Kammbacks & tail aero info

[aerohead]

Originally Posted by DifferentPointofView

Yeap, that's America for ya.

I wanna try that disk Idea... it'd be a lot easier for me I think.

On the surface,the disc looks inviting.I've only seen these depictions at 0-yaw angle.
Since the discs themselves have Cd 1.11,if the vehicle encounters any crosswind,the flow separates immediately into an enormous wake.I believe there is a depiction of a flat plate in the "FLOW-IMAGES."
I would consider the potential of the disc system dubious at best.I really do believe them to be only of academic interest and relegated to the wind tunnel,not the real world.

[aerohead]

Originally Posted by Bearleener

diesel_john, what does your stepped tail look like? How is it fastened/reinforced? Thanks!

Has anyone thought about a retractable Kammback? Sure would be nice if it would fold together at low speeds, reducing vehicle length for easier parking and making it slightly more street-legal.

One possibility would be, say, 3 panels that fold out at the top and sides, perhaps deployed with a pull string arrangement through eyelets at the corners.

Or how about a clear plastic, inflatable, conical bag (something like those temporary bouys used for sailboat regattas), which would roll up into a little box when parked? Maybe you could even use the low-pressure area back there at speed to inflate it. Or exhaust gases at the tailpipe: pump directly to inflate, use a venturi with side tap to create vacuum to deflate.

In the U.S.,the DOT allows length statutes to be violated by 48-inches if for fuel economy.
The inflated conical bag has already been patented by a fella in Lubbock,Texas,which is inflated by ram-pressure of the moving rig.
If you can get a look at Hucho's "Aerodynamics of Road Vehicles",there is a depiction of the best inflated(extensible) tail I've seen yet,from 1930s Germany.I believe the design is by Koenig von Fachsenfeld/Kamm.
A very crude fan-inflated envelope helped my pickup to see over 30-mpg at 65-mph,with air conditioning on.

[JeepNmpg2]

Originally Posted by aerohead

In the U.S.,the DOT allows length statutes to be violated by 48-inches if for fuel economy.

Thanks for this man, I forgot to mention, that your post is where I based my 48 inch from the bumper length is off of. Not that I am too long, just that I wanted something to cite if I ever got pulled over and questioned by the authorities.

[aerohead]

Originally Posted by JeepNmpg2

Thanks for this man, I forgot to mention, that your post is where I based my 48 inch from the bumper length is off of. Not that I am too long, just that I wanted something to cite if I ever got pulled over and questioned by the authorities.

yeah Jeep,I don't think it will ever be an issue.Some truckers running boat-tails are doing it for the allowable extra length and potential for greater paying loads.
Safety should always be on your mind,so should you ever get into some hellacious crosswinds,you might want to watch your speed until you feel she's sure-footed in these blasts.
The tail has altered your center-of-pressure in relationship to your center-of-gravity so your 'gust' sensitivity is an unknown 'til you feel her out.
I've never experienced any ill-efects but you want to ere on the side of safety.
'love the tail! Glad you did it.We're one tiny step closer to getting off foreign oil.And everything else.

[jime57]

Originally Posted by aerohead

In the U.S.,the DOT allows length statutes to be violated by 48-inches if for fuel economy.

Phil, I wonder if you have additional references for this. It is a question of some concern to those building boattails.

[MetroMPG]

I thought the context (of the over-length allowance) was for commercial trucks, but I could be wrong.

[JeepNmpg2]

Originally Posted by MetroMPG

I thought the context (of the over-length allowance) was for commercial trucks, but I could be wrong.

I think you're right Metro as I've not seen mention of a maximum vehicle size for standard automobiles, at least other than rentable moving vans. I'm obviously nowhere near that size but I suppose it is somewhat up to the officer's discretion as to whether or not it is safe for you or others on the road to operate your vehical with said modifications.

[comptiger5000]

Jeeps like to try to change lanes in a good crosswind anyway, so definitely be careful the first time you drive it in a good wind.

[kurtis1971]

its tempting to build my own car this way

[aerohead]

Originally Posted by jimepting

Phil, I wonder if you have additional references for this. It is a question of some concern to those building boattails.

I just called Washington D.C. back in the 1970s.The Energy Research & Development Agency had an understanding with the US dept of Transportation that arbitrary length limits could be violated if the extra length had to do with fuel efficiency.
And yes it was intended for commercial vehicles.
Since DARPA has let millions for R&D of 'longer' more efficient 18-wheelers it implies that the understanding remains.
As to passenger vehicles,I would limit the length to that of a 1-ton crew-cab dualie pickup.
If somebody wants to make an issue of it,they'll have a discrimination lawsuit.
Highway Patrol and Police and such are authorized and expected to intervene when they suspect an unsafe vehicle.
Only one time was I almost stopped,it was in the CRX,and maybe it had to do with not removing the racing numbers from the car between Bonneville and Los Angeles.At Inyokern,California,a Chippy came up close from behind out of nowhere,tailed me for a time,then faded,did a u-turn and sped away in the opposite direction.
I've displayed my vehicles at energy events around Texas,with plenty of the constabulary represented on site,non that I visited with even brought up the topic of the extra length.
Same,coast-to-coast.I just don't think it's an issue,especially with two oil wars going on.
Last December,Midland Police tailed me through the downtown area for awhile.They no doubt had never seen a trailer like mine,and after a few blocks they just turned off on another street.
I'll follow Teddy Roosevelt's lead and 'take the mods and let Congress argue!'