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What Is 4WD and How Does It Work?

What Is 4WD and How Does It Work?

All-wheel drive (4WD, 4×4, AWD) is a type of car construction in which torque from the engine drives both axles of the car. Four-wheel drive (4WD) is used on SUVs to increase cross-country ability. Its use on ordinary cars improves driving, not off-road qualities.

Types of 4WD

There are several types of 4WD. They differ in the scheme of operation. For complete information, you can also check out here.

Part-time 4WD

Under normal conditions, engine power is transmitted to one axle (rear or front). If necessary, the four-wheel drive is activated using a special lever or button. This is the simplest and cheapest type of “four-wheel drive” used for offroad driving. This approach usually does not involve a differential that distributes the torque between the axles. Therefore, it cannot be operated constantly on the highway. Otherwise, fuel consumption increases and tire and transmission wear accelerates. On a hard surface, you need to turn off the four-wheel drive. It should be used only in mud, sand, ice or snow. Its disadvantage is that the lack of a differential between the axles worsens the car’s handling on ice and wet asphalt.

Automatic 4WD

Like the Part-time system described above, this type of 4WD is activated only when necessary. However, automation does this instead of the driver. Communication is implemented using a viscous coupling or multi-disc clutch under electronic control. The second axle is activated when the wheels of the main driving axle slip. The system provides good driving qualities on sand, dirt, or a rough winter road. However, it is poorly adapted for off-road driving: the second axle is connected too late when the first one has already slipped. A connected drive based on a viscous coupling cannot be used offroad for a long time. The unit may fail due to overheating. Some models are equipped with a clutch pre-lock button, which allows you to easily overcome a problematic section.

Full-time 4WD

In cars with this type of 4WD, all power is always transmitted to four wheels. They are divided by means of an inter-axle differential, which improves handling, reduces tire wear and reduces the load on the units. To improve offroad capabilities, “Full-time 4WD” machines can be equipped with additional locking differentials (between the wheels and between the axles). This function is implemented in two versions: automatic or manual. This type of machine is the least prone to skidding and has the best offroad driving. If there is a differential lock, it should be turned on only before overcoming mud, snow, sand, or a long slippery climb. In other cases, it only worsens driving characteristics and reduces the service life of tires and units.

Selectable 4WD

This is the best type of 4WD, and it combines the advantages of all of the above. Its only drawback is the high price. A car with multi-mode four-wheel drive can drive with one or two driving axles. The driver himself chooses the state of the differentials. On asphalt, the front axle is enough. On a slippery road, you should turn on full 4-wheel drive, and while offroad, lock the differential (on the most difficult sections, all three – inter-axle and inter-wheel differentials).

4WD: principles of operation

The all-wheel drive system with a viscous clutch is most widely used. It includes manual transmission or automatic transmission, clutch, transfer box, cardan and main gears, inter-wheel and inter-axle differentials.

This option of all-wheel drive is used on cars with front- and rear-wheel drive. In the first case, the gearbox is installed across the axis of the car; in the second – along it. This affects the design features of the “dispenser” and cardans.

The clutch on the manual transmission performs two functions:

  • It protects the transmission from overloads
  • Ensures a short-term disconnection of the engine and gearbox during gear shifting

Automatic transmissions are equipped with a torque converter that performs a similar function.

The transfer case, which includes a reduction gear and an inter-axle differential, distributes the torque between the axles and increases it when the “reduced gear” (low range) is engaged.

To improve off-road characteristics, the transmission is equipped with an interaxial differential lock. In the simplest case, it is automatically blocked by a viscous coupling unit. More advanced models use a multi-disc friction clutch and a self-locking Torsen differential.

On cars designed for off-road driving, automatic or manual locking of differentials between the wheels is installed. The system works as follows:

  • Torque from the engine is transmitted through the clutch to the gearbox
  • Engine power is distributed to the axles through the transfer case
  • Cardan gears actuate the inter-wheel differentials of the rear and front axles

Which type of 4WD drive is better?

All-wheel drive, connected in manual mode, is rarely used. Activation of the second axis using a friction clutch is more common. It can be controlled by electronics, read data on the speed of rotation of the wheels, or be blocked when heated due to slipping. For rare off-road trips, you can buy a car with permanent four-wheel drive and a differential that is locked with a viscous coupling. If you expect long trips on sand and dirt, it is worth overpaying for a multi-mode four-wheel drive, which behaves equally well on the highway, in snow or on a demineralized dirt road.

Advantages of 4WD

Compared to cars with one driving axle, four-wheel drive cars have the following advantages:

  • Improved acceleration on slippery surfaces
  • Increased offroad capabilities
  • Good driving stability.

The last statement is only true for full 4-wheel drive systems. An automatic 4WD system with a high clutch can bring unpleasant surprises, unexpectedly connecting a second driving axle.

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