All-wheel drive not only ensures better off-road mobility, but also for better and faster drivability. But actually there is no „den“ All wheel drive. In reality, there is a wide variety of systems that allow driving both vehicle axles and one of them is the so-called Haldex coupling, so that the drive is often referred to as Haldex all-wheel drive. This technique is now widespread, although it was not until 1995was discovered by Volkswagen at the Swedish company Haldex for the general market. In the following article we will discuss the most important properties of this type of drive and what advantages, disadvantages and problems are associated with it.
Contents
The function of the Haldex coupling
In order to be able to discuss specific advantages and disadvantages, we must first deal with the function of the Haldex coupling. The basis of this system is that the standard drive axle remains normally connected to the engine. This is usually the front axle of the vehicle. The other axle is then added to the Haldex all-wheel drive via the Haldex coupling.
The main focus of this type of drive is that the torque, which is also transmitted to the front axle to the rear axle, can be regulated by adjusting the pressure . This allows precise adjustment to the current road and traffic conditions. After the Haldex drive was ready for series production in 1998its specific design and thus also the way it works has been changed several times. However, the basis is a cardan shaft, which supplements the drive shaft for the left or right vehicle wheel on the vehicle drive axle (usually the front axle). The Haldex coupling, which is a corresponding axle differential, is now used to transmit the desired torque to the rear axle. This consists of an oil-pressure-driven multi-plate clutch. This clutch and thus the torque transmission is activated by building up sufficient oil pressure.
How is the Haldex clutch activated and controlled?
There are now several generations of Haldex couplings. These differ primarily in the way the oil pressure was built up to close/activate the clutch. With the older clutches of generations 1 to 3 mechanical constructions were primarily used for this. These came mainly in the years of construction 1998 to 2008for use. A speed difference between the front and rear axle drives an oil pressure pump and the clutch transfers power to the rear axle. The speed difference arises above all when the front axle does not have enough grip and spins as a result. In this way, the torque is automatically better transferred to the road. The regulation was done electronically via a valve that could change the applied pressure.
The Haldex Generation 4eventually phased out mechanical Haldex pumps. Instead, the Haldex pumps for the Haldex clutch oil are now electrically driven. An advantage of this design is that the Haldex differential can now be fully electronically controlled. A speed difference between the front and rear axles is not required first, but the system can only build up the required oil pressure prophylactically, so to speak, via the Haldex pre-loading pump. With the 5 generation Haldexyou benefit above all from a weight reduction of the entire system. In addition, it has smaller dimensions, making it easier for vehicle manufacturers to integrate it into their vehicles. In addition, the integrated Haldex controller allows the additional drive train to be completely separated, so that fuel can be saved in this way. Because the Haldex clutch works with an oil pump, there is of course also a corresponding Haldex filter, which is one of the wearing parts on your car. An oil change is also necessary with a Haldex four-wheel drive from time to time.
What are the advantages of the Haldex coupling?
As mentioned at the beginning, the integration of the Haldex all-wheel drive requires little space. Especially for small cars, which anyway primarily drive the front axle, the dynamic integration of the rear-wheel drive is a decisive advantage. The additional weight is only fifty to one hundred kilograms. The response time of this drive concept is very fast and the torque distribution can also be regulated comparatively easily using Haldex Control, because this is only done using oil pressure. In the latest generation, the second drive train can be electronically decoupled, which, among other things, enables a possible towing process (otherwise a four-wheel drive vehicle must not be transported with a classic tow truck).
Disadvantages and limitations of Haldex four-wheel drive
But where there is light, there is also shadow, and so each vehicle system also has specific disadvantages and problems. Because this four-wheel drive is not continuously active, some experts claim that the Haldex clutch cannot be called a true four-wheel drive at all. Whether this is more of an advantage or a disadvantage depends entirely on the situation. Ultimately, however, this design comes with the disadvantage that the off-road capability of such a vehicle is significantly worse than that of a car with full-time four-wheel drive. Ultimately, the focus here is on suitability for everyday use.
Another disadvantage is that the specific design only allows a maximum of half of the torque to be transmitted to the second drive axle. This is because the front-wheel drive always remains active and at least half of the engine torque is transferred to the corresponding two wheels. The engineers achieve smaller displacements by integrating metered braking of individual wheels. However, it is not possible to control the drive in a particularly rear-biased manner.
Conclusion
The demand for all-wheel drives has grown significantly over the past few decades. This has led to the fact that such drives are also to be installed more and more in smaller vehicles. However, this was difficult with the classic all-wheel drives because they take up too much space. The Haldex coupling is an interesting alternative in this environment and even allows the engine torque to be distributed between the front and rear axles as desired. This has made the Haldex clutch one of the most popular all-wheel drives of our time.
A tip from CarTipsandmore: If you decide to buy an Audi with four-wheel drive, you should know that this manufacturer markets a Haldex four-wheel drive as permanent four-wheel drive. The technical reason for this is that with these vehicle models, a small proportion of the engine torque (a few Nm) is always transmitted to the rear axle. However, the same limits, limitations, and disadvantages that we described in this article apply.