#How 4x4, all wheel drive works

04 / May 2022
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How 4x4, all wheel drive works

Seletron Performance

All-wheel drive, how does it work?

 

There are 4WD, AWD, 4Matic, 4drive, and others, but what do these acronyms mean? They all essentially mean the same thing, namely all-wheel drive, but these types of powertrains are not all the same. Let's take a look at how they differ and how they perform!

 

The acronyms indicate that all wheels are driven, 4WD stands for 4-wheel drive, AWD stands for all-wheel drive, 4Matic and 4drive, the acronyms used by Mercedes and Seat, respectively, say the same thing. However, as we mentioned, the systems do not all work the same. The first big difference comes from the word "permanent" when discussing all-wheel drive.

 

Non-permanent electronically engaged all-wheel drive.

 

Let's look at an example: earlier, we mentioned Seat's traction. Using the Alhambra 4drive as an example, this traction, while all-wheel drive (i.e., involving all wheels), is not permanent. The car in question is a front-wheel drive with a classic differential and with the addition of a transmission linkage to the rear axle (via a classic driveshaft). It reaches an electronically controlled clutch system connected to the input of a second classic differential with semi-axles connected in turn to the rear wheels.

 

This is a simplified all-wheel-drive system suitable for navigating low-grip surfaces. The electronic control unit detects possible front-wheel slippage (the ones always in drive), and in the event of excessive differential in the speed of rotation of the front wheels relative to the rear wheels, it controls the clutch lock, which also transmits torque to the rear axle, effectively becoming (temporarily) an all-wheel drive. The differentials, both front, and rear, are not self-locking. This effect can be simulated through the intervention of the braking system, which, in this case, is the electronic control unit. It can individually actuate the brake calipers to limit the slippage of the wheels that are not gripping in order to transfer drive torque to those of greater grip.

 

audi quattro ultra

Non-permanent all-wheel drive with viscous coupling.

 

There are even simpler systems that do not require the use of electronics. This system is used on some 4x4 hatchbacks that use a viscous coupling between the transmission-differential unit and the rear differential. In the presence of a substantial difference in the speed of rotation of the 2 axles resulting from slippage, the fluid contained within the viscous coupling overheats due to the sliding of the lamellae connected between the transmission PTO and the rear differential. This changes its viscosity and locks the lamellae, transferring torque to the rear differential. When the sliding stops (i. e., the vehicle regains grip and drives), the fluid cools and restores its viscosity, freeing up a contained speed difference between the 2 axles (a normal situation at every curve). This system is also suitable for steering out of trouble on low-grip surfaces.

 

Mechanically driven non-permanent all-wheel drive.

 

These are the less refined all-wheel drives in which a single axle ordinarily handles the drive. If necessary, torque can also be transferred to the second axle by operating a mechanical control that engages a drive shaft at the second axle differential. This type of traction has almost always been used in older off-road vehicles. It is suitable for low-grip roads but not for sports cars or luxury cars.

 

Permanent all-wheel drive, how it works, and why it is the best.

 

Let us start with a bit of theory. Every wheel that adheres to the ground has a road grip, which we can denote as the force in kilograms it can sustain before it slides on the ground and thus loses grip. The wheels have to handle two different forces: the centrifugal force (the transverse force felt when turning) and the longitudinal force that can be negative or positive if you are talking about braking or acceleration (if it is also a driving wheel).

 

These two forces give rise to a resultant force which is the vector resultant, a value that in a curve without acceleration or braking can only be transverse, while in a straight line, it can be zero (advancing at constant speed and direction), or completely longitudinal (in the case of braking or acceleration without change of direction). What matters is that in turning and acceleration (also in braking, but we do not care about that in this context), the resultant is a sum of the two forces. This means that the maximum road holding in kg must be divided between transverse force (turning) and longitudinal force (acceleration). It also means that the stronger the acceleration (torque transferred to the wheel), the less the road holding capacity when handling curves (transverse forces) will be.

 

 Trazione integrale

That's where permanent all-wheel drive comes in.

 

Now imagine that we have the same car with the same engine (thus with the same final drive torque) but with traction permanently distributed to 4 wheels and not just 2. Suppose we have a 50% torque distribution to the two axles (although there is usually a slight asymmetry in the transfer, e.g., 45% front and 55% rear or even 40/60 to have oversteering behavior). In this case, the driving force of each wheel will be halved compared to the 2WD (or 2x4 if you prefer) situation. As a result, the wheel will have more transverse grip when handling curves since it will be less engaged (remember the vector resultant?) in bearing the longitudinal forces generated by the driving torque.

 

Thus, cars with permanent all-wheel drive offer a safer and more balanced ride. Cars with 2x4 traction under acceleration have strong understeer (front wheel drives) or oversteer (rear wheel drives) behavior, and cars with permanent all-wheel drive (except in rare cases such as some turbocharged Porsches that intentionally have a distribution close to 30/70 on the front and rear axles respectively) have much more neutral and predictable behavior, as well as better traction in all driving conditions.

 

For permanent all-wheel drives, there may be additional devices such as self-locking differentials or systems that use brakes-to transfer torque to gripping wheels by taking it away from wheels with poor grip.The center differential serves to compensate for the difference in the speed of the two axles when turning. Also, its geometry allows the traction system to be configured by defining the percentage of torque transfer to the two axles to determine oversteer, neutral, or understeer behavior.

 

If you liked this article about the main differences between all-wheel drives, please share it with your friends. We also invite you to read the many other articles we have written about chip tuning modules, electronic tuning in general, turbocharger operation, various diesel and gasoline injection systems, and many other automotive-related technical topics!

 

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