What’s the Difference Between All-Wheel Drive and Four-Wheel Drive?

iStock/PeopleImages
iStock/PeopleImages

As the weather turns nasty, you may start to think more about your car’s transmission—particularly, its ability to get you up and down muddy, snowy, or icy roads. While you may know that two-wheel drive isn’t the best option for driving in harsh weather, other terms are more confusing. Like, for instance, all-wheel drive versus four-wheel drive. Don’t all cars have four wheels (at least when it comes to sedans, SUVs, and other consumer vehicles)? As Jalopnik explains, the difference between all-wheel drive (AWD) and four-wheel drive (4WD) is more than just semantics—and which type you need depends heavily on the kind of driving you do on a regular basis.

When a car manufacturer specifies something as two-, four-, or all-wheel drive, the company is referring to the tires that receive power from the engine. With two-wheel drive, the engine powers either the front axle or the back axle of the car, meaning that the engine is moving only the front tires or the back tires. The other axle is just rolling along. When four-wheel drive (sometimes referred to as 4x4) is engaged, meanwhile, the engine rotates all four wheels. This helps give the car extra traction on slippery surfaces like sand, ice, and gravel—so that even if one set of wheels can’t get traction on a slippery surface, the other wheels might be able to.

The key word there is “engaged.” Traditionally, four-wheel drive means that your car can drive with all four wheels, but you have to manually choose that option. The rest of the time, you’re in two-wheel drive mode. As Ray Magliozzi of NPR’s "Car Talk" once explained to a listener, four-wheel drive is “designed to be engaged when you're already stuck, or in a specific situation where you know you might get stuck—like in snow, sand, or mud. It’s not designed for normal road use, and must be disengaged before you drive on dry, paved roads.”

In all-wheel drive, meanwhile, the car figures out what kind of traction you need all on its own. It has sensors that figure out how much power should go to each wheel to give you the best traction without any input on your part. This is a useful feature for driving in a variety of different conditions, while a dedicated four-wheel drive mode might be better for serious off-roading.

Be warned: There are downsides to all that extra traction. A car’s engine has to work harder to power all four wheels. Those engines are also heavier, which itself takes more power to move. As a result, four-wheel or all-wheel drive cars usually use significantly more fuel than your typical two-wheel drive model.

There are other caveats to this explanation: Full-time four-wheel drive cars do exist, but they differ slightly from all-wheel drive vehicles because of how the two axles of the car move in relation to each other. In four-wheel drive, the front and rear wheels are locked together so they spin at the same rate; in all-wheel drive, the wheels aren’t locked like this—different amounts of power go to the different wheels as needed.

All this said, even if you live somewhere with snowy winters, it doesn’t necessarily mean you should spring for all-wheel drive. If you’re driving a lot in inclement weather—say, if you live in South Dakota or Wyoming rather than sunny Southern California—winter tires may actually be a lot more important than which wheels the engine is powering. Need proof? Watch the video comparison here.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

What's the Difference Between Mold and Mildew?

iStock.com/AndreasReh
iStock.com/AndreasReh

We’re all familiar with colorful spots of something growing in our showers and in other dark, damp areas in our homes, but you may not know what to call it. Is it mold, or is it mildew? What is the difference between the two, anyway?

Both terms refer to fungus, but as it happens, it’s a squares-versus-rectangles situation. Mildew is a type of mold. The term typically describes fungi that grows flat, on surfaces like the walls of your shower or window sills. There are also several types of mildew that are specific to plants—powdery mildew and downy mildew are parasites that grow on certain trees, flowers, and crops, for example. While mold might be a colorful green or black, mildew is typically white.

The word mildew originally came from honeydew, a term for sticky secretions aphids and other insects leave on plants, which people used to think came from the sky, like dew. Eventually, the word came to refer to the mold caused by the fungi that fed on these secretions.

Leaves covered in white powder
Powdery mildew on maple leaves
iStock.com/kazakovmaksim

Most of the household growths we refer to as mold belong to just a few families of fungi species. According to the CDC, the most common indoor molds are Cladosporium, Penicillium, Aspergillus, and Alternaria. Household molds can be a variety of colors, from orange-brown to green to gray to black. (Note that not all mold that is black in color is the more toxic species we call “black mold,” or Stachybotrys.) In contrast to the powdery texture of mildew, molds are typically fuzzy or slimy.

In nature, mold can play an important role in the ecosystem, breaking down dead plants and leaves. In your house, those decomposition abilities aren’t quite so welcome. Mold spores fly through the air, and when they land in moist places, they start to grow—whether that’s on food, your ceiling, paper products, wood, carpet, leather, or elsewhere around your house—and in the process, destroy whatever they're growing on. Unlike mildew, most molds grow down into the surface of its habitat, making them more difficult to remove. In porous materials, mold grows into all the empty crevices, which is why it is often impossible to remove all the mold from ceiling tiles (or soft foods like bread).

Mold growing under a windowsill and near the carpet of a home
Mold growing in a Nashville home following a flood
Martin Grube, FEMA // Public Domain

Getting rid of the unsightly growth in your damp bathroom is more than just a matter of aesthetics. Indoor mold can cause allergic reactions, such as a stuffy nose or itchy eyes, and can lead to infections for people with compromised immune systems. Some people are more sensitive to mold than others, and may experience more symptoms when exposed to it. Generally speaking, though, mold spores are everywhere, so you’re never going to live a totally mold-free life. Spores will come into your home through windows, doorways, ventilation and climate control systems, and via your clothing, shoes, and pets.

But there’s only one way to effectively inhibit mold growth at home: Get rid of the moisture. That means fixing leaks, getting better ventilation, and possibly running a dehumidifier, according to the CDC’s recommendations on mold.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

What Happens to Films Selected for Preservation by the Library of Congress?

iStock
iStock

On Wednesday morning, the Library of Congress announced its latest slate of movies selected for permanent safekeeping in the National Film Registry. As always, the picks varied widely. The National Film Registry’s class of 2018 includes Cinderella (1950), My Fair Lady (1964), Jurassic Park (1993), The Shining (1980), Smoke Signals (1998), and the animated short Hair Piece: A Film for Nappy-Headed People (1984), which was produced by Ayoka Chenzira, one of the first black female animators.

Originally established in 1988, the National Film Preservation Act tasks the board with selecting American films that are “culturally, historically, or aesthetically" significant. They can pick up to 25 per year, and the movies must be at least 10 years old. The National Film Preservation Board is made up of representatives from a number of industry organizations, including the Academy of Motion Picture Arts and Sciences, the Directors Guild of America, and the National Society of Film Critics. With the new selections, there are 750 films in the registry.

Selection for the registry is an honor, of course, but what does it mean beyond that? How does the Library of Congress, the U.S. legislature’s storage agency for documents and media, go about preserving movies?

According to Steve Leggett, program coordinator for the National Film Preservation Board, selection implores the Library of Congress to get the best possible copy of the film in its original format and store it in their vaults at the National Audio-Visual Conservation Center in Culpeper, Virginia. This ensures the film will be available to future generations.

For Hollywood movies, the process is usually pretty easy. “We simply ask the studio to donate a copy,” Leggett told Mental Floss in 2015. In some cases, that isn’t even necessary. The Library of Congress has more than 1 million films on file, many of them sent by studios or filmmakers for the sake of copyright registry. When the original Star Wars was selected in 1989, Leggett says, congressional librarians simply checked that the 35 millimeter print submitted with Lucasfilm’s copyright application was in good shape. It was, so no further action was needed.

For older and more esoteric selections like newsreels, silent films, documentaries, and early technical achievements in filmmaking, Leggett says the library often seeks out a copy from the community of preservationists. Universities, private foundations, and hobbyists that preserve old films might get a call from the Library of Congress if they have a good copy of a National Film Registry selection. In rare cases, the library will barter for the film, using redundant materials on its shelves. Other times, it will make a copy or pay the archivist to make a new 35 millimeter copy for them. The Culpeper facility stores nitrate prints, the original film stock for many early movies, in specialty lockers because the material is highly volatile and flammable.

Silent films can be tricky because studios often released, revised, and then re-released versions of the film. When one is selected, Library of Congress archivists collect as many aspects and versions of the film as they can, which might mean contacting several studios and archivists.

Of particular challenge in 2015 was the induction of Symbiopsychotaxiplasm: Take One, William Greaves’s quasi-documentary of his 1968 theatrical project staged in Central Park. The film was screened often through the years, as Greaves gained a cult following. It was released on DVD in 2006, but the National Film Preservation Act specified that the library should seek a copy in the original format, which it didn’t have. Leggett said Greaves’s 1968 original cut was “lost,” but the library worked with the late filmmaker's estate to create a new 35 millimeter version that resembled it.

The Audio-Visual Conservation Center itself, buried on a mountainside, has storage space controlled to stay cool and dry. “A film could survive for hundreds of years there,” Leggett says. He admits the audiovisual center wouldn’t survive a nuclear strike—in the event of World War III, the world might lose its best copy of Buster Keaton’s The General—“but it did survive an earthquake with all materials intact.”

An earlier version of this article ran in 2015.

SECTIONS

arrow
LIVE SMARTER