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 Caused Pangea to Break Apart?

iStock.com/alfimimnill
iStock.com/alfimimnill

Emily Devenport:

There's another way to look at this question. People tend to think in terms of supercontinents forming and then breaking up again due to convection currents in the mantle, hot material rising and causing rifts in weaker spots, possibly in old sutures where the continents were shoved together—but what is really happening is that ocean basins are opening and closing, and the ocean has an active role in subduction.

The opening and closing of an ocean basin is called a Wilson Cycle. It begins when hot material rising from the mantle stretches the overlying crust. As molten material rises, a rift is formed. The rift is widened as material continues to squeeze into it. If that rifting goes on long enough, through a broad enough swath of a continent, ocean water will eventually flow into it, and an ocean basin begins to form. The upwelling of hot material will continue to rise through that thinner area of crust, pushing the plates apart. The Atlantic Ocean is an example of a basin that is well along in the Wilson Cycle; eventually subduction is going to begin at its margins, and the whole shebang will pivot.

This will happen because at the edge of continents, sediments accumulate. The weight of those sediments, combined with the weight of the water, drives the heavier, denser edge of the oceanic plate under the continental crust, which is fatter and lighter. Eventually subduction begins, and the basin begins to close again. The Pacific Ocean is an example of a basin that's closing.

If you look at a map of the oceanic rift zones, you'll notice that the one in the Atlantic is pretty much in the middle of that ocean, but the Pacific rift zone has been pulled all the way over to North America above Central America. Subduction is actively occurring on all margins of that plate.

The simple picture is that the continents are moving toward each other across the Pacific Ocean while the Atlantic Basin continues to widen. The truth is more complicated. When plates subduct, the water in the crust lowers the melting point of those rocks, so partial melting occurs. The partially melted material begins to rise through the overlying rocks, because it's less dense, and decompression melting occurs. Eventually, the upwelling of hot material forms plutons and volcanoes above the subduction zones. Fore-arc and Back-arc [PDF] basins can form. As the oceanic crust is pulled under the continental plate, island chains and other chunky bits get sutured to the edge of the continent along with sediments, making it larger. Our world is ~4.6 billion years old, so our continents are really large, now. They're unlikely to rift through the ancient cratons that formed their hearts.

What will happen if subduction begins on the eastern side of North America before the Pacific Basin closes? The margin next to California is a transform fault; it's not subducting. Will it eventually push itself under that part of North America again, or will the transform zone get bigger? The hot spot that was driving the ancient Farallon Plate under North America was eventually overridden by the southwestern states (Arizona, New Mexico, etc.) forming a rift zone. Will it continue to rift or poop out?

There are computer models predicting what supercontinent may form next. They will continue to change as our understanding of tectonic processes gets more accurate.

This post originally appeared on Quora. Click here to view.

What Do Those Recycling Symbols and Codes Mean?

iStock.com/ChrisSteer
iStock.com/ChrisSteer

Earth Day is here again, serving as an annual reminder of the need to reduce, reuse, and recycle our way to a better planet.

When it comes to the last part of that familiar three-”R” mantra, most people know enough to separate certain items from the rest of their garbage, but much of our modern recycling routine remains a mystery. From the recycling symbol itself to what those numbers on plastic containers actually mean, there's a lot you can learn from your trash before it becomes someone else's treasure.

An International Symbol With An Earthy Origin

The universal recycling symbol—three folded arrows that form a triangle, with the head of one arrow pointing to the tail of the next—was created in 1970 by University of Southern California student Gary Anderson as part of a contest tied to the very first Earth Day. Each arrow of the design represents one of the steps in the recycling process: collecting the recyclable goods after use, breaking them down and reforming them, and then packaging new products in the containers.

Originally designed as an inverted triangle, the symbol was later rotated to the pyramid-like orientation commonly used now.

The Number Game

The American Society of Plastics Industry first began using numbers inside the recycling symbols on plastic containers in 1988 as a way to assist with sorting them. The "Resin Identification Code" uses seven numbers to identify the type of synthetic material used to manufacture the container, with the higher numbers representing less commonly used plastics.

Here's a primer on each of the codes:

1. Polyethylene Terephthalate (PETE/PET)
Usually accompanied by the letters "PETE" or "PET," this resin is generally used for soda bottles and other containers for edible and non-edible goods. When it's not being used to manufacture containers, you might recognize it by another name: polyester. (Yes, it's the stuff that insulates your jackets.) It's also one of the most widely accepted forms of plastic in curbside recycling programs, though the amount of useable material available for new products after breaking down this plastic is relatively small.

2. High Density Polyethylene (HDPE)
The second most widely used resin for plastic bottles, HDPE is a stiff, strong material with a high resistance to chemicals, which has made it the go-to plastic for food items like milk and juice, as well as household cleaners and trash bags. It's also easy to break down in the recycling process and easy to reform, making it one of the most efficient consumer plastics. Most curbside recycling programs have no problem with accepting products made from this plastic.

3. Polyvinyl Chloride (PVC)
First discovered in the 19th century, PVC is commonly used in building materials today—especially pipes and plumbing material—due to its strength and chemical resistance (although it's occasionally used for some household products). It has a nasty habit of releasing highly carcinogenic toxins into the atmosphere when it's burned, so recycling is a significantly less appealing option for PVC disposal, and it's usually not accepted by curbside recycling programs.

4. Low Density Polyethylene (LDPE)
This plastic is becoming more common today, especially for manufacturing squeeze bottles and grocery bags. Plastics made from LDPE are usually very strong, and they're regularly used as sealants because of this quality. While they weren't included in curbside recycling programs at first, plastics made from LDPE are now becoming more commonly accepted.

5. Polypropylene (PP)
Regarded as one of the “safest” plastics produced today, PP is generally used for squeezable bottles, bottle caps, and straws. Along with LDPE, it's also used for food-storage containers that can be reused over time. It has an extremely high melting point, so it's one of the best consumer plastics for items that will be exposed to heat. Like LDPE, it's becoming more common for curbside recycling programs to accept items made from this plastic.

6. Polystyrene (PS)
More commonly known as styrofoam, this type of plastic is not only notoriously difficult to recycle, but it's also been shown to leach dangerous toxins over time into anything packaged in it—and even greater amounts of toxins when it's burned. This is the resin usually found in disposable serving trays, egg cartons, and cups, and it's rarely accept by curbside recycling programs due to the danger it poses and the difficulty of recycling it. Basically, this is the worst of the bunch.

7. Everything Else
There are countless other plastics, but very few of them are easily recycled in curbside programs, making this category the catch-all for everything that could conceivably be broken down and reformed, but might be better off reused or reformed in some way that doesn't require a chemical process. This category encompasses everything from bulletproof material to those large water jugs on office coolers, and is rarely included in curbside recycling programs.

Safety In Numbers

For anyone wondering which plastics are safe to reuse in their current form, it's widely accepted that HDPE (2), LDPE (4), and PE (5) can be reused multiple times for edible items, as they're generally resistant to chemicals, haven't been shown to degrade, and don't leach dangerous substances into their contents.

This story first ran in 2013.

SECTIONS

arrow
LIVE SMARTER