GRM Aerodynamics Question

I just picked up a rear spoiler for my Trooper. Here's what it looks like (not my Trooper):

I know its primary purpose is to prevent exhaust gases from entering and to keep the rear window clean, but how will it affect fuel mileage?

On the one hand, it obviously creates drag. But on the other hand, it fills in a very low pressure area, possibly making the vehicle more aerodynamic overall.

So, which one wins? :D

Everyone knows MUD makes a truck run better, longer. Add mud, everything else is inconsequential.

I'm laughing at the Irony of you owning a vehicle with that Aero profile and concerning yourself with MPG.

I'm not really concerned, just curious. I'm installing it either way, but thought it was an interesting question.

Tom,
There are 3 laws of Thermodynamics that rule the world we live in.
"You can't get anything for free" is the one you are fighting with. The fact that the spoiler turns the air to the back glass is going to require more energy than allowing it to find the path of least resistance. Basically you will pay for a smaller wake by having to use a bit more gas. It may not be measurable over the distance of a tank with typical driving. If you are trying to hyper-mile the truck then you may start to see a difference (a small one).
NoCones is right about the general shape of the truck and MPG chasing, but if you really want to get some better mileage, look at the underside of the truck and think about fairing in some of the mechanicals under there.
Steve Stafford.

I doubt you will be able to notice a difference on that vehicle. Your adding what 2" x 48" of Aero exposure to a vehicle with a ~36 sq. ft. frontal area. Unless it dramatically alters the Cd it's just going to be noise in your ~14mpg. The most I would think it's going to do is improve or worsen your MPG by 5% or so which for your is <1mpg which unless you do an ultra scientific MPG study going to be relatively difficult to validate.

Break out the tape and yarn, and photo/video/document the results. I smell a magazine article ...

I'm thinking of a figure mentioned in Brock Yates's book about the early years of the Cannonball Run. The first couple of runs they made were in a full-size van; seems like they did one trip in stock trim, and the next trip with a spoiler like yours on the back. There was a percentage mentioned (and I forget what it was), but they blamed it for dramatically reduced top speed and fuel economy on the second trip.

Edit to add: +1000000 @ slantvaliant's comment.

its a brick... how much better can the mileage get?

Either way it's 100% guaranteed to make it faster. Looks like it adds about 2mph, whether fully stopped or hard on the throttle.

The theory is that this kind of thing reduces drag by scooping air from above the car and filling the low pressure area behind it. The downside is that they create a bit of drag and usually create lift (on some cars with a "liftback" shape, they can kill more lift than they make and the only cost is drag.) You might get a little MPG boost out of it.

Tom Suddard wrote: But on the other hand, it fills in a very low pressure area, possibly making the vehicle more aerodynamic overall.

That.

Back in December 2008 , C&D did a test of how various roof attachemnts (crossbars on vs. cross bars off vs. cargo carrier) affected fuel mileage. Ultimately, they determined that the crossbars alone reduced fuel mileage by around four percent. I would imagine the spoiler you mounted will have a slightly greater impact than the crossbars. However, you already have crossbars on the car so the impact may be mitigated.

nocones wrote: I'm laughing at the Irony of you owning a vehicle with that Aero profile and concerning yourself with MPG.

This. I bet you'll notice a bigger difference if you slow down 2 mph!

Bryce

I doubt you'll notice any MPG difference at all. Should help keep the rear window cleaner though.

Mmadness wrote: Back in December 2008 , C&D did a test of how various roof attachemnts (crossbars on vs. cross bars off vs. cargo carrier) affected fuel mileage. Ultimately, they determined that the crossbars alone reduced fuel mileage by around four percent. I would imagine the spoiler you mounted will have a slightly greater impact than the crossbars. However, you already have crossbars on the car so the impact may be mitigated.

I always wondered about that. Everyone is chasing miliage these days.. yet I still see minivans and SUVs with roof racks (and station wagons too) and rarely do I ever see anything strapped to them.

Mmadness wrote: Back in December 2008 , C&D did a test of how various roof attachemnts (crossbars on vs. cross bars off vs. cargo carrier) affected fuel mileage. Ultimately, they determined that the crossbars alone reduced fuel mileage by around four percent. I would imagine the spoiler you mounted will have a slightly greater impact than the crossbars. However, you already have crossbars on the car so the impact may be mitigated.

This

mad_machine wrote:

Mmadness wrote: Back in December 2008 , C&D did a test of how various roof attachemnts (crossbars on vs. cross bars off vs. cargo carrier) affected fuel mileage. Ultimately, they determined that the crossbars alone reduced fuel mileage by around four percent. I would imagine the spoiler you mounted will have a slightly greater impact than the crossbars. However, you already have crossbars on the car so the impact may be mitigated.

I always wondered about that. Everyone is chasing miliage these days.. yet I still see minivans and SUVs with roof racks (and station wagons too) and rarely do I ever see anything strapped to them.

As soon as I saw the mileage drop 2mpg on my IS300 from my roof rack, it was off. Besides the noise, I don't get why people would drive around with huge mpg killing bars on their roof either.

Had a customer swear that his MPG dropped considerably after changed his oil. I'm SURE it had nothing to do with that giant cargo box you mounted on your roof after it left.

Thanks for all the input, this is interesting for sure!

I already have crossbars with bike mounts and a Hi-Lift on the roof, so I'm not really chasing mpgs. If I was, the first thing I would do is not drive an Isuzu Trooper....

Cone_Junky wrote: As soon as I saw the mileage drop 2mpg on my IS300 from my roof rack, it was off. Besides the noise, I don't get why people would drive around with huge mpg killing bars on their roof either.

I do it because I throw a bike or two on top of the car and go mountain biking several times a week. Around town, the hit isn't as bad either.

If I didn't go biking often, I'd take the roof rack off in a heartbeat. They really are noisy, and the mileage hit at speed is definitely noticeable.

Hey guys!! I've got an aerodynamics question about my toaster!!

I've often thought about this too. Our 2008 4Runner has a similar (albeit much more stylish-looking with integrated high brake light) "spoiler" in the same spot - as did all 4Runners that weren't SR5 models, for a number of years (IIRC).

IDK about mileage (we have a roof rack with a fairing on the front, so air may not hit it much anyhow), but when I roll down the rear hatch window with the driver's window down, it's virtually silent back there and up front....almost no wind noise at all....great flow. Not sure if the spoiler has anything to do with it. With the window up in the rain, I can see the wind blowing raindrops off the rear glass, downward. I never use the rear wiper. Never need to.

mad_machine wrote: I always wondered about that. Everyone is chasing miliage these days.. yet I still see minivans and SUVs with roof racks (and station wagons too) and rarely do I ever see anything strapped to them.

After reading this, I took the crossbars off the van that very day. As a result, the car is also much quieter and is less susceptible to crosswinds. I estimate it saved over $200 a year with the number of miles we pile onto it. It really isn't difficult, four bolts that can be taken out with an Alan key. On some Jeeps, the crossbars have knobs on them so they can be removed without tools.

SVreX wrote: Hey guys!! I've got an aerodynamics question about my toaster!!

If you use thick sliced bread it will help plug the slots in the top for smoother airflow. I am curious if sesame seeds could work like small vortex generators.

Can I assume this trick would work for a first gen 4Runner? What spoiler would you recommend? I figured there was a massive low pressure area back there, as the exhaust fumes were nasty with the back window down.