Full-time all-wheel drive has long been considered one of the most advanced transmission schemes. In such a design, traction is continuously transmitted to both axles, and for normal operation, the system requires three differentials: two inter-wheel and one center differential. The latter is especially important because when cornering, each wheel moves along its own trajectory and rotates at a different speed.
Without a center differential, a car on dry asphalt starts to behave extremely unpleasantly: the transmission experiences increased loads, tires scrape the surface, and the car itself tends to go straight even when turning the steering wheel. That is why rigidly connected part-time cannot be constantly used on a regular road, and the 4WD mode there is speed-limited.
The full-time system solved this problem, but along with the advantages, it also brought new disadvantages.
Why full-time began to be abandoned
The first problem was fuel consumption. Constantly rotating driveshaft, transfer case, half-shafts, and three differentials create significant losses. Compared to a front-wheel-drive version, the car consumes approximately 1.5–3 liters more fuel per 100 kilometers.
For a driver who spends most of their time driving on city asphalt, this translates into constant unnecessary expenses.
The second reason is the weight of the structure. The transfer case, driveshaft, center differential, and rear differential are heavy components that the car has to carry constantly.
The third disadvantage was maintenance. Owners have to regularly change oil:
- in the transfer case
- in the rear differential
- monitor the condition of the driveshaft
- control the wear of universal joints and oil seals
Front-wheel-drive cars simply do not have such expenses.
At the same time, in urban conditions, the advantages of full-time were not so obvious. An all-wheel-drive crossover brakes no better than a front-wheel-drive one, since grip depends primarily on the tires. The main advantage of all-wheel drive is a more confident start and stable acceleration on slippery surfaces.
But over time, even this advantage became less noticeable due to the development of electronics.
How electronics changed all-wheel drive
Modern stabilization systems have learned to:
- brake the slipping wheel
- simulate locking differentials
- correct the trajectory of movement
As a result, front-wheel-drive crossovers with effective ESP began to behave quite predictably on asphalt even in winter.
An additional factor was environmental requirements. In Europe, automakers began to be severely fined for exceeding CO2 emission standards. Every extra liter of fuel meant additional grams of carbon dioxide, so full-time all-wheel drive gradually turned from a technical advantage into a problem for manufacturers.
Evolution of Audi Quattro
One of the most famous examples of full-time for a long time remained the Audi Quattro system. Starting in 1980, Audi used a mechanical Torsen differential on models with a longitudinal engine.
Over 35 years, the company produced about 3.3 million all-wheel-drive vehicles with this scheme. Torsen worked completely mechanically and independently redistributed traction between the axles depending on road grip.
However, in 2016, Audi introduced the Quattro ultra system. Instead of Torsen, it used an electronically controlled clutch. During calm driving, the rear axle was completely disconnected along with the driveshaft.
Fuel savings turned out to be relatively small — about 0.3 liters per 100 km. But the system became:
- 4 kilograms lighter
- cheaper
- more flexible in traction distribution
The clutch could transmit 0 to 100% of the torque to the rear, while Torsen worked in a fixed 40:60 scheme.
The classic Torsen was retained only on powerful S and RS versions with V6 and V8 engines, where the load is too great for the clutch.
Similar path for BMW and Mercedes
BMW began moving in the same direction even earlier. In 1985, the BMW E30 featured true full-time all-wheel drive with a center differential and viscous couplings.
Later, in 1991, the BMW E34 was equipped with an electronically controlled multi-plate clutch with a 36:64 distribution.
And already in 2003, xDrive debuted without a center differential. The system used only a multi-plate clutch with a basic 40:60 distribution in favor of the rear axle.
During parking, the clutch completely disengages, and the car effectively becomes rear-wheel drive. In case of wheel slip, the system activates in about 100 milliseconds.
A similar evolution occurred with Mercedes-Benz. The first Mercedes-Benz 4Matic appeared in 1987 on the W124 and used mechanical locks along with hydraulic clutches.
The modern 4Matic+ on AMG models can already completely disconnect the front axle, turning the car into a rear-wheel-drive vehicle for fuel economy.
Why crossovers switched to clutches
Mass-market crossovers began to abandon full-time all-wheel drive even earlier. In the 1990s, viscous couplings were widely used — cheap and simple solutions. But they had a serious drawback: they did not combine well with electronic stabilization systems.
In a turn, the car first behaved like a front-wheel-drive vehicle, and then the rear axle suddenly engaged. Because of this, understeer could unexpectedly turn into oversteer.
Later, viscous couplings began to be replaced by electro-hydraulic systems like Haldex.
The first generation of Haldex appeared in 1998 on:
- Audi A3
- Volkswagen Golf 4Motion
- Audi TT
Modern versions of the system work much more complexly. Electronics preemptively send a small portion of traction to the rear axle even before slippage. In winter, engagement occurs preventively, even taking into account air temperature.
When driving on the highway, the rear axle can be completely disconnected, which reduces fuel consumption.
However, such systems also have a weak point: under serious load, the clutch overheats and temporarily disengages. For heavy off-road, this is a serious drawback, although most crossover owners do not encounter such conditions.
Who retained true full-time
Not all manufacturers have completely abandoned classic full-time all-wheel drive.
Among the models where it is still used:
- Lada Niva
- Toyota Land Cruiser 300
- Range Rover
- Mercedes-Benz G-Class
- Subaru models with manual transmission
- Mitsubishi Pajero Sport with Super Select system
Interestingly, full-time all-wheel drive on the Niva appeared not for off-road records, but due to design features. For a passenger car, the part-time scheme turned out to be inconvenient, so engineers had to use a center differential.
But such simplicity resulted in high fuel consumption. An injected Niva with a 1.7-liter engine and 83 hp consumes 12.1 liters in the city according to the passport. Owners often report 12–15 liters, and in winter, figures reach 16–17 liters per hundred.
Why electric vehicles brought back all-wheel drive
Unexpectedly, all-wheel drive received a new lease of life in electric vehicles. But now it is arranged completely differently.
Instead of complex mechanics, two separate electric motors are used: one on the front axle, the other on the rear.
Onboard electronics distribute traction between them up to a thousand times per second. This eliminates friction losses in driveshafts, transfer cases, and differentials.
Some manufacturers have gone even further. For example, Rivian and GMC Hummer EV use separate motors on each wheel. Thanks to this, the wheels can rotate in different directions, and the car can almost turn on the spot.
In fact, electric vehicles have made complex mechanics, which for decades were considered the pinnacle of engineering thought, unnecessary. The transfer case, driveshaft, and center differential have gradually given way to electric motors and software control.
Full-time did not disappear because it was a bad system. It's just that modern technologies allowed achieving a similar result in cheaper, lighter, and more economical ways.