)
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I'm with the aerodynamic kitteh. Your cap may be suboptimal aerodynamically, but it looks sturdy and practical, and there a lot of virtue in those two words.

I had a wedge cap for years and it wasn't movable. That's why I don't still have it.

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Old 01-11-2013, 03:41 PM   #12 (permalink)
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Quote:
Originally Posted by ChazInMT View Post
Indeed this is an excellent build, and is probably the strongest looking cap I've seen.

If you do a Gen2, you'll probably want to consider going at about an 8 angle, this would raise the back 6 to 8 inches off the top of the tailgate. It has shown this will have a good combination of Cd reduction and also reduce the lift coefficient considerably which will make your truck feel much more planted at highway speeds. Additionally you'd gain some under cap volume.

It is a common misconception that you want the back as small as possible, and while quite good, it is not ideal. You're probably getting 70% of the benefit of a very well designed cap.

Sorry to nitpick, you have done a great job. I just want this here as a guide for anyone else who looks in on this and it will help them design a better cap.

Thanks for sharing!

What did you use for a cover? Is that coroplast?

Here's a pic of my angle analysis. The wing shape is the Ideal Template overlayed on the truck. If you don't curve the top, it's best to go with less angle to compensate. The line is at 8.


And here's a link to a Bigger Version of the illustration.
Thank you for your input!
I might have to give it a try (when time permits).
The cover material is .040" black anodized aluminum.
I over built it.
I'd rather give up a couple .10th's in mpg than have to spend the time/money to rebuild something that wasn't structurally sound from the get go.
Must be a holdover from my racing days.
Nothing worse than saving a couple pounds on a car, then having it fall apart 'cause you built it too light! Like we always said though, In order to win you have to finish!
it's not nitpicking it's constructive criticism. There is a difference.
Nitpicking is is when you critcise with nothing good to add. See that a lot.
Bob
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Old 01-12-2013, 03:40 PM   #13 (permalink)
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"Wedge"

For a back of the envelope calculation,if your mpg was 17.53,and went up to 18.93,that's a 7.98% mpg improvement.That would suggest about a 15.97% drag reduction.If starting at Cd 0.41,your 'Wedge" would put you in the neighborhood of Cd 0.344.
Not too shabby for a single mod! And you probably saved at least 300 man-hours in fabrication time versus an 'ideal' cap.
I like it! Thanks for doing it and sharing the data.
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Old 01-12-2013, 05:31 PM   #14 (permalink)
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Quote:
Originally Posted by aerohead View Post
For a back of the envelope calculation,if your mpg was 17.53,and went up to 18.93,that's a 7.98% mpg improvement.That would suggest about a 15.97% drag reduction.If starting at Cd 0.41,your 'Wedge" would put you in the neighborhood of Cd 0.344.
Not too shabby for a single mod! And you probably saved at least 300 man-hours in fabrication time versus an 'ideal' cap.
I like it! Thanks for doing it and sharing the data.
Thanks,
your calcs for improvement are pretty much spot on for what I gained (difference) in mpg.
When I bought the truck I was getting 13.6 mpg so overall I'm happy with where it was to where it is.
Like everyone knows though the first big improvement numbers are the easiest, once you get down to the final "small" improvements it can be rather difficult.
Just like in racing taking the first steps for weight reduction are easy it's the last 20#'s that are a pain.
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Old 01-14-2013, 02:25 PM   #15 (permalink)
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Sure pleased to see this over here (also posted on Dodge Cummins Diesel Forum)

The utility of a truck might be changed some, but to reduce it would be a mistake, overall. This bed topper meets that challenge nicely!

It would be interesting to see how this type would do against a half-tonneau (where air pressure is equalized against the tailgate from both sides) as the very low cost of such a device (time + money) versus a higher cost (same) of the OP's design would appeal to truck owners wishing to keep functionality high, but to lower operating costs as well.

I'm no DIY'er of note (more a fixer/adaptor), but this design is the one for me.

Thanks for posting.

And, welcome!

.
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Old 01-14-2013, 03:01 PM   #16 (permalink)
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Quote:
Originally Posted by slowmover View Post
Sure pleased to see this over here (also posted on Dodge Cummins Diesel Forum)

The utility of a truck might be changed some, but to reduce it would be a mistake, overall. This bed topper meets that challenge nicely!

It would be interesting to see how this type would do against a half-tonneau (where air pressure is equalized against the tailgate from both sides) as the very low cost of such a device (time + money) versus a higher cost (same) of the OP's design would appeal to truck owners wishing to keep functionality high, but to lower operating costs as well.

I'm no DIY'er of note (more a fixer/adaptor), but this design is the one for me.

Thanks for posting.

And, welcome!

.
Thank you again.
So far as fabrication the only tools used that alot of people wouldn't have is a chop saw, mig welder, sheet metal brake and metal shear.
The last two COULD be improvised. The rest was done with regular shop tools nothing extraordinary.
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Old 01-14-2013, 04:25 PM   #17 (permalink)
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I thought of builing a similar cap for the ranger but have the front edge low enough so i could see over it , perhaps this would be equal due to the angle being shallower, and could save some material and maybe add a visor on the rear of the cab to push the flow back.
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Old 01-14-2013, 07:06 PM   #18 (permalink)
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And let the Gen2 version (if there is one) be where it "mates" with a hitch receiver mounted box that continues the the "topper" out further yet (for those reading this thread with an interest to their own).

A box of that sort would interfere with a weight-distribution hitch of the sort I have (maybe most), even via short extension for the box (lengthening the distance from TV axle to TT axles is a basic no-no for trailer handling/braking). But it would be hell for solo highway trips . . 28-mpg here we come!!

.
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Old 01-14-2013, 07:58 PM   #19 (permalink)
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Quote:
Originally Posted by slowmover View Post
And let the Gen2 version (if there is one) be where it "mates" with a hitch receiver mounted box that continues the the "topper" out further yet (for those reading this thread with an interest to their own).

A box of that sort would interfere with a weight-distribution hitch of the sort I have (maybe most), even via short extension for the box (lengthening the distance from TV axle to TT axles is a basic no-no for trailer handling/braking). But it would be hell for solo highway trips . . 28-mpg here we come!!

.
I'm liking that idea!! alot!
Never even thought of it but you got the gears turning even for this one.
It would be easy enough to use something similar to the hitch rack deals with an aluminum frame covered in .040 take it off when I need to tow!I think I know what I'll be doing in the spring!
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Old 01-15-2013, 08:48 AM   #20 (permalink)
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Nice build!

The skin is aluminum?

I literally take my cap off to you (sorry, no pix). That's a build level we don't see here very often. Coroplast is sturdy and all, but, dang.

I've noticed the gigantic gaping wheelwells with nothing in them for many years now, and the truck designers continue to leave them wide open. You'd think they'd offer the option, just some kind of spats or something for the taller-standing HD trucks.

Skin over that yawning crevasse and you'll be deep into the 20+mpg territory, I'm sure. Airdams, nut adjustment and you can be chasing Big Dave.
[edit]
Whoops, gas engine, maybe not. But still I think you're well on your way.

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