Four-Wheel, Front-Wheel, All-Wheel Drive
- Making 'Sense' out of Making 'Traction' -
By Steve Ford - The Car Guy®
If you understand why it's better to have the inside lane when you race in a 440-yard dash, then you'll certainly grasp this next concept.
To refresh your memory, when you run against several other individuals on an oval track then each of you starts in a separate lane. The person on the inside lane is actually running on a slightly smaller oval; with each of the runners on the outside lanes running on slightly larger oval paths. That's where that cliché term of being on the "inside track" comes from.
Okay, so what's this got to do with cars? Well, this "inside track" principle is what requires your automobile or truck to have a differential. Whenever you make a right or a left-hand turn in your vehicle, the wheels on the inside of the turn rotate fewer times (smaller arc) than the wheels that follow the turn on the outside.
WHAT'S THE 'DIFFERENCE"?
When making a right-hand turn through an intersection, for example, the right-hand (inside) set of wheels might rotate 10 times to complete the arc -- while the left-hand (outside) set of wheels rotate 12 times -- resulting in "different" rotation speeds for the wheels.
Now, what would happen if both sets of wheels were rotating on the exact same shaft? Clearly, the tires would wear more quickly (as every right or left turn would bring tire scuffing) -- caused by the resistance of the tire rotation differential (e.g., on the intersection turn described above: 10 rotations of the inside wheel versus 12 rotations for the outside wheel). To solve this 'differential' challenge, early automotive engineers invented a mechanical solution: the "differential."
It's hard to have been a motorist and not heard the term "differential," but yet few actually understand its purpose. Now you know why vehicles need to have the device to accommodate wheel speed/rotation differences, or perhaps this is just a reminder. Though the differential is an asset on turns -- balancing drive power between the left and right-hand wheels (left and right-hand axles) -- it has a weakness when it comes to distributing power.
DISTRIBUTION OF DRIVE POWER
You'll quickly relate to this next consideration if you've ever found yourself stuck in snow or mud. Since the nature of the differential is to split drive power, if one wheel begins to lose traction it actually transfers more power to that very slipping wheel. So you can actually be sitting in the snow with a right (or left) drive wheel spinning uselessly with no traction, while the opposite-side wheel sits still and yet has traction!
Automobile engineers have created another technical feature to help in slippery traction conditions and it's called the "limited slip" differential. As its name implies, this device limits slipping by helping ensure equal power distribution to both left and right drive wheels. The limited-slip type differential is generally an optional feature and makes the drive wheels function more like they're on a single axle (while still enabling them to rotate at different speeds when the vehicle rounds a corner).
Now that you're more familiar with the terms "differential" and "limited-slip", let's move to front-wheel drive, four-wheel drive and all-wheel drive.
During this most of the past century and into the 21st Century, both automobiles and trucks have mostly been propelled by rear-wheel drive. While front-wheel drive systems have been offered on some models, the rear drive design has been "conventional." More recently, emphasis on fuel economy (lighter weight/fewer parts) and economies of manufacturing have resulted in a proliferation of front-wheel drive vehicles.
FRONT 'PULL' OR REAR 'PUSH'
Rather than having an engine up front, a transmission mid-way and a differential/rear axle assembly in the back, front-wheel drive consolidates all this hardware in the front of the car. Today, front-wheel drive has become dominant in newer cars, making rear-wheel drive more of the exception.
In addition to the advantage of incorporating both the transmission and the differential up front along with the engine in a "transaxle" to simplify construction, this design offers a traction advantage. Since all of the weight of the power system hardware is directly over the driven wheels, this enhances traction in mud and snow. The front-wheel drive layout also takes up less space than the rear-wheel drive arrangement, enabling increased space for cargo and passengers.
At this point then the concept of four-wheel drive seems pretty straight forward, right? How about this: If you purchase a four-wheel drive vehicle that is not equipped with either rear "limited-slip" or front and rear "locking" differentials, then you're not really getting a true four-wheel drive to all 'four' wheels. What?
'ALL-FOUR' FOUR-WHEEL DRIVE
Again if you're driving on mud or snow, then even with available power going to the front and rear wheels -- those front and rear differentials will still ultimately drive only one of the front wheels and one of the rear wheels (whichever wheels have less traction!). Of course, you'll still be better off than with conventional two-wheel drive, as you'll at least have drive power from both the front (pulling) and the back of the vehicle (pushing).
When a four-wheel drive vehicle adds limited-slip or available locking differentials, only then does the system actually live up to engaging all four wheels at the same time. But wait! What about "all-wheel drive," isn't that the same thing as four-wheel drive? Nope.
While most four-wheel drive systems allow shifting from two- to four-wheel drive (disengaging the front-wheel drive system), all-wheel drive is a bit of a hybrid "four-wheel drive." An "all-wheel drive" system incorporates engineering that actually directs power to the wheel, or wheels, that have the most traction -- so that you get optimum road grip at all times. Instead of delivering power simultaneously to both the front and rear axles, if only the front wheels have traction, then the all-wheel drive design transfers the majority of power up front -- where it's needed most (or the rear, or a combination of both).
PRACTICAL ALL-WHEEL DRIVE
The important note about all-wheel drive is that it does not necessarily mean four-wheel drive, as it can engage either front wheels or rear wheels. The nature of the "all-wheel drive" system is really "best-wheel drive," since it's more of a thinking drive system. This is why you are seeing more new passenger cars and vans offering all-wheel drive systems.
Four-wheel drive systems are generally heavier-duty than all-wheel drive systems, and are more suited for off-road driving where terrain requires the complete engineering of four-wheel drive. But whether your vehicle is front-wheel drive, four-wheel drive or all-wheel drive, when it comes to the differences at least you now have the 'inside track'.
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Entire Contents - Steve Ford, The Car Guy - Est. 1983
® The Car Guy - 1983 CA; Interstate Registrations 1985; 1997 - Trademark Registered: United States Patent and Trademark Office