How do Haldex couplings work?

Sometimes, it is considered the inferior version of the Audi quattro system. Some people say it’s not a real AWD system. Yet many cars with permanent all-wheel drive, such as the Volkswagen group cars with transverse engines (e.g. Volkswagen Golf, Audi A3…), use this traction system developed by Haldex Traction, because of the advantages it provides. It is lighter than a Torsen-based AWD system, and allows for better fuel consumption than Torsens.

Yet there is a great deal of mythology and contradictory information online about this system. For example, a commonly quoted bit of data is the fact that a Haldex system can transfer “up to 50%” of the torque to the rear wheels, which is technically not true, since under front-wheel slip conditions, Haldex-driven cars can send 100% of the torque to the rear wheels.

But if that isn’t correct, then what is the torque split of a Haldex coupling system? Why is this figure (the 50/50 torque split) quoted so often then?

To explain this, let’s take a look at how Haldex couplings work.

Disclaimer: Haldex is a brand name. I am not affiliated with Haldex Traction, BorgWarner, Audi, or any of the trademarks/companies that appear in this article. The use of the Haldex brand in this article is purely due to name recognition, as this type of multi-plate clutch coupling is commonly referred as “Haldex all-wheel drive system” or similar names.

Inside the Haldex unit

Another common misconception is that a Haldex system is a differential. The Haldex coupling doesn’t include a differential. Instead, it uses a multi-plate clutch that allows for the progressive engagement of the rear axle.

If we simplify to the core parts of the unit, we have:

• An input shaft driven by the engine
• An output shaft that drives the rear axle
• A multi-plate clutch controlled by a hydraulic pistons
• A hydraulic pump that engages the pistons
• A control unit that actuates the hydraulic pump

This clutch is controlled electronically. The Haldex control unit reads wheel speed data, acceleration… and determine whether the clutch needs to be disengaged, fully engaged, or partially engaged to a certain extent. Then sends a signal to the clutch piston, which will actuate this clutch and can engage in a matter of milliseconds. In fact, modern cars can even react before the wheels start to slip.

This means that the engine can make the output shaft of the Haldex rotate as fast as the input shaft, but not any faster. When the clutch is disengaged, the car will behave as a front-wheel drive car and the rear wheels will not be driven. When fully engaged, the rotational speed of the input and output shafts will match.

How does this translate to torque?

Since the control unit is constantly altering the level of pressure of the clutch, the torque split is dynamic. But we can take a look at a few different scenarios to see how the system works and how torque (and force) will be transferred to the ground in each of them.

Constant Speed

Let’s start with the main source of confusion about the infamous “50/50 torque split”. If we have a car driving at a constant speed with the haldex clutch fully engaged, we will have something like this:

The front and rear axle turn at the same speed, and all the wheels turn equally. This is the case where the torque would be split equally amongst all the wheels. Fairly easy, right?

However, one of the main benefits of the Haldex coupling is being able to engage the clutch electronically when wheel slip is detected. And unless the road you’re using to move at a constant speed is an ice rink, chances are there won’t be any slippage.

In this case, the Haldex unit will decouple the rear axle, so all torque will be transferred to the front, looking more like this:

Therefore resulting in all of the torque being put to the ground through the front axle. This is how the vehicle will operate most of the time, essentially behaving like a front-wheel-drive car.

Front Two Wheels Slipping

Now let’s see another theoretical case, where the two front wheels have no traction whatsoever. This is what happens for example when we place the car on two rollers:

The sensors detect wheel slip and the Haldex unit engages immediately, achieving matching speeds in both axles.

This is precisely the case where the Haldex coupling can transfer more than 50% torque to the rear wheels, which is exactly what is happening here.

If we delve a bit (but not too much, don’t worry!) into the physics of torque and movement for a wheel, we have that torque equals Force multiplied by the radius of the wheel (τ=F⋅r). Any torque applied would translate to the wheels applying force against the ground, which would provoke wheelspin immediately. However, we can see in the video above that as soon as the Haldex engages, the front wheels do not move at all.

This means that 0% torque (or very close) is being transferred to the front wheels, and the remaining 100% (or near 100%) is sent to the rear wheels.

Why does this happen? Easy. By virtue of the clutches being engaged, the front wheels cannot move unless the rear wheels move. For the wheels in both axles to move at the same speed, the car needs to apply a far greater torque in the rear wheels, as there is more friction to overcome. This is a mechanical constraint: the engine is simply sending power (a rotational speed and a torque) to the drivetrain, and all a fully-engaged Haldex does is equalise the speeds in both axles. Simply by doing that, more torque will be sent to whatever axle needs to overcome more friction.

Heavy Acceleration

Except for very few cases (Lamborghini Aventador, Bugatti Veyron), most cats with a Haldex system are front-wheel-drive biased. In this type of car (e.g. Audi S3), it is typical to have wheelspin in the front wheels when accelerating from a standstill.

This type of slippage appears when the drivetrain sends to the wheels more torque than the wheels can put to the ground through their traction force. When accelerating heavily, weight is shifted towards the rear of the car, therefore unloading the front wheels even more and further reducing the tractive force the wheels can apply. Traction control systems combat this by limiting the throttle input and reducing torque.

However, the Haldex has a trick up its sleeve: since it works together with the traction control system, it has information about many parameters of the vehicle. Since it’s controlled electronically, the clutch can be engaged even before slipping. Modern Haldex systems will send some power to the back as soon as a heavy acceleration is detected, for example if you slam on the throttle from stopped, or if you engage the Launch Control.

Rear Two Wheels Slipping

Easy! Whenever the clutch is fully disengaged, all power will be sent to the front wheels. In this case, the Haldex will not act (or disengage the clutch to cut power to the rear wheels, if it was already engaged) and the vehicle will work just like any front-wheel drive car.

Different grip on left to right

This is a bit problematic, both for the case of loss of traction on two and three wheels. Haldex systems cannot send power to the wheels independently, as they can only “see” their input and output shaft. In this case, the behaviour of the car will be determined by whether the vehicle has a locking differential (either fully-locking or a limited slip diff).

As an example, if the car has a fully locking differential in the rear, we will have a similar case to the one with the front two wheels slipping: all torque will be sent to the wheel with more traction. If this is not the case, the Haldex system will match the speed in the front axle and the rear axle differentials. This might not be enough to get the vehicle out of this situation.

Drift Mode and other tricks

This is not all! Since they are electronically controlled, there are lots of other trickery when it comes to Haldex systems.

One of them is the Drift Mode that cars like the Focus RS, Mercedes A45 AMG or Audi RS3 have. In this mode, the vehicle makes the wheels in the rear axle spin faster than the ones in the front axle, even though the Haldex is front-biased.

How does it do this? There are two tricks. One of them is a pinion (a fixed gearing) that allows the rear axle to spin faster than the front axle whenever the Haldex is fully engaged. By doing this, oversteer becomes a lot easier.

The second bit of magic is what Audi calls the “RS torque splitter control units”. As you can see in the video below, the system contains a multi-plate clutch (therefore replicating a Haldex coupling) for each rear wheel.

By doing this, the Audi (and the other models with Drift Mode, which work in a very similar way to this) can selectively lock the multi-plate clutches in the rear wheels, and send more torque to the outer rear wheel to initiate a wheelspin. Once the drift has started, they equalise the speed between both rear wheels (essentially acting like a rear locking differential) for easier control of the slide.

While multi-plate clutch couplings are far from a perfect four-wheel-drive system, they have their own advantages that make them simply a “different” rather than “inferior” technical solution. After all, there is a reason Bugatti and Lamborghini use it!