[QUOTE=mendo;237835]you can do it with a little latch.
clip the latch on the U shaped bar that is on the window, push the loop that is on the latch into the lock.
Do you realize that in the manual it states not to have the rear open when the engine is ON... the exhaust fumes WILL get into the cabin. Have you ever heard of Carbon Monoxide poisoning??? maybe you should refrain from giving people dangerous ideas that could kill them just to sound smart!
you can do it with a little latch.
clip the latch on the U shaped bar that is on the window, push the loop that is on the latch into the lock.
Do you realize that in the manual it states not to have the rear open when the engine is ON... the exhaust fumes WILL get into the cabin. Have you ever heard of Carbon Monoxide poisoning??? maybe you should refrain from giving people dangerous ideas that could kill them just to sound smart!
Yes, I do. I personally have enough sense to only do this with both windows open.
I am not telling people to do it, I am only telling them how.
as in all things, there is personal responsibility and common sense.
if we mandate that any activity that could possibly maybe hurt someone is banned then what? we pretty much do not change any part of the car because it was not designed that way and "they" know better than us.
as in all things, there is personal responsibility and common sense.
Common sense is rare these days. Telling someone how to do something and having the common sense not to do it yourself without both windows down doesn't keep someone else from doing it with the windows up and fan on "1."
Having disclaimers to make sure people follow common sense sometimes help.
"Warning: This window screen is not solid and may push out leading to a fall. Keep small children away from screen." Sad on so many levels.
Anyone who has ever had an SUV or station wagon with an electric rear window can relate to this. No matter how many side windows you open, you can never achieve the breeze gained by additionally cracking the rear window.
Thanks for the idea Mendo. However, should I decide pursue this, I want to be able to open and close it once moving.
Well, it's real easy to relax when you're brewing up a case of CO2 poisoning! (laugh, or be flogged) But, what about this...
Empirical Data:
Someone try this (I'll do it myself when I have the time). Get some tape and about 20 (or more) little pieces of string, about 3-inches in length. Open the hatch and, starting at the outside-lower corners of the glass, tape the strings so the hang down over where the hatch is cracked. You might also consider a few up the sides of the glass.
Now, go drive the car.
Rear Window:
The rearview mirror would work, but I'd rather let someone else drive so I could devote full attention to the dynamic behavior of the stringlets and not be concerned with being The Next Guy To Rear-End Someone In A SmartCar. Remember to test from low speeds up to at least 60mph-or-so. This will give us an immediate/intermediate answer to our question. We already know what the airflow does with the hatch closed, but this test will, by the direction in which the little stringlets dance and point, tell us what the air is doing with the hatch open.
Thought-The Final Frontier:
He then began a process that he had become accustomed to calling (and wrongfully so) THOUGHT, then turned to me and said, "But it gets better mileage with the tailgate down so the air can pass through without being blocked."
I, of course, did not press the issue. If he couldn't understand my question, then he surely wouldn't understand the thought process that would lead him to empirical testing.
Manual Shift:
If the manual says don't drive with the rear hatch open, well; I certainly understand that. There is a considerable pressure/flow fluctuation zone in that area when at road speeds and who knows (we soon will!) what could really be going on back there. However, it does no harm to empirically check what the manual says. It could be right on the money, or it could be playing it safe. Me? I like to test things... at least those things that are within my power to test (safely), and this is certainly one of those test-able things.
Tailgate Tally:
This whole idea came to me because of all the pickup trucks that run around here in the USA with either netting in place of the solid tailgate, or tailgates simply left in the down position for "aerodynamic" reasons, yielding "better gas mileage." I saw a fellow at the gas station who was so-equipped, and asked him why he did it. "Better gas mileage," was the answer. Then I kinda popped his balloon.
I told him, "When I was a kid on the farm, I used to ride to town in the back of pickup trucks all the time, and I noticed that, when sitting at the very back of the pickup bed, up against the tailgate, my hair blew FORWARD, not backward as one might reasonably expect. This is because of a reverse eddy in the airflow (HINT: watch water navigate around a rock or stick in a creek) that HEAVILY suggests better mileage could be had by leaving the tailgate in the UP position, thereby foiling (to some extent) this contrary current."
Thought-The Final Frontier:
Our brave pickup jockey began a painful process that he had come to think of as (and wrongfully so) THOUGHT, then turned to me and said, "But it gets better mileage with the tailgate down so the air can pass through without being blocked."
Full Court Press:
I, of course, did not press the issue. If he couldn't understand my question, then he also wouldn't understand the thought process that would lead him to a simple empirical test procedure.
Eddy's Such A Drag:
Aerodynamic drag is induced when the air does not (can not) stay attached to the surface of the solid it must flow over. Eddys are proof positive of the absolute worst kind of airflow problems, and pickups have them in spades. Many new aircraft I see at my local airport/hangout are beginning to sprout the oddest-looking bulges and indentations one could imagine on a high-speed "streamlined" form of transport. These come from empirical testing, and not just testing each piece (say, an engine cowling), but rather, how everything--as a whole--reacts together. It's called Area Rules and, to put it simply (or simplistically), you let the air tell you what it wants to do. We've wasted a lot of energy over the years trying to make air do what we think it ought.
Sidebar-An Aero To The Heart: Factoid of the day: Coefficient of Drag (CD) is a measure of the aerodynamic efficiency of a given shape when indexed to that of a simple "known" shape. The lower the number, the more efficient. The CD of a 1960's Volkswagen bus is LOWER than that of a Jaguar XK-E. Now you know.
The Light At The Wind Of The Tunnel:
IE: The aircraft's wing is wind tunnel tested and is as close to laminar flow (air stays attached) as it can be. Engine nacelle is like-tested and is near perfect. But what happens when you stick THAT engine under (or, in case of the new Hondajet, over) THAT wing? That's an altogether different set of problems. We can predict some of it, but empirical testing is still the last word, and area rules is the last-syllable-of-that-last-word.
11 Rock-Tapped Out:
This whole thing reminds me of a scene from the movie Spinal Tap where the dim-bulb rocker is pointing out to Rob Reiner that, "Our amplifiers go to '11'" and, no matter how elegantly Reiner explains it to him, our dull rocker simply could not conceive that the number "11" could mean anything other than "louder than '10'."
Breathe Easy:
I think that, under the right (wrong?) conditions, that exhaust could find its way into the cabin, but this situation is not a static one. When you open the hatch, you have introduced a different set of variables. Air needs to rush from somewhere to fill the space behind the smart's Kamm tail. With a side window rolled down (or partially so), this space may be filled with air emanating from the point of least resistance (to flow), which may be from the side window to right out the HatchCrack (should I trademark that term if this works?). Or it may not... at least not sufficiently, in which case, you might take an unwanted nap while driving with using the now-patented HatchCrack open. (my lawyers are checking...)
Novacaine Butt Overload (or "Another Dead End")
Then there's crosswinds to account for. Oh, forget it.
Blessed By The Pope:
For those of you who need to believe that owner's manuals are written by soft-hearted, altruistic engineers, with no input from the advertisement and/or legal departments, this whole exercise is a no-brainer. You simply should not as much as even THINKabout testing this idea since the thought police will surely descend upon you, laying waste to you and yours, all because you "Trusted Not The Force, Young Skywalker!"
Going Regimental:
Also, for the reason stated above (semi-unpredictability of a dynamic airflow situation), you really should not bypass the testing regimen by simply propping the hatch open and driving away.
The Big Closer (or: "Tanks, Mom!"):
I'm right in the middle of my best tank ever. Got to 294 miles when 1/2 tank (4 hash marks on, 4 off) first appeared.
...and I did it all with my hatch closed.
ADiF
PostScript:
Sorry for this War And Peace-like tome, but my arthritis medication kicked in and I needed an outlet. My life and the SCOA drew straws over who would be that outlet, and you lost. Just remember to rejoice that my fingers began to ache before I took the time to expound upon the works of Herr Professor Wunibald Kamm, and whether his work bears its prodigious fruit as applied to our favorite little car.
Personally, I don't feel like a have a problem with cabin air, and also wouldn't drive with the rear window cracked.
But, in lieu of rigging strings and trying to watch them, one could go to the hardware store section where they sell smoke detectors, et al, and get one of the little button-type, passive CO detectors. It involves a fuzzy button about the size of a shirt button. Normally the button is tan colored, but in the presence of CO it turns grey. It will retain the grey color for some time before returning to tan in fresh air.
I use them at home during hurricane season in case the generator has to be used (in a utility shed that abuts the house), and also in my sailboat cabin which has a small, cast iron, solid-fuel parlor stove.
Well, it's real easy to relax when you're brewing up a case of CO2 poisoning!
General cure for CO2 poisoning - breathe.
CO poisoning is a whole different issue.
The "cut off" back works better for mileage than trying to bring the rear airflow back together in a tear shape partly because of the vorticies generated by the airflow back there. By towing our own little whirlwinds, we actually reduce the amount of drag from separation of flow (turbulent flow) by having much of the airflow use those vorticies as virtual aerodynamic surfaces, joining the flow back together more effectively than with fixed surfaces.
If there was no vortex and the air just flowed one direction, albeit with turbulence, there wouldn't be an issue with CO getting back in through an open tailgate. With the air being whipped around in circles, it's easy to pick up exhaust and bring it into the rear area of the car. Empirical evidence can be used to determine of the flow from open windows pushes the whole vortex out further from the car or only augments one edge of the vortex allowing exhaust back in the car even through there's flow from the windows around the sides.
What I do know is that aerodynamics can be counter intuitive. Because of the nature of the negative pressure circular flow behind the car, I would suggest never trying to improve the airflow by leaving windows or tailgates open back there.
- John Handwork
B.S. Aerospace and Ocean Engineering, VA Tech
General cure for CO2 poisoning - breathe.
CO poisoning is a whole different issue.
Et "2", Brutus? But what's a couple of digits between friends, eh? As long as they're not middle ones.
Quote:
Originally Posted by John_H
The "cut off" back works better for mileage than trying to bring the rear airflow back together in a tear shape
Not necessarily, but far be it from me to argue with a man of letters (and as seen above, also of numbers)
The Kamm backs on production cars are almost always for show, and--historically--are not designed according to Kamm's work. There may be some passenger cars that do use the Kamm-tail "correctly," but I'm not aware of what's what, one way or the other. Wikipedia mentions this, but no citations beyond,
"Most often, however, (the Kamm-tail) only insinuates streamlining when used in production cars. It is then only a design trick to make the vehicle 'look fast.'"
I first saw it on Porsche race cars, which then became copied on street cars, but only as noted above: "to look fast." I've never seen a street Kamm-tail that employed anything remotely resembling the Porsche race car's rendition of a genuine Kamm design but, again, I haven't even a single wind tunnel with which to verify my suspicions.
A fellow hyper-miler near me has a Honda Civic to which he has grafted a long, cone-shaped mylar tail piece. He says it now runs 70mph with next-to-no throttle, and the coast-down is incredible. He looked up my phone# when he saw an article in the local paper here where I took the reporter on a 1.5 hour ride in my Prius (big loop) and averaged 96.4 mpg for the entire trip. Still, I've yet to meet him, and have only seen his car once on the street (and believe me, you can't miss it!).
If the Civic pilot can be believed, there's much to gain from gently letting the air settle back to where it wants to be at the rear, but I've not driven or ridden in the "Mylar Condor" Civic.
I do know what they do with the tail-end of a Bonneville car when the rules permit, but I don't know how "clinically tested" these designs are. Owing to my coastdown measurements (in which I'm not able to accurately divide aero drag out of rolling resistance) the smart fortwo is a lousy design compared to the Prius. But that is of no consequence to me and changes not my admiration for its engineering. I think the smart's Kamm-tail is complete happenstance ("Well Wolfgang, vee are finished buildink "car" now, so just stick zee hatch on hier und let 'er rip), but would be happy to learn otherwise.
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but at last I can really dig into this fun little machine. 
A2Jack
RBsmart
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