How Your Vehicle Works
Four-Wheel / All-Wheel Drive
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There are almost as many different types of four-wheel drive systems as there are four-wheel drive vehicles. It seems that every manufacturer has several different schemes for providing power to all of the wheels. The language used by the different carmakers can sometimes be a little confusing, so before we get started explaining how they work, let's clear up some terminology:
Usually, when carmakers say that a car has four-wheel drive, they are referring to a part-time system. For reasons we'll explore later in this article, these systems are meant only for use in low-traction conditions, such as off-road or on snow or ice.
These systems are sometimes called full-time four-wheel drive. All-wheel drive systems are designed to function on all types of surfaces, both on- and off-road, and most of them cannot be switched off.
Part-time and full-time four-wheel drive systems can be evaluated using the same criteria. The best system will send exactly the right amount of torque to each wheel, which is the maximum torque that won't cause that tire to slip.We need to know a little about torque, traction and wheel slip before we can understand the different four-wheel drive systems found on cars.
Torque is the twisting force that the engine produces. The torque from the engine is what moves your car. The various gears in the transmission and differential multiply the torque and split it up between the wheels. More torque can be sent to the wheels in first gear than in fifth gear because first gear has a larger gear-ratio by which to multiply the torque.
The interesting thing about torque is that in low-traction situations, the maximum amount of torque that can be created is determined by the amount of traction, not by the engine. Even if you have a NASCAR engine in your car, if the tires won't stick to the ground there is simply no way to harness that power.
For the sake of this article, we'll define traction as the maximum amount of force the tire can apply against the ground. These are the factors that affect traction:
The weight on the tire - The more weight on a tire, the more traction it has. Weight can shift as a car drives. For instance, when a car makes a turn, weight shifts to the outside wheels. When it accelerates, weight shifts to the rear wheels. The coefficient of friction - This factor relates the amount of friction force between two surfaces to the force holding the two surfaces together. In our case, it relates the amount of traction between the tires and the road to the weight resting on each tire. The coefficient of friction is mostly a function of the kind of tires on the vehicle and the type of surface the vehicle is driving on. For instance, a NASCAR tire has a very high coefficient of friction when it is driving on a dry, concrete track. That is one of the reasons why NASCAR race cars can corner at such high speeds. The coefficient of friction for that same tire in mud would be almost zero. By contrast, huge, knobby, off-road tires wouldn't have as high a coefficient of friction on a dry track, but in the mud, their coefficient of friction is extremely high. Wheel slip - There are two kinds of contact that tires can make with the road: static and dynamic. Static contact - The tire and the road (or ground) are not slipping relative to each other. The coefficient of friction for static contact is higher than for dynamic contact, so static contact provides better traction. Dynamic contact - The tire is slipping relative to the road. The coefficient of friction for dynamic contact is lower, so you have less traction. Quite simply, wheel slip occurs when the force applied to a tire exceeds the traction available to that tire. Force is applied to the tire in two ways: Longitudinally - Longitudinal force comes from the torque applied to the tire by the engine or by the brakes. It tends to either accelerate or decelerate the car. Laterally - Lateral force is created when the car drives around a curve. It takes force to make a car change direction -- ultimately, the tires and the ground provide lateral force.
Let's say you have a fairly powerful rear-wheel-drive car, and you are driving around a curve on a wet road. Your tires have plenty of traction to apply the lateral force needed to keep your car on the road as it goes around the curve.
Most people don't even come close to exceeding the available traction on dry pavement, or even on flat, wet pavement. Four-wheel and all-wheel drive systems are most useful in low-traction situations, such as in snow and on slippery hills. In the next section, we'll see how four-wheel drive systems can help in these situations.
The benefit of four-wheel drive is easy to understand: If you are driving four wheels instead of two, you've got the potential to double the amount of longitudinal force (the force that makes you go) that the tires apply to the ground.
This can help in a variety of situations. For instance:
In snow - It takes a lot of force to push a car through the snow. The amount of force available is limited by the available traction. Most two-wheel drive cars can't move if there is more than a few inches of snow on the road, because in the snow, each tire has only a small amount of traction. A four-wheel drive car can utilize the traction of all four tires. Off road - In off-road conditions, it is fairly common for at least one set of tires to be in a low-traction situation, such as when crossing a stream or mud puddle. With four-wheel drive, the other set of tires still has traction, so they can pull you out. Climbing slippery hills - This task requires a lot of traction. A four-wheel drive car can utilize the traction of all four tires to pull the car up the hill.
There are also some situations in which four-wheel drive provides no advantage over two-wheel drive. Most notably, four-wheel drive systems won't help you stop on slippery surfaces. It's all up to the brakes and the anti-lock braking system (ABS).
The main parts of any four-wheel-drive system are the two differentials (front and rear) and the transfer case. In addition, part-time systems have locking hubs, and both types of systems may have advanced electronics that help them make even better use of the available traction.
A car has two differentials, one located between the two front wheels and one between the two rear wheels. They send the torque from the driveshaft or transmission to the drive wheels. They also allow the left and right wheels to spin at different speeds when you go around a turn.
When you go around a turn, the inside wheels follow a different path than the outside wheels, and the front wheels follow a different path than the rear wheels, so each of the wheels is spinning at a different speed. The differentials enable the speed difference between the inside and outside wheels.
There are several different kinds of differentials used in cars and trucks. The types of differentials used can have a significant effect on how well the vehicle utilizes available traction.
The transfer case is the device that splits the power between the front and rear axles on a four-wheel drive car.
Back to our corner-turning example: While the differentials handle the speed difference between the inside and outside wheels, the transfer case in an all-wheel drive system contains a device that allows for a speed difference between the front and rear wheels. This could be a viscous coupling, center differential or other type of gearset. These devices allow an all-wheel-drive system to function properly on any surface.
The transfer case on a part-time four-wheel drive system locks the front-axle driveshaft to the rear-axle driveshaft, so the wheels are forced to spin at the same speed. This requires that the tires slip when the car goes around a turn. Part-time systems like this should only be used in low -traction situations in which it is relatively easy for the tires to slip. On dry concrete, it is not easy for the tires to slip, so the four-wheel drive should be disengaged in order to avoid jerky turns and extra wear on the tires and drivetrain.
Some transfer cases, more commonly those in part-time systems, also contain an additional set of gears that give the vehicle a low range. This extra gear ratio gives the vehicle extra torque and a super-slow output speed. In first gear in low range, the vehicle might have a top speed of about 5 mph, but incredible torque is produced at the wheels. This allows drivers to slowly and smoothly creep up very steep hills.
Each wheel in a car is bolted to a hub. Part-time four-wheel drive trucks usually have locking hubs on the front wheels. When four-wheel drive is not engaged, the locking hubs are used to disconnect the front wheels from the front differential, half-shafts (the shafts that connect the differential to the hub) and driveshaft. This allows the differential, half-shafts and driveshaft to stop spinning when the car is in two-wheel drive, saving wear and tear on those parts and improving fuel-economy.
Manual locking hubs used to be quite common. To engage four-wheel drive, the driver actually had to get out of the truck and turn a knob on the front wheels until the hubs locked. Newer systems have automatic locking hubs that engage when the driver switches into four-wheel drive. This type of system can usually be engaged while the vehicle is moving.
Whether manual or automatic, these systems generally use a sliding collar that locks the front half-shafts to the hub.
On many modern four-wheel and all-wheel drive vehicles, advanced electronics play a key role. Some cars use the ABS system to selectively apply the brakes to wheels that start to skid - this is called brake-traction control. Others have sophisticated, electronically-controlled clutches that can better control the torque transfer between wheels.
The type of part-time system typically found on four-wheel drive pickups and older SUVs works like this: The vehicle is usually rear-wheel drive. The transmission hooks up directly to a transfer case. From there, one driveshaft turns the front axle, and another turns the rear axle.
When four-wheel drive is engaged, the transfer case locks the front driveshaft to the rear driveshaft, so each axle receives half of the torque coming from the engine. At the same time, the front hubs lock.
The front and rear axles each have an open differential. Although this system provides much better traction than a two-wheel drive vehicle, it has two main drawbacks. We've already discussed one of them: It cannot be used on-road because of the locked transfer case.
The second problem comes from the type of differentials used: An open differential splits the torque evenly between each of the two wheels it is connected to. If one of those two wheels comes off the ground, or is on a very slippery surface, the torque applied to that wheel drops to zero. Because the torque is split evenly, this means that the other wheel also receives zero torque. So even if the other wheel has plenty of traction, no torque is transferred to it.
Previously, we said that the best four-wheel drive system will send exactly the right amount of torque to each wheel, the right amount being the maximum torque that won't cause that tire to slip. This system rates fairly poorly by that criterion. It sends to both wheels the amount of torque that won't cause the tire with the least traction to slip.
There are some ways to make improvements to a system like this. Replacing the open differential with a limited-slip rear differential is one of the most common ones - this makes sure that both rear wheels are able to apply some torque no matter what. Another option is a locking differential, which locks the rear wheels together to ensure that each one has access to all of the torque coming into the axle, even if one wheel is off the ground -- this improves performance in off-road conditions.
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