DarbyWalters said:
Big Square ended vehicles would probably benefit the most. The problem is placement...how far away from the trailing edge should they be placed based on speed ranges (mph) where you want the most improvement. I know from racing for Mazda for over 10 years that other devices such as spliters ect do work, but you have to place them precisely to get the desired effect. I definitely think these would work in a few places on my Jeep Liberty...in front of wheelwells, on fuel tank skid, back section of roof and sides, ect. Wishbone shaped air ducts work great getting air in hood, fenders for brakes and windows for cool air. Just thought they were something different. If you do a lot of highway travel, they would probably show a slight increase in mpg on a "Block Shaped" vehicle like a Commander. If they lessen dirt on the rear window and tailights and help with "buffitting" on the highway, that is just an added bonus.
I'm going to have to disagree with you. Maybe splitters and vortex generators and the like work at the high speeds attained by race cars (I used to race, know racers, and have extensive background with Cobras, Cobra replicas and Mustangs. I've had one of my Cobras - stock windscreen, no aerodynamics and 1965 styling - at 165mph), but it's my layman's opinion that adding the wishbone thingys won't do anything to enhance the aerodynamics of the Commander.
In fact, I'm of the opinion that adding anything like vortex generators, wishbones, wings or anything to modify the airflow off the rear of the vehicle will actually decrease fuel economy and ruin the aerodynamics and increase drag coefficient - something you most certainly would not want on a Jeep Commander, which already has a staggeringly-high .42 Cd.
And here's my reasoning why...
As I'm sure you know drag is based on airflow. Drag Coefficient (Cd) is based off a complicated formula that I can't pretend to explain. I can make it work, I just don't have technical language chops to write it all down in a smart way. All I can say is that aerodynamics in aircraft are significantly different than in automobiles.
Further, it's been shown that by changing the shape of big, flat things like Peterbilt and Mack trucks by adding on the giant fiberglass curved airdam on the roof of the tractor (at the
front end of the rig) and putting the trailers closer together and closer to the tractor you can significantly improve the Cd of these vehicles. All without adding vortex generators, wishbones or wings to the last trailer.
It's been shown that pickup trucks get worse mileage and have a higher Cd if you lower or remove the tailgate - and the number gets even higher if you add a tailgate net.
Race cars, to get better mileage will draft other race cars. And, more importantly, 'racecar' spelled backwards is 'racecar' ... Okay, that has nothing to do with anything, but I thought it was a neat little palindrome. And using words like palindrome make others think I know what I'm talking about, and that I have a better grasp of the english language than I do.
The reason why I don't believe that the wishbones will at all help or improve aerodynamics on the Commander are because of all above examples. When you're traveling down the road (especially in a pickup truck or an SUV like the Commander) the air breaks around the vehicle, rejoining at the rear. With the square corners and large flat area (or truckbed) there's a misconception that the air is wicking around those corners and increasing the surface area the air has to pass by - which increases drag. This is not how the airflow actually works.
I'm going to have to use the pickup truck airflow as an example, because it's easily verifiable (I'm not going to cite examples of the research, though. Anything I cite could be considered slanted. Do a search on tailgate aerodynamics and choose your own example).
As speed increases, a phenomenon known as a "stagnant air bubble" is created. It gets larger the faster the vehicle goes. This air is virtually motionless. If you've ever driven behind a pickup truck with the tailgate up and leaves or light trash in the back, you've undoubtedly seen leaves that seem to 'float' in the air in the bed, occasionally going high enough to be grabbed by the air moving over the surface of the air bubble to be whipped around. If you've ever been behind a pickup truck with the tailgate down or off, you know that nothing stays in the bed for long; it all gets sucked out and inevitably hits your car (I'm sure there's a Murphy's Law or formula that proves that the cleaner your car is the more road debris hits it, rather than the dirty 1972 Pinto driving next to you).
This is the same thing that happens at the back of the Commander. There's a big stagnant air bubble chasing it down the road. The way the air moves over the vehicle, it moves down the body and - instead of moving around the rear corners and down the back window - it flows
over the air bubble. Once the airflow is off the rear of the vehicle, it no longer creates drag on the vehicle.
Changing the way air flows over the vehicle (again, citing the truck tailgate up vs. down) - effectively 'breaking' the airflow the way the wishbones does - destroys the stagnant air bubble. Without that, the air still has to come back together behind the vehicle. Without the air bubble, you create a vacuum directly behind the vehicle. The airflow no longer joins behind the air bubble, it now joins at the rear of the vehicle, which means that it flows down, around and up from under the car and creates
more drag as the air flows over the entire rear surface of the car. This is why trash gets
pulled out of a truck with the tailgate down.
Those little wishbones will ADD square feet of drag to the rear of the Commander. Adding drag decreases airflow efficiency. Decreasing airflow efficiency lowers gas mileage.
...That's how I see it.
And besides...they look funny.
Your pal,
Meat.
