• 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.
   

• Four-wheel drive - 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.
   

• All-wheel drive - 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.

Traction

• For the sake of this article, we'll define traction as the maximum amount of force the tire can apply against the ground (Or that the ground can apply against the tire -- they're the same thing). 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.

• 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 gear set. 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 drive train.
   

• 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 (8 kph), 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.
   

• First, let's see how the most basic part-time four-wheel-drive system works.

• 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. The animation below shows how a system like this reacts under various conditions.

• 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.