wing on the back of a car increasing MPG? Color me dubious.

May 5, 2006 8:12 AM

Last night on ABC news there was a guy with a VW beetle (silver) who put a wicked-huge wing on the back and claimed some ridiculously-high MPG figure. Alas, my google fu fails me. Why would this wing possibly work? They guy wasn't a tuner, AFAICT.
posted by Wild_Eep to science & nature (27 comments total)

this one ?
posted by Baud at 8:18 AM on May 5, 2006

sorry, no. I found that one too. The guy you link to put a jet engine in his beetle, I'm looking for a guy who bolted on a homemade looking wing.

The horizontal portion was large, and may have connected to the car directly under the rear window. It had a large surface area (IMO, about the same horizontal surface area as a VW Jetta trunk lid.)

The wierdest thing about it though were the *large* side-surfaces of the wing. They seemed to be ~18" guitar-pick shapes on each side, standing vertically.

I would think the vertical bits would make the car more stable at speed (go in a straight line better), but I don't know why it would have *any* effect on mpg. If anything, I'd think their weight would be a hinderance.
posted by Wild_Eep at 8:24 AM on May 5, 2006

Was it this?
posted by TonyRobots at 8:26 AM on May 5, 2006

TonyRoberts' linked article attributes a 5-8% increase in mileage to the wing. Not bad, but not miraculous.
posted by MrMoonPie at 8:34 AM on May 5, 2006

So, it's a turbo diesel that gets like 50mpg without the wing. Looks like the wing takes it up to 60mpg. Maybe less stunning because of the high baseline mpg?
posted by Mid at 8:34 AM on May 5, 2006

Wow, great job!

How'd you find it?
posted by Wild_Eep at 8:37 AM on May 5, 2006

The wing plus new, low-rolling-resistance tires takes it up to 58.8 mpg, from a starting point of 52.5. That's a 12% increase, about half of which is due to the wing.
posted by MrMoonPie at 8:41 AM on May 5, 2006

I don't know if the numbers are true or not, but TonyRobots' article summarizes how the energy is saved pretty well.

As to how the wing would work, for the kind of drag improvement described the spoiler would be preventing or reducing flow separation from the back end of the car. The Beetle's hidquarters have a pretty bad aerodynamic shape (put there in the name of style) so there is much room for improvement there.
posted by cardboard at 8:45 AM on May 5, 2006

Google: vw wing mpg
posted by TonyRobots at 8:48 AM on May 5, 2006

sorry, slight correction, it was: vw wings mpg
posted by TonyRobots at 8:49 AM on May 5, 2006

Wow, I wonder how much LRR tires cost, and what the break-even point is for them.
posted by Wild_Eep at 8:55 AM on May 5, 2006

Wild_Eep:

This PDF explains:

"The highest and lowest rolling resistance tires we tested differed in efficiency by 60%, indicating that tire choice can have a bigger impact on fuel economy than most people realize. Rolling resistance differences of 20 to 30% are not uncommon among tires of an otherwise similar size, type, and level of performance. This means an individual vehicle could save up to 6% of its gasoline use if it were fitted with very efficient tires, paying for the modest additional cost of low rolling resistance tires in approximately a year of fuel savings.

In other words, a typical compact car such as a Ford Focus can improve its mileage from 30 mpg to 32 mpg simply by using lower rolling resistance tires. For a car averaging 15,000 miles per year the fuel savings is about $50
(at $1.50 per gallon)."
posted by odinsdream at 9:38 AM on May 5, 2006

Wow, I wonder how much LRR tires cost, and what the break-even point is for them.

LRR tires are almost certainly a false economy. You are trading traction for fuel economy. While decreased acceleration may not bother many people, the decreased stopping power will bother you as soon as the person in front of you slams on their brakes.
posted by b1tr0t at 10:22 AM on May 5, 2006

Rear-mounted spoilers take effect only when you hit about 90 mph. Also, they push the rear down, so that the tires are pressed harder against the pavement, increasing traction. If anything, this would increase rolling resistance, since it would increase the amount of flexing in the sidewalls.

In words of one syllable, it's a hoax.
posted by KRS at 10:23 AM on May 5, 2006

KRS, are you an aerospace engineer that collected and published data to support your hypothesis?
posted by parallax7d at 10:37 AM on May 5, 2006

traction ! = acceleration, except at the drag strip, which is pretty irrelevant in a discussion about fuel economy.
posted by Kirth Gerson at 10:39 AM on May 5, 2006

The guy gives a rather cursory explanation on how it works here.

IANAAE, but probably what it does is reduce the turbulence of airflow coming off of the end of the car. The winglets probably work as they do on aircraft, to reduce drag caused by the wing itself.
posted by zsazsa at 11:01 AM on May 5, 2006

its not a hoax. this isnt a spoiler in the traditional sense, its designed to smooth the airflow over the back of the car, to prevent friction losses due to turbulence.

although he's kind of cheating since he went for the LRR tires as well :)
posted by joeblough at 11:09 AM on May 5, 2006

traction ! = acceleration, except at the drag strip, which is pretty irrelevant in a discussion about fuel economy.

False. Reduce your tire's traction, and you increase stopping distance.
posted by b1tr0t at 12:12 PM on May 5, 2006

Even if acceleration traction were equivalent to stopping traction (it isn't) - stopping distance has what to do with fuel efficiency?
posted by Kirth Gerson at 12:20 PM on May 5, 2006

Kirth - I'm not sure what physics fantasy land you live in, but in the real world, friction doesn't care if you are accelerating or decelerating. It resists slippage in either direction. Since most people don't have cars with engines that will spin the wheels, or if they do, don't drive in a manner that would cause wheel spin, maximum traction is a more important in a panic-stopping situation. LRR

Wild_Eep wondered about the break even cost of LRR tires, and I suggested that LRR tires are probably a shortsighted solution because they reduce your ability to stop your car. While they may save you a few dollars every time you pump some gas, they increase the chance that you or your car will be destroyed the next time you find yourself in a sudden traffic jam.

You can try the following experiment yourself, ideally on a long stretch of empty road: With normal tire inflation, measure your minimum stopping distance from 55 MPH to 0. Now overinflate your tires to simulate LRR tires. You could drive a few hundred miles to measure the fuel savings, but you will probably damage your tires (the most likely result is uneven tread wear caused by the overinflated tire shape) so I don't recommend doing so. Again measure your stopping distance from 55 MPH to 0. You will find that the stopping distance has increased. The time it took you to reach 55mph in the first place will probably increase as well, but that is tangental to the discussion at hand.
posted by b1tr0t at 12:37 PM on May 5, 2006

B1tr0t, there is a difference between Rolling Resistance, and Static Friction. A tire with low RR does not essentialy have to also have low static friction.
posted by nomisxid at 1:10 PM on May 5, 2006

There are enough exotic materials out there that I could imagine one with the properties you suggest. In most applications where a LRR tire is used, a narrow tire is typically used, and it is often run at a high pressure (Insight, Prius), compared with ordinary cars.

Does the LRR compound make up for the traction lost by using a much smaller contact patch?
posted by b1tr0t at 1:20 PM on May 5, 2006

That's a false comparison, bitrot. Regular tires are designed to run at a particular inflation and using them out of spec certainly doesn't "simulate LRR tires" in any meaningful way. LRR tires are designed though particular textures and tread patterns to have less rolling resistance while preserving stopping ability.

They may indeed suffer somewhat but given how litigious our society is I find it impossible to believe they are simply inferior tires.
posted by phearlez at 1:26 PM on May 5, 2006

b1tr0t; the PDF I linked to explains much of what you're concerned about. The government rates stopping distance on a scale, which the authors of the PDF use in judging the tire selection. Tires that scored too low aren't recommended, even if their rolling resistance were very low.

So, the PDF does an excellent job of providing you with LRR tire options that also perform well with regard to stopping distance and other factors. In short, it's written with the average concerned consumer in mind, not the fuel-efficiency fanatic.
posted by odinsdream at 4:02 PM on May 5, 2006

It looks like I was wrong. My objection should be to narrow-width tires and high pressures, not LRR tires.

I jumped the gun and assumed LRR tires included all methods of lowering resistance, rather than a particular tire design.
posted by b1tr0t at 4:07 PM on May 5, 2006

Rear-mounted spoilers take effect only when you hit about 90 mph. Also, they push the rear down, so that the tires are pressed harder against the pavement, increasing traction. If anything, this would increase rolling resistance, since it would increase the amount of flexing in the sidewalls.

Maybe in a car that wasn't design primarily for athsthetics, like the VW beetle, rather then aerodynamic performance, like the Honda Insight or Toyota Prius. The wing, in this case, corrects an aerodynamic flaw.
posted by delmoi at 11:47 PM on May 5, 2006