Prius Removable Kammback
 

I bought a "towing package" for my little black peanut car (no, not for towing a mobile home-- for my bike rack and for my little aluminum rack that I use to haul my paramotor around). In looking at how easily these weight-bearing accessories slide into and out of the "towing hitch" receptacle, I have drawn up a design for a removable kammback that works as an aero mod, improving mileage, while at the same time not hurting resale value and providing more cargo space.

The so-called towing package fits great is available on ebay for about $100 or so, it bolts right on with only a minor cut in the plastic underpan and removal of the rear frame tow attach points.

The design for my removable unit is a basic kammback shape, with plexiglass on the sides to allow visibility for rear lights and plexiglass on the top to allow for driver visibility. To keep it from wobbling around in the hitch or rubbing against the paint, a soft foam rubber gasket would be used to make a seal against the car, and the unit would have to be pushed against the car tightly in order to put the rear pin into the hitch receptacle. Provisions for accessing the hitch need to be made, as well as a removable lid for hauling large items (like my paramotor!).

It should be made from aircraft grade aluminum tubing, with some sort of paintable skin that is rigid in sheets and flexible (id est, not coroplast). Maybe aluminum sheeting or fiberglass?

Using this idea, can any of the talented artists here post some conceptual drawings of what this thing should look like?

Can any engineering people post recommendations for dimentions? Framing specifications and load limits for stock rear springs?

Can an aero whiz suggest computer modeling ideas? Incorporated rear wheel treatments?

Ideas about exhaust considerations?

Ideas on how to make it crumple in an accident?

Anyone want to build one of these with me?

I'm not sure how to design one but the Aerocivic has one of the best Kammbacks I've seen so far. One revision even sacrificed itself in an accident to protect the car it's attached to.

That's the general shape I'm going for, but removable and with a higher WAF rating. (Wife Acceptance Factor)

If you want that much help, you probably shouldn't specify the materials yourself. There are a dozen good ways to get similar results, but most potential craftsman will only be comfy with one or two of them.

My ideas are a starting point to be improved upon or perhaps scrapped altogether for a better direction.

The more people that comment on this, the better.

Hi,

What version of the Prius are you working with?

Its an '07

The difficult thing is going to be having a way to lift it on and off the car easily, since I expect it would be large and cumbersome.

As such, do you have a goal for the weight of it?

We could certainly keep it under 50 pounds without too much of a problem I would think... that would limit its load capacity a lot I'm sure but I don't need to haul rocks. :)

I guess since it's just a kammback and not a full boattail that's possible. Might be difficult though

Sheee-oot! Very good idea! The concept probably would work on many cars since you should be able to put a hitch on nearly any car, I'd think.

"Plexiglas on top" ?? You already have a kammback with attached flow over the rear window. The thing to do is to just continue the same line to a smaller terminus. To keep the signal lights visible from both sides, you'd probably want the extension all clear anyway. It isn't easy to do that with light weight and neat appearance. It might be better to go all opaque, and put trailer lights on the tailcone.
The design could work well with just flat-wrapped sheet stock and no compound curves. If you don't like Coroplast, sheet PVC or coloured polycarbonate would probably work well at a reasonably light gauge. Craig Vetter is happy with his thin solid stock, but I can't remember the compound.

To get a really nice appearance, a mold is probably best. It could be filled with fiberglass or vacuum-formed thermoplastic, and then reinforced with "hat sections" bonded to the inside. Those can also be used if sheet stock is just held in a simple jig to give it the right shape. The can fan out from a metal or 'glass tube that goes in the hitch receiver. You might want a long tongue that goes right through the hitch receiver to make mounting easier, with an extension handle for tightening.

I would like to deposit some encouragement.

Yes, this sounds like it would be a great idea as long as there's not a lot of play between the hitch and the adapter, otherwise your cammback may have a bit of sway to it.

Just as a side note, if anyone's interested in trying this out, I still have the trailer hitch I took off my Prius sitting in my garage ;)

SUMMARY

OK, we can go with what will actually be less of a kammback and more of a removable boattail.

So it will be functional like this:
http://www.racksolid.com/images/thule/684.jpg

And much more aerodynamic than this:
http://priuschat.com/forums/attachme...r-img_2810.jpg

But, while removable, will be a boattail like this:
http://forum.ecomodder.com/attachmen...p;d=1202612003

The upper portion of the boattail could be opaque since I don't think its legal to have something without proper lights or safety markings go past your rear bumper by more than 2 feet... this would plug-in much like any trailer would and require a simple wiring harness.

The body could be made without any compound curves of some flat sheet, but ideally should be molded, and reinforced internally with hat sections.
(I didn't know what a hat section was, so I looked it up, as follows)
http://www.indiamart.com/frontlinero...n_10529477.jpg

Lights can be added in the rear, like the Aerocivic, but stylized to match the lines of the prius.

This "Removable Boattail" design could be adapted to any number of different cars, and kept under 100 pounds.

Tomorrow I'll to to the local raw metals wholesaler and pick up a small stick of aluminum box tubing to form the portion of the Removable Boattail that inserts into the hitch. Once that's bent and drilled I can get out the cardboard and start making a mess!

"Opportunities" yet to figure out:
1. How will the removable boattail attach to the body(besides the hitch)? Will it use a foam gasket? Is it important to attach at all, or should it be spaced apart from the upper trailing edge of the body by 1/2"?
2. How to open the trunk without removing the boattail? An extra lift-lid attached to the spoiler?
3. How to open the boattail itself for cargo access and light weight?
4. How to maintain driver visibility?
5. Bracing layout for "crumple zone" safety provision.
6. Anyone else's suggestions or ideas, I'm sure I'm overlooking something important.

Thanks all for your help!!!

First, I think this is a neat idea.
Second, the "WAF" is humorous and indeed important; I dread the day when I go to a large EM gathering and my wife gets together with other wives/girlfriends for a group rant. :)

I do have one criticism. This would be best suited for a much larger, less aero, vehicle. I am doubtful of the FE gains you will see from this, and the added utility/safety will be nearly the only gain from this mod - maybe not enough to pay for itself.
I do empathize with you; I criticize Honda, Toyota, and SMART for chopping their cars too short. Adding another foot or two at the back gives you some aero benefit, increases cargo room substantially, and has the potential for HUGE gains in rear-ending safety. Instead they want their cars to be easy-to-park for people who can't drive. The Honda Fit and the SMART fortwo are the worst offenders - no cargo room, wasted aero, and no crumple zone!

I forgot to make a suggestion for a vehicle to adapt this to - What about making this for the Chevy Equinox or the Ford Escape?
Both are popular, medium-size SUVs and would stand to gain a good amount with a Kammback, and both appeal to drivers who want MPGs as well; my neighbor is getting an Escape to replace his full-size van and my wife wants an Equinox, due to GM's claims of next year's model getting 32 mpg highway with the 4-cyl.
(I'm still dragging my feet - if we buy a new car it should be capable of getting 40 hwy IMO)

I rather like the idea of implementing something like this on SUV's.. wasn't that the intended purpose of the aeropack? (Did that project ever come to fruition? There was mention of it on here at some point, I remember.)

Yeah I think this design can be easily adapted to fit almost anything. Hopefully someone else will try this too!

UPDATE:

This morning I purchased what will become the central backbone of this removable boat tail: Three feet of 6061 aluminum 2" square pipe, 1/4" wall. It fits very tightly inside the hitch receptacle, with almost no play-- without adding a roundover on the edges of my square pipe. I drilled the pin hole 3 1/2" on center from the end of the pipe that goes into the hitch, which leaves plenty of material inside the hitch for less wobble. 7 3/4" from the hitch end, I cut the pipe at a 10 degree angle, so that by inverting the pipe and re-welding I will have a 20 degree bend in the pipe which extends upwards about bumper-level. This should yield a 250 pound max cargo capacity, so long as the load is placed within 2' of the bumper.

Now, lighter gauge pipe can be added as required. I will try to find some scrap for free.

SPECS SO FAR FOR THE INCOMPLETE PROJECT
Cost: $27
Weight: 6 pounds with hitch pin
W.A.F. rating: Neutral

Hopefully I can still find some good 6061 scrap today.

To open the hatch, you might operate a lever that does the final tightening of the connection. Or, just lift a lid with a rear or side hinge. If there is a lid, you need some serious framing. It might be better to just make a really convenient way to slide it back for access from the front. A lip at the bottom would make it a bin.

The example given of a red car with a horizontal boattail instead of a Kammback is not ideal for aero. The angle at the bottom should not be more than about 4 degrees because it only has a limited air supply, unlike the top. The angle shown is good for downforce, not mileage.

I would try using a continuous extension of the sheet material under the bumper to prevent independent roll motions, and use a tightening wedge or something to snug the rest of it to the body with a foam gasket. It is probably easier and lighter to just keep the bin full of foam or cargo than to get serious about an extra crumple zone. You can probably get used to the length, or else stick small antennas on the back corners so you can see them. Be sure to streamline them - round is awful.

In fiberglass work, hat sections are reinforcements added to a surface by laying wet cloth over a minimal mold, such as foam pipe insulation or a cardboard tube with a spiral slit in it. They are versatile, easy to do, and quite efficient if done with that in mind. To get the most out of them, use cloth strips cut on the bias, with a thick, narrow strip of unidirectional fiber at the top.

Thanks that's a bunch of good info!

I'll go out now and check my nominal departure angle, to make sure I can get in and out of my horrible driveway. With my 1/4" wall pipe at a 20 degree angle from the bottom of the bumper, I'll be able to build the underbelly any shape I like, as long as its less than a 20 degree departure angle.

Once I get this welded, I'll start mocking up some cardboard and post a pic to get some more feedback... but we're looking now to be mindful that this boat tail should not act like a down-force wing.