The History of the AWD Drivetrain: Types and Details

The AWD drivetrain can now be found on various cars - from subcompacts to MPVs and SUVs. However, it took half a century of technological development and engineering to design a truly efficient light-weight all-wheel drive system.

Along with the growing popularity of SUV, the demand for all-wheel drive cars is also growing. However, the AWD drivetrain can be different, and it happens that even several different transmission options with AWD are available for the same model at once. In total, there are four main types of AWD systems. Let's look at each of them in more detail. But its brief history comes first.

The History of the AWD Drivetrain: Types and Details photo 2

The first AWD cars began to appear at the dawn of the automotive industry. In 1903, the Dutch company Spyker introduced the AWD Spyker 60/80 HP racing car. It is interesting that this car already had an inter-axis differential in those years (more on that later).

The 1993AWD Spyker 60/80 HP The 1993AWD Spyker 60/80 HP

1907 can be considered the year of birth of the first SUV, because Daimler designed the Dernburg Wagen commissioned by the German Colonial Office (what was not there at the beginning of the 20th century!). This car was used for long-distance travels on the African off-road. It had an AWD system. Its suspension parts and steering mechanisms were protected by covers, which reduced the angle of wheel rotation to only 23 degrees. With a length of almost five meters, this significantly worsened the car’s maneuverability. To solve this problem, the rear wheels were also made steerable. This makes the Daimler Dernburg Wagen also the first AWD SUV with a fully-controlled chassis. And this happened in 1907! Well, everything new is well-forgotten old.

The Daimler Dernburg Wagen is the first AWD SUV with a full-controlled chassis The Daimler Dernburg Wagen is the first AWD SUV with a full-controlled chassis

The Daimler Dernburg showed that AWD is required to improve cross-country performance. However, its need to improve driving characteristics on good roads would remain in question for many years to come. So, in the 1920s and 1930s, a number of AWD racing cars were created, such as the Miller 91 and Bugatti Type 53. They had an advantage over their RWD and FWD rivals in grip. However, large power losses, high fuel consumption, and complex handling made it clear that the time for AWD passenger cars had not yet come.

It took half a century of technology and engineering expertise to create a truly efficient 4×4 lightweight system. In 1980, it appeared in a production form on the legendary Audi quattro, which also became one of the most successful cars in the rally group B and twice won the WRC. It is Audi’s motorsport success that forced the automotive world to take a fresh look at the prospects for AWD passenger cars. Today, AWD drivetrains can be found in both harsh SUVs and small hatchbacks.

The rear and front axles of the car travel different distances during a turn The rear and front axles of the car travel different distances during a turn

One of the main problems when creating an AWD car is that front and rear wheels of the car roll along different paths when turning. This means that the AWD system must have a device that will divide the moment between the axles in the right proportions, allowing them to rotate at different angular speeds. This device is called a differential.

However, there are AWD schemes without differentials. We have already found that there are four main types of AWD systems: part-time all-wheel drive, on-demand all-wheel drive, permanent all-wheel drive, and electronically controlled all-wheel drive. The part-time all-wheel drive system is the simplest. Let's start with it.

Part-time all-wheel drive

Cars with this type of transmission are usually rear-wheel drive by default. The front axle is connected here using a transfer case, which ensures equal distribution of torque between the front and rear axles. This is exactly the AWD scheme, in which there is no center differential. «Then, how does the car turn?» you may ask. With all-wheel drive on a dry surface – it turns very badly. Due to the fact that the front and rear axles rotate at the same speed, while driving a different distance, the wheels of the axle, making a smaller radius, are constantly trying to break into a slip, which leads to a huge turning radius, increased tire and transmission wear on a dry surface. Therefore, it is recommended to use part-time all-wheel drive on slippery surfaces, like sand, snow or mud. RWD and FWD cars can be used for asphalt driving, which requires the driver to have stable skills of handling a rear-wheel drive car.

Part-time AWD is usually used on ‘harsh’ SUVs and pickups Part-time AWD is usually used on ‘harsh’ SUVs and pickups

The advantages of such a transmission include simplicity, strength and reliability. If the absence of the center differential when driving on asphalt creates a lot of problems, then its presence can cause a lot of problems off-road. The differential is arranged in such a way that if the rotation speed of one axle falls by 20%, it will be increased by 20% on the other axle. But if one axle slips and the second one will stand on hard surface, then all the disturbing moment will be transferred to those wheels that have less grip and the car will drive itself into a trap. This cannot happen to cars with rigid transmission, because they simply do not have an inter-axle differential, and an inter-wheel one, as a rule, has a lock (when the lock is on, the differential ceases to redistribute the moment). This AWD is great for conquering severe off-road, but when you normally drive on good roads, it is of little use. This is why this system is used on serious off-road vehicles.

On-demand all-wheel drive

The on-demand all-wheel drive system is mostly used in compact and mid-size SUVs. This transmission usually has three modes. The car, as in the case of part-time AWD, has a transmission only on one axle by default. If there is a slippery section or light off-road in front of the car, the driver can switch to automatic transmission mode. In this case, when the main driving wheels, e.g. the front wheels, slip, the rear axle will automatically connect. And the third mode is locking the center coupling. In it, the moment is distributed equally between the axles. This is necessary for passing sections of serious off-road terrain.

Three on-demand AWD modes: 2WD, Auto, and Lock Three on-demand AWD modes: 2WD, Auto, and Lock

It seems that this system combines advantages of AWD and 2WD. But in fact, it turns out that this transmission is much inferior to systems with a rigid FWD on off-road vehicles due to the fact that the visco-coupling, which here plays the role of an inter-axle differential and redistributes the moment along the axles, quickly overheats during prolonged skidding. And when driving normally on slippery surfaces, it does not always have enough response speed, which is why the AWD is connected with a delay. So, the driver, in fact, uses just a 2WD car but with more features.

Permanent all-wheel drive

As the name implies, a car with such a system will always remain all-wheel drive. The center differential is usually used in such transmissions. Its presence means that if one wheel slips, it will transfer all the disturbing moment and the car will become stationary. Therefore, these transmissions are often equipped with various differential locking systems. One of the most popular systems of permanent all-wheel drive is a transmission based on a Torsen differential (Torque sensitive LSD). This is a self-locking differential, but it is often supplemented with a forced locking system to improve the redistribution of moments along the axes.

Cars with a well-tuned permanent AWD can perform well both off-road and on asphalt and slippery roads.

Electronically controlled AWD systems with torque vectoring can distribute the moment between the wheels so that the car can make a turn Electronically controlled AWD systems with torque vectoring can distribute the moment between the wheels so that the car can make a turn

Electronically controlled all-wheel drive

The development of technology has led to the creation of advanced AWD systems that combine the advantages of all the above schemes. There, the control of the moment distribution over the wheels is entrusted to smart electronics. There are a huge number of all-wheel drive systems of this type today. The most common is a transmission with a Haldex clutch. The role of the center differential is performed by a multi-disc hydraulic clutch with electronic control. The 5th generation Haldex sends only 5% of the torque to the driven (usually rear) wheels by default. If the driving wheels slip, it can send up to 50% of the torque to the other axle.

Transmissions with torque-vectoring systems are also increasingly used. There, electronically controlled hydraulic couplings are installed instead of the rear inter-wheel differential. When turning the car, it distributes the moment in such a way as to forcibly slow down the inner wheel. There are also systems in which it is possible to disable one of the axles, such as in the current BMW M5.

Modern AWD drivetrains are complex systems that, with the help of advanced technologies and hydraulics, can provide excellent grip on the road and decent off-road performance when driving at high speeds. We should also mention hybrid systems where one axle is driven by an internal combustion engine and the other one — by an electric motor. It seems that such AWD will soon become very popular. Which AWD system is the best to your mind?

Sign up or log in to post a comment