Full and not quite full drive. How all-wheel drive systems differ

All existing all-wheel drive options can be reduced to three main categories: manually engaged (Part Time), permanently engaged (Full Time), and one that is activated by the car itself (On Demand Full Time).

Manually engaged drive

Part Time, as this type is also called, is not designed for prolonged driving in 4x4 mode. Its design lacks a center differential that would smooth out the difference in rotation speeds between the axles. If you drive with the drive engaged on hard surfaces, transmission components quickly fail.

It makes sense to use such a drive only on short difficult sections and at low speed.

Most often, a transfer case lever is used for this. However, on some modifications, to engage the front axle, you have to leave the cabin and manually turn special hubs on the wheel hubs.

This scheme is used exclusively on true off-road vehicles that are operated for their intended purpose. Classic examples are the Jeep Wrangler and Nissan Patrol, which are in no hurry to entrust drive control to electronics.

In addition, "temporary" drive is installed on almost all Chinese copies of popular off-road vehicles from the nineties.

Off-road vehicles with honest Part Time are gradually disappearing — they are being replaced by newer all-wheel drive systems.

Permanent all-wheel drive

Full Time in most manufacturers does not allow the driver to disconnect one of the axles themselves.

Thanks to the center differential, such a transmission always operates in all-wheel drive mode, regardless of conditions. In modern versions, this differential is additionally equipped with its own electronics.

Such a design allows distributing torque between the axles not only equally, but also in other proportions. If a wheel starts to slip, the "smart" differential quickly transfers traction to the axle with better grip or even to a separate wheel that holds the road.

Among all 4x4 systems, this type is considered the most advanced.

Modern systems with an abundance of electronics can adapt to specific surfaces — asphalt, gravel, sand, and so on. The driver just needs to press the appropriate button.

The most well-known examples of permanent drive are Audi with the Quattro system and Subaru with AWD.

Interestingly, such a drive is also installed on completely non-off-road sedans, coupes, and hatchbacks. This once again confirms the versatility of such a scheme.

Automatically engaged all-wheel drive

The On Demand Full Time system leaves the car front-wheel drive and only engages the rear axle when the drive wheels slip. In modern versions, this happens almost instantly, as soon as the first signs of loss of traction appear.

Depending on the specific implementation, the torque between the axles can vary over a wide range — from 10/90 to 90/10.

The electronic stability control (ESP) ensures that the car does not lose control when the drive character changes sharply — from front to rear or vice versa.

On most versions, for difficult sections, there is a possibility to forcibly fix the torque distribution at 50/50. For this, there is a button labeled Lock or 50/50. But as soon as the speed reaches 40-50 km/h, the lock is removed, and the system returns to normal mode.

Such a car can also be made completely front-wheel drive — just press the button marked 2WD. Disconnecting the all-wheel drive saves fuel, and in the city four drive wheels are not always needed.

Automatically engaged drive is the youngest of the 4x4 systems.

It is this one that is installed on most crossovers sold on our market. It can even be said that such a drive has become an integral feature of a modern crossover. The new class of cars required a new type of drive — everything is logical.

Which drive is better?

Choosing an unequivocally best option is not easy — each type has its pros and cons.

For serious off-road, off-road vehicles with manual drive engagement and rigid mechanical locks of all differentials are optimal. But in the city, such cars do not bring driving pleasure.

City crossovers with automatically engaged drive, on the contrary, are practically helpless off-road, but handle like regular passenger cars.

A compromise solution looks like permanent all-wheel drive — it copes with off-road and behaves confidently on the highway.

However, it will not be possible to interfere with its operation: neither save fuel nor overcome a particularly difficult section (despite the electronics) may not work out.