Receiver-hitch boat tail

[chillsworld]

Originally Posted by freebeard

Split tailgate.

Each side could swing open into a storage locker, with depth equal to half the tailgate width and a width equal to the length of the boattail.

That was my original thought too! I would rather the design not utilize my receiver, for a multitude of reasons. The first being, that this will hold my spare tire so I see no need for this to be removable once it is built.

Could split the weight between two hinges, allowing for a stronger structure and a heavier payload for storage of plus size tire and equipment. Would also allow for a true key lock on the split to gain access to the inside of the hatch rather than trying to reach between boat tail in truck to release the arm. BUT, my problem was I didn't know if the seam would create bad flow? Or worse yet, how would I seal it to prevent air flow into the structure and causing a "parachute" effect? It increases the number of variables, and would necessitate a more accurate design of both the structure and the shell. Of course, that's not that big of a deal. I've only started thinking about his in a serious fashion lately, it's probably far simpler than my first impression of it.

Example of load capabilities on split swing bumpers, they're used a lot on trail rigs and overland vehicles:


And the latching mechanism that could be extended via metal subframe to the rear of the boat tail (this could be hidden behind a locked hatch). I would probably secure one side to the bumper, an then lock the opposite to it... Like you do with double door in a house:


As far as mounting to a factory bumper goes, it's a bad idea... Very few are meant to handle the weight and even fewer are designed in a manner that you can weld the hinge to it. But you can weld a subframe that hides behind the stock bumper. Here is an example of one, I found it here Gray Goose Build Thread - JeepForum.com , but pretty much no information other than to show the following pics. I was kind of bummed there weren't more pics or a proper write up on it

He unbolted the factory hitch, and welded a cross beam that goes in the bumper and two runner welded to the arms which bolt to the frame. Pretty clean/simple once you determine correct placement.





his simple description:
"It's an A to Z hinge (off road fab company), 3/4" bolt with a "t" welded to it to latch the carrier"

And the amazing "factory" look he was able to achieve with this design (even color matched the swing away arm):





I would imagine this wouldn't be cheap... But cost would probably be far cheaper when comparing what you get. The hinges are $52, add $25 for a larger HD version. So on a split version, you have $100 to $150 in the hinges (Tire Carrier Swingout Heavy Duty Hinge Kit) . Latches can be purchased in various sizes and costs, and metal costs would be determined by type and source. But considering the high cost of thule slide aways, and hitch mount versions, I think it would be worth it. Plus it would allow peace of mind knowing it can handle ANYTHING you would build onto it. It might be overkill for some or even most of you guys, but I spend a lot of time driving forest service roads in the mountain... Two weekends ago I spent a few hours trying to pull a 150' downed tree out of the roadway And I have a plus size spare that won't fit under the truck anymore, got's to have somewhere for it to go. I don't like having it inside, since it becomes an unsecured mass in the case of an accident... "Death by spare" doesn't appeal to me

As soon as I can get a decent photo of my truck, I'll start drawing/designing the tail structure to mount on my contraption. I really wanna make this happen, just a matter of time and resources haha

~C

[chillsworld]

Originally Posted by aerohead

You can do better than a 'Trailer Tail.'

But isn't your original post very similar in design to a "trailer tail"?

your posted design:

Originally Posted by aerohead

Trailer tail:


Would this not net me FE gains on the back of an SUV? Or are you simply saying that I should try to do better because I can?


~C

[kafer65]

How about for a rough shape 2 pieces of coroplast held together with a piano hinge with torque springs to hold outward tension as you pull sides plush with the SUV sides. Make a box frame for them to mount to. The piano hinge is the point of the backwards boat bow shape.

[freebeard]

Sounds like you've thought about it more than I had. Split like this, two ways, the height of the storage would be cut in half.



You could have something like the tailgate area above pushed back to mimic the trailer tail. The bottom half could swing 225° to put the shelving on the corners when it's open. That would put the hinges in the new construction.

Insofar as the Trailer Tail—it is a box cavity (for stowability). A flat truncation (flat, concave or convex but with a sharp separation) will perform better, though not as well as a 'later' truncation or full boattail.

[aerohead]

Originally Posted by chillsworld

But isn't your original post very similar in design to a "trailer tail"?

your posted design:


Trailer tail:


Would this not net me FE gains on the back of an SUV? Or are you simply saying that I should try to do better because I can?


~C

AT Dynamics' Trailer Tail is designed for collapsability and minimum displacement when nested against the trailer.It's way out ahead of nothing,aerodynamically,but is intentionally compromised for operator ease of operation and also DOT length limitations.
My rendering comprises the most complicated geometric structures as far as fabrication goes.There's not a flat surface anywhere and the subtle curvilinear contours needed for boundary layer protection are everywhere.
Kamm's last project has this complex form

Tou can see how NASA went gently into the 'slopes'

I took advantage of VW's existing curvature when I did my Transporter

These curves are very important to the boundary layer.

[chillsworld]

Originally Posted by aerohead

AT Dynamics' Trailer Tail is designed for collapsability and minimum displacement when nested against the trailer.It's way out ahead of nothing,aerodynamically,but is intentionally compromised for operator ease of operation and also DOT length limitations.
My rendering comprises the most complicated geometric structures as far as fabrication goes.There's not a flat surface anywhere and the subtle curvilinear contours needed for boundary layer protection are everywhere.
Kamm's last project has this complex form

Tou can see how NASA went gently into the 'slopes'

I took advantage of VW's existing curvature when I did my Transporter

These curves are very important to the boundary layer.

So it's not that they aren't appropriate design, they are just to "simple" with straight lines and flat panels... Got it.

I got out my protractor and a pic of my Beast, it appears that I have about a 30 degree departure angle with my hitch. I would need to maintain that steep angle for my non-paved adventures... I routinely drag my hitch, so a lower angle would probably be a quick trip to catastrophe with a boat tail. I know the steepest angle on the "teardrop" aero template is 22 degrees, so how screwed am I with my need for 30 degrees? I'd hate to build this thing and then see no improvements due to my design limitations Could maintaining correct angle/curve on the sides and top be good enough? Could I do some sort of rear belly pan/diffuser that would assist in directing air up along the bottom of the tail?

-C

[basjoos]

On the Aerocivic, I've had to compromise on the bottom angle to keep from dragging on a steep driveway entry. As long as you keep the top and side angles correct you be ahead of the game. Having a smooth underbelly transitioning to a diffuser on the underside of the boattail will help at reducing the drag increase of too sharp of an angle under the boattail.

[freebeard]

Compare C and D. The inverted fins are called 'skegs'. If they were retractable, with a hinge at the front and skateboard wheels at the tips, that would preserve your departure angle. And it could be combined with F, similar to the Trailer Tail.

[chillsworld]

Originally Posted by basjoos

On the Aerocivic, I've had to compromise on the bottom angle to keep from dragging on a steep driveway entry. As long as you keep the top and side angles correct you be ahead of the game. Having a smooth underbelly transitioning to a diffuser on the underside of the boattail will help at reducing the drag increase of too sharp of an angle under the boattail.

Sweet, Game on

I want to take advantage of no longer having a spare hanging behind the rear axle! I'll hopefully get under there in the next week or two and figure out what I'm working with.

Originally Posted by freebeard

Compare C and D. The inverted fins are called 'skegs'. If they were retractable, with a hinge at the front and skateboard wheels at the tips, that would preserve your departure angle. And it could be combined with F, similar to the Trailer Tail.

True! I guess you could even have the fins spring loaded and retractable upon pressure being applied... Have them go up into a slit in the boat tail? That's got some serious potential for experimentation if I ever get this thing built.

Would this have the potential to cause lift under the tail? Or would this actually create down force the way a diffuser works?

~C

[freebeard]

Exactly the way a difusser works, or the endplates on a wing; by imposing order on the air which would just do whatever it wants otherwise.