Understanding All-Wheel Drive: Types of AWD
All AWD systems are not created equal. That's a commonly understood fact nowadays. Before you buy an AWD vehicle you should learn a little bit about the many different types of systems. We've created an introduction for you.
Let's imagine the rear wheels of a vehicle. The vehicle has a standard differential. The wheel on the left has less traction. It spins more as more power is transferred to it. However, it's not pushing the vehicle forward well; it's just spinning. The wheel on the right, which has more traction, spins more slowly. For the front wheels, it's the same. The wheel with the most traction spins more slowly.
The power that gets transferred to the wheels of a vehicle with limited-slip differential will be very different than that of one with standard differential. Power is transferred to the wheel with more traction, which helps the car accelerate even faster and, sometimes, handle better.
Here's a video that explains the mechanics of limited-slip differential.
Just as these differentials split power between the left and right wheels, they can also split power between the front and rear wheels. This is one very common differentiating aspect of popular AWD vehicles. Some always split power between the front and rear. Others send all of the power to the front and only power the rear wheels under certain circumstances. Here are some examples of different configurations.
Mazda CX Series
The CX-7 and CX-9 usually power only the front wheels, in most cases. Under certain circumstances, it sends as much as 50% of the engine's power to the rear axle, as needed.
A number of manufacturers equip their AWD vehicles with on-demand AWD. Many Hyundai, Ford, and Lexus vehicles offer it. The majority of these vehicles transfer less than 50% of their power to the rear wheels. That is, they do not have "50/50 Split" AWD systems. This kind of AWD is efficient but it doesn't always benefit the driver in less-than-perfect driving conditions by improving traction or handling. Although, each model has different specifications.
4Matic, xDrive and other Performance AWD Systems
BMW's AWD system pushes most of the power to the rear wheels. A clutch sends power to the front as needed but the majority of the torque usually goes to the rear. Sometimes, the rear may get 50% of the power, but it never gets less than that. This is touted as a performance racing enhancement.
Other manufacturers offer similar drive trains. Mercedes' product is called "4Matic." Often, this type of AWD is found in luxury vehicles that are marketed as performance machines. They are "rear-weighted," because the majority of the torque always goes to the rear unless it's going 50/50.
Various high-performance vehicles employ torque vectoring. Basically, it has the same goal as limited-slip differential, which is to transfer power to the wheels with the best traction. Limited-slip is entirely mechanical, whereas torque vectoring has electronic components. Vectoring requires special hardware to monitor the wheels of a vehicle.
Manufacturers have claimed that vectoring improves handling. Car and Driver's tests support that claim, as other tests do. Although, we wouldn't assume that all torque-vectoring-equipped vehicles handle better than all non-torque-vectoring-equipped vehicles.
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