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.

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Why Does Hand Sanitizer Have an Expiration Date?

Hand sanitizer does expire. Here's why.
Hand sanitizer does expire. Here's why.
galitskaya/iStock via Getty Images

The coronavirus pandemic has turned hand sanitizer from something that was once idly tossed into cars and drawers into a bit of a national obsession. Shortages persist, and people are trying to make their own, often to little avail. (DIY sanitizer may not be sterile or contain the proper concentration of ingredients.)

If you do manage to get your hands on a bottle of Purell or other name-brand sanitizer, you may notice it typically has an expiration date. Can it really go “bad” and be rendered less effective?

The short answer: yes. Hand sanitizer is typically made up of at least 60 percent alcohol, which is enough to provide germicidal benefit when applied to your hands. According to Insider, that crucial percentage of alcohol can be affected over time once it begins to evaporate after the bottle has been opened. As the volume is reduced, so is the effectiveness of the solution.

Though there’s no hard rule on how long it takes a bottle of sanitizer to lose alcohol content, manufacturers usually set the expiration date three years from the time of production. (Because the product is regulated by the Food and Drug Administration, it has to have an expiration date.)

Let's assume you’ve found a bottle of old and forgotten sanitizer in your house somewhere. It expired in 2018. Should you still use it? It’s not ideal, but if you have no other options, even a reduced amount of alcohol will still have some germ-fighting effectiveness. If it’s never been opened, you’re in better shape, as more of the alcohol will have remained.

Remember that sanitizer of any potency is best left to times when soap and water isn’t available. Consider it a bridge until you’re able to get your hands under a faucet. There’s no substitution for a good scrub.

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Why Are Sloths So Slow?

Sloths have little problem holding still for nature photographers.
Sloths have little problem holding still for nature photographers.
Geoview/iStock via Getty Images

When it comes to physical activity, few animals have as maligned a reputation as the sloth. The six sloth species, which call Brazil and Panama home, move with no urgency, having seemingly adapted to an existence that allows for a life lived in slow motion. But what makes sloths so sedate? And what horrible, poop-related price must they pay in order to maintain life in the slow lane?

According to HowStuffWorks, the sloth’s limited movements are primarily the result of their diet. Residing mainly in the canopy vines of Central and South American forests, sloths dine out on leaves, fruits, and buds. With virtually no fat or protein, sloths conserve energy by taking a leisurely approach to life. On average, a sloth will climb or travel roughly 125 feet per day. On land, it takes them roughly one minute to move just one foot.

A sloth’s digestive system matches their locomotion. After munching leaves using their lips—they have no incisors—it can take up to a month for their meals to be fully digested. And a sloth's metabolic rate is 40 to 45 percent slower than most mammals' to help compensate for their low caloric intake. With so little fuel to burn, a sloth makes the most of it.

Deliberate movement shouldn’t be confused for weakness, however. Sloths can hang from branches for hours, showing off some impressive stamina. And because they spend most of their time high up in trees, they have no need for rapid movement to evade predators.

There is, however, one major downside to the sloth's leisurely lifestyle. Owing to their meager diet, they typically only have to poop once per week. Like going in a public bathroom, this can be a stressful event, as it means going to the ground and risking detection by predators—which puts their lives on the line. Worse, that slow bowel motility means they’re trying to push out nearly one-third of their body weight in feces at a time. It's something to consider the next time you feel envious of their chill lifestyle.

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