Anyone tried using Hidden Hitch...?
 

Has anyone fashioned a removeable Aero Device that slips into thier Hidden Hitch? I was thinking that a flat piece extenting out from the underside for highway use only would be kinda kewl. You could make it about as large as you needed since you could take it off when not traveling.

I've heard some SUV and PT cruiser owners report that when using a luggage box or platform with a large tote on on their hitch they have seen improved MPG...

This is due to a couple of effects I think, first the surface about halfway up the back of the vehicle allows for a more sedan like clean separation of air flowing over the top. This improves form drag or shape drag, due to allowing the air to depart the vehicle at a more horizontal angle, thus effectively "tilting the airfoil" that the vehicle body forms forward and causing the shape "drag" or aerodynamic lift to act in a forward direction.

Secondly, the sides of the box being narrower than the vehicle, at the minimum so that the lights are visible, creates a step that will fill with a fairly stable vortex, which has the effect of rounding the back of the vehicle and allowing more air from the sides to tuck around behind it. This reduces the base drag. Some of this type of effect may also be seen on the top as well, dependant on how the tailgate/roofline curves down.

As for just a flat plate... it's usually more beneficial to encourage air from underneath to flow upwards behind the vehicle. This does make for more spray on the back window if not balanced by flow from the top but it also helps the bulk airflow to be generally more horizontal than downward. Therefore a lower down plate or hitch load platform with nothing on it, may be more likely to have a detrimental effect. Raising it to around half the height of the vehicle would allow it to act like a splitter and separate air off the back cleanly.

It's possible that rear tire carriers can have some of these effects, but they may be a little lopsided if carried off center or absent if the tire is on the small side. For instance on your Liberty, you might be getting some drag reduction on the passenger side, but I think the amount it extends from the back is too short to have much effect on air coming over the top, and it's too far from the drivers side. I wouldn't be surprised if you told me it tends to pull right slightly at speed.

On that vehicle, I might be inclined to investigate making a lightweight box affixed to the tire carrier, that is about the size of the tire top to bottom, about twice as wide front to back and fits between the lights/bumper ends.... though maybe a little shorter than the tire, so it's poking out the bottom and top... guess it could be mounted on the hitch and be a kind of shell that slips over.

Hmmm thinking on some rule of thumb stuff I hashed out a while ago, if you can get about an 8"-10" step on the sides, and then have a second box inset another 8"-10" inches, with the top and bottom left level, you'd have a sort of boatail round the sides where vortices would round out the effective surface.

I was thinking a lightweight "bobtail" or similiar device could be made for the back of an SUV out of paper mache for trial purposes. Then later and aluminum frame could be formed in the desired shape with some sort of fabric or rubber covering to create the "tail". Just a thought...I wonder if you roemed something shaped like a "staple" that is pointed legs down. The top side would be about 1/3 the way tp the rear of the vehicle. The sides (say the device is 2 feet long---away from the vehicle) could slant inward to the back...hummm

The Spare Tire on a Jeep Liberty is centered just under the rear window...I don't think it makes a big difference in mpg.

I see what you are saying about mounting the device where the spare mounts. I don't carry a spare most of the time and the bolts that are already present for the carrier could be used as an anchor.