Four-Wheel Drive Vs. All-Wheel Drive: What’s the Difference?
Four-Wheel Drive Vs. All-Wheel Drive
Today I’m looking out my window in beautiful Boulder, Colorado and watching the snowfall. We got about 5 inches overnight, and a couple of friends had come to stay the night after barhopping in Denver. As with most of my articles, this one started with a question; as my friend James was looking outside, he was considering the drive to work in his Jeep.
Another friend, Gene, was cleaning off his Subaru in my driveway. James remarked, “We’re lucky we both have four wheel drive, otherwise the drive to work would be terrible.” Time passed, they left, and I realized that there has to be a difference between drivetrains in vehicles, but I had no idea what they were.
In the spirit of spreading knowledge, here is what I learned.
Two-Wheel Drive
This is the most common drive-type in cars. What this means is that energy produced by the engine is fed through the transmission, which turns one axle containing two wheels. Hence, two-wheel drive is when two wheels on the same axle are permanently powered.
The benefit to this type of drive-type is that it is cheap to produce. Most other drive-types require specialized parts to properly deliver power to the wheels. Having a two-wheel drive system cuts down on the production cost of the car, which is usually then translated to the consumer as a lower retail price.
The faults with two-wheel are many; the most obvious of which is that the more wheels that have power driven to them, the more traction your car has. A two-wheel drive vehicle is only taking advantage of the traction from two tires, leaving the other two as free spinning. Another problem, not exclusive to two-wheel drive vehicles, is tire wear.
Take a length of string, and cut it exactly in half. Place them on a table parallel, as if you were simulating the tracks your tires would make on a road. Now, to finish the experiment; try bending them both to the left as if your car was making a left turn. The outside string won’t cover the same amount of distance as the inside. In car-terms, for a 90 degree left turn, most inside tires will rotate 10-15 times depending on size, whereas the outside will rotate 12-17 times. This creates more wear on that outside tire, which is the reason for needing to rotate tires 3-4 times a year.
This is most pronounced in two wheel drive vehicles, because two tires are completely power-free. Tires that endure the stress of transferring the energy from the engine to the ground will wear faster than ones that don’t. What this means is if you turn in one direction more than the other, all four tires will wear in a different way.
Four-Wheel Drive
There are two different categories of four-wheel drive; part time, and full time. It was researching these two drive-types that I really was surprised, because the assumption that these two categories are the same is dead wrong. While there are always exceptions to these rules, here are the basics.
Part time four-wheel drive was originally intended to give a car more traction to tow things, carry heavier loads, or drive in adverse conditions. These cars are two-wheel most of the time, and have a lever or knob to select four-wheel drive. Since this capability was meant to be used only when needed, using it on regular dry pavement will actually cause extra mechanical wear and damage your car. Regularly, two wheels get power to the ground; but when you stop and select four-wheel drive, the power from the engine is transferred not just to one axle, but both. All four wheels are powered this way, giving you double the traction. The important thing to remember about part time four-wheel drive is that it’s designed to be very good at its jobs, so it sacrifices the ability to use it on regular pavement.
Full time four-wheel drive is self-explanatory. In cars that have this feature, all four wheels are driven by power delivered to both axles. However, a vehicle with full time 4WD will be worse in adverse conditions than its part time cousin. The reason for this comes down once again to production cost. To make a four-wheel drive vehicle active full time would require an extremely robust transmission. these transmissions are usually not stock items on the production line. Since buying in bulk saves money, they just reduce the amount of power that can be delivered to all four wheels so the same transmission can be used in part time and full time vehicles. Usually, these cars don’t even have a two-wheel drive option, and some rough terrain competence is retained. The reason for rolling this out is to provide added stability to everyday driving, making the morning commute safer for everyone.
All-Wheel Drive
A vehicle equipped with all-wheel drive is essentially a modified 4WD car. PR departments of car companies to create another selling point for the vehicle created the term all-wheel drive. When you think about the meaning of the words, it simply is stating that all of the wheels have power delivered to them. A 4WD vehicle would also qualify as having all-wheel drive.
The difference is in the computer. A four-wheel drive vehicle delivers the same power to both axles all of the time. When a car with all-wheel encounters slippery terrain, the car’s computer gets involved. The computer starts running calculations at the rate of hundreds per minute; comparing variables such as speed, friction, tire wear, tire rotation, road quality, and road slope. When some of those variables cause the wheels on one axle to start to lose traction, the computer decreases the amount of power going to that axle, and redirects it to the other. You may remember the old Subaru saying “It takes power from the wheels that slip to the wheels that grip.”
Aside from the computer involvement, the only other major difference is that all-wheel drive is generally installed on passenger vehicles. Almost all vehicles with all-wheel drive lack a low range transfer case in their transmission. This reduces the amount of torque, or pulling power from a stop, making a 4WD vehicle better for towing.
Pete Wise is a White-Hat SEO Jedi who is in love with Content Creation. This article was researched and written for the AAMCO Centers of Colorado, who provide expert transmission repair and auto repair. If you like what you read, follow Pete on Twitter: @MySEOHeadache
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