Is that the thing that directs air down the back window to keep it clean? If not, how about a pic
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| Will Liberty CRD Rear Air Deflector Help MPG? http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=21114 |
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| Author: | VACRD [ Mon May 28, 2007 3:09 pm ] |
| Post subject: | Will Liberty CRD Rear Air Deflector Help MPG? |
The aerodynamics is the Liberty is not great, even so still get 27 mpg average (70% HW and 30% City) Just installed front air defelector hoping to improve my mpg a bit more. A rear air defelector costs $98.00 on the jeep site. Does anyone know if the rear air deflector can help mpg? |
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| Author: | crackercrd [ Mon May 28, 2007 5:42 pm ] |
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Is that the thing that directs air down the back window to keep it clean? If not, how about a pic
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| Author: | VACRD [ Mon May 28, 2007 6:23 pm ] |
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Yes - that is what the description says. |
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| Author: | ATXKJ [ Mon May 28, 2007 6:56 pm ] |
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The rear air deflector that clears the rear window - increases drag and will decrease mileage ( that's if you see any effect at all - it should be pretty small) Rear aerodynamic panels that would improve mileage would be around all 4 sides and gradually taper the rear cross sectional area. something like this http://whyfiles.org/shorties/190truck/ |
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| Author: | VACRD [ Mon May 28, 2007 9:37 pm ] |
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ATXKJ Thanks for the info. Looks like rear deflector doesn't make any sense. |
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| Author: | Weegie5 [ Tue May 29, 2007 12:37 am ] |
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I got one in hopes that it could help keep my back glass clean. It didn't do much. As far as fuel economy, negligible difference. I kind of like the look, so it'll stay. |
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| Author: | techTim [ Tue May 29, 2007 1:10 am ] |
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here's a super scientific link for something that might help, but with the box on a box design of the liberty, maybe no..... http://www.mitsubishi-motors.com/corpor ... 16E_03.pdf bought a motorcycle from an engineer type who had a bunch on the back of his cube van, he swore by 'em, so I dunno........ 
I'm pretty thrilled that I can get near econo-car mileage, turn around and tow a bunch of weight and/or go out 'wheeling the same day in the same rig. can't say that for anything else out there. |
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| Author: | ATXKJ [ Tue May 29, 2007 12:12 pm ] |
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This site had some ideas - not many are applicable to Liberties http://www.recumbents.com/car_aerodynamics/ and this also has good ideas http://www.gassavers.org/forumdisplay.php?f=4 |
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| Author: | Cowpie1 [ Sat Jun 02, 2007 1:28 am ] |
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The rear deflector was a no brainer for me as I live on rural gravel roads and the pile up of dirt on the rear of any vehicle is a problem. The rear deflector on my Libby has done a fantastic job. I only get the dirt buildup down by the bumper. Haven't tried checking mpg without the deflector, but since I got 31.6 on my last trip with four adults and cargo, I don't think I have lost anything substantial by having it on. Most people would probably not have a need for such an item. Those of us that live on dirt/gravel roads find them indespensible. Even in comparison to other Libby owners in the area that don't live on gravel roads, we do not have near the buildup and therefore cleaning problems. We hardly ever use the rear window wiper. The Libby is a rolling brick, aerodynamicaly. Not sure the aerodynamics of having a rear deflector means anything. I view it as just a tool to do what I need to have done. |
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| Author: | csukoh78 [ Sat Jun 02, 2007 7:44 am ] |
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They DO work..... Here's why.... At lower speeds, they force air down and keep dust, gravel, exhaust fumes away from the back of the vehicle. They were originally designed just for dust, but manufacturers in the 70's and 80's discovered that they also work on exhuast fumes. Stationwagons that had tailgate windows that could roll down would suck them in, so they were installed so that the occupants would not choke on exhaust fumes and potentially pass out from CO2 poisoning. BUT, at higher speeds, the liberty (and any other large "flatbacked" vehicle) creates a vacuum with its passage through air on the back of the vehicle. Air goes around the vehicle, but swirls and cavitates in an effort to fill the empty space near the back of the window. Its a fruitless effort, and the volume of air that you disturb and tow along with you is three times as big as the liberty itself. The net effect of this is that the faster you go, the more air you "pull" along with you, and this decreases gas mileage exponentially. Think of it as a parachute. What this thing does, however, is force fresh air down into this vacuum, effectively filling it with a large enough volume of air that the vacuum is neutralized. At speed, this creates a large self-replacing air pillow attached the back of the vehicle, in which air coming around the vehicle just slides off on all sides. Mythbusters did this and proved that it is in fact "confirmed." Also, my physics professor did a similar experiment with a smoke chamber and the smoothness of airflow with it was phenomenal. The pitch (or angle) of the wing can be altered, but most manufacturers choose the angle to be most effective at highway speeds. The pitch the Mopar wing uses is most effective between 68-80 mph, right in the highway zone. So yes, it does work great. Get one. PS This is essentially the opposite of a spoiler. A spoiler on a rear drive car (like Nascar) forces air up in an attempt to create more "weight" or downforce on the back of the car for more traction. It does this because the pressure to displace air upward, when it wants to go down, produces downforce. What it also does is create a much larger vacuum in each vehicle, so when you hear the term "drafting", you will know that this means one car is getting into the vacuum of another car, and is being sucked forward along with it! The physics of 200mph, however, dictate that it is more beneficial to create drag and downforce, rather than lift and reducing vacuum drag. This deflector does the opposite. It forces air down, filling the vacuum instead of making it bigger. The physics of highway speed dictate that an wind deflector would be more beneficial. Technically, it does reduce the weight on the tires, because the deflector is actually pulling up on the vehicle at speed. But at over two tons, I don't think she'll get airborne any time soon! |
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| Author: | Ranger1 [ Sat Jun 02, 2007 8:29 am ] |
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So how many miles per gallon go you gain in fuel economy with the deflector ? |
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| Author: | csukoh78 [ Sat Jun 02, 2007 9:07 am ] |
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with a deisel, Im not sure. Deisels are already pretty efficient and the torque they produce can much more easily overcome drag than the gassers (if they are geared correctly, deisels are not designed to rev). The wind deflector helps gassers more than deisels because the torque curve drops off pretty quickly in gassers, forcing that much more fuel to be burned to get the torque high enough for highway speeds. Either way, its a good investment. It will definitely help towing mileage too. |
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| Author: | Katmandu [ Mon Jun 04, 2007 2:16 pm ] |
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csukoh78 wrote: They DO work..... If they do help MPGs, WHY don't manufactures install them as OEM ???
Certainly higher MPG ratings would be more cost effective in light of adding $100 to the base price! |
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| Author: | csukoh78 [ Mon Jun 04, 2007 2:28 pm ] |
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Katmandu wrote: csukoh78 wrote: They DO work..... If they do help MPGs, WHY don't manufactures install them as OEM ???Certainly higher MPG ratings would be more cost effective in light of adding $100 to the base price! Are you serious? Since when have manufacturers ever been concerned about doing what's right versus keeping costs as low as possible? |
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| Author: | rpapkey [ Mon Jun 04, 2007 3:06 pm ] |
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csukoh78 wrote: Katmandu wrote: csukoh78 wrote: They DO work..... If they do help MPGs, WHY don't manufactures install them as OEM ???Certainly higher MPG ratings would be more cost effective in light of adding $100 to the base price! Are you serious? Since when have manufacturers ever been concerned about doing what's right versus keeping costs as low as possible? Thats a fact. You always have to remember that engineering, marketing and the beancounters are always fighting over this kinda stuff. It might be something as simple as the manufacturer that makes the spoiler couldn't produce enough to supply the assembly line. So instead of making it standard they tacked it on as an extra. Just a completely uneducated theory. I can vouch that it'd help some. I don't think it'll be much, but some. |
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| Author: | ATXKJ [ Mon Jun 04, 2007 7:58 pm ] |
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csukoh78 Do you have any data from Mythbusters? - they don't list air deflectors or gas mileage on their web - although it could easily be a subset of another title. I don't expect manufacturers to do anything that's not cost effective - so the return on installing an air deflector would have to exceed the cost - and if it's a small amount of mileage that would be a hard argument to make. However there are areas where trivial mileage improvements matter - i.e. interstate trucking companies spend a lot of money on trivial improvements because they are cost effective, also universities and Gov agencies modeling aerodynamics - they are looking at the extensions on the back of the trailer, not air deflectors. Rear deflectors that go up - like a racecar - or down to clear the window - should increase drag (and frontal area) - although as I said before - the amount should be small enough that it'd be hard to measure. |
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| Author: | csukoh78 [ Mon Jun 04, 2007 10:53 pm ] |
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we were shown a clip from our physics professor. The other crew did it (not jamie and the other guy) I dont know if it ever aired, but it was a definite show done by mythbusters. I cant copy it and post it because its encoded on my university site. |
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| Author: | ATXKJ [ Tue Jun 05, 2007 3:05 pm ] |
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FYI - while I was looking for data I found a website http://www.airtab.com/ tape on Turbulators - lots of testimonials they look weird - I'm not sure if I do enough highway driving to try them. photo page has lots of examples - even a Corvette that says it helped. Note - I know testimonials are not data they aren't statistically valid |
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| Author: | VACRD [ Tue Jun 05, 2007 6:57 pm ] |
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Just looked on Jeep Site. Side window deflectors cost $91 for four. My front deflector (black on silver) looks pretty good. I think I will get the side window deflectors. I have read on this site that does help mpg. Looks like the rear window deflector is questionable from a mpg perspective. |
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| Author: | BVCRD [ Wed Jun 06, 2007 10:13 am ] |
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It won't help. Anything that makes more drag will hurt. Maybe less than 1 mpg but it won't help. Only way the side deflectors will help is if you have your windows down a bit. With windows down, it really messes up the smooth air. Car acts like a parachute. |
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