Fuel efficient AWD vehicles offer a fantastic combination of handling, safety and fuel efficiency. But they are not all created equally.
Different manufacturers use different approaches to achieving AWD functionality. Some manufacturers even refuse to publish detailed information about how their systems work. There are many different terms (many are trademark terms) used by different manufacturers to describe their various approaches with only a limited amount of agreement on what these terms mean. Let’s introduce three basic terms and discuss their tradeoffs.
4WD (Four Wheel Drive)
4WD refers to the classic heavy-dute four wheel drive system first used decades ago in vehicles intended for heavy off-road use. Think of Jeep, Land Rover, Toyota Land Cruiser, 4WD pickups etc. 4WD generally offers the best traction performance in extreme off-road situations. But unless you are a farmer, use your vehicle in off-road construction sites, or are a heavy off-roader, these systems are generally overkill. They are additionally heavy, have lower fuel economy and in fact may not perform as well as other systems in slippery on-road conditions (due to high weight performance penalties).
AAWD (Automatic All Wheel Drive)
This system is by far the most common in AWD passenger cars. All AAWD system have a means by which power can be transmitted to any or all of the four wheels. Nearly all AAWD systems available in modern passenger cars rely heavily on inputs from various sensors (accelerometers, wheel speed, throttle position etc.) and engage the vehicle’s stability control system (or traction control system) to adjust the power sent to each of the wheels based on the constantly changing traction conditions. AAWD systems are lightweight, low friction (because they are often not engaged) and represent only a very modest fuel economy penalty.
AAWD provides modest improvement in traction and handling compared to regular two wheel drive vehicles. Under some conditions however they do take some time to function, and the computer algorithms can be observed to “hunt” for the best solution to the tractions conditions. Under some conditions, AAWD systems simply fail to deliver the traction of a 4WD system or of a FTAWD (discussed next).
FTAWD (Full Time All Wheel Drive)
Full Time All Wheel Drive is by far the best solution for passenger cars where traction under difficult conditions is the priority. They are generally very light in weight (like AAWD) and represent only a small penalty in terms of system friction and therefore also offer very small penalties for fuel economy.
FTAWD systems distribute power to all four wheels all the time unlike AAWD system that only engage under certain conditions. For many buyers, FTAWD represents an optimal tradeoff between traction, handling, safety and fuel economy in comparison to all other options.
See For Yourself
In the following test certified by the United States Automobile Club (USAC) five vehicles in the same class but with differing approaches were test on a ramp with rollers to simulate slipper conditions. The front rollers and the left rear rollers are free to spin (simulation loss of traction on these three wheels. The right rear rollers were covered allowing the right rear wheel to maintain traction.
Vehicles tested include the Nissan Rogue, Honda CRV, Ford Escape, Toyota RAV4, and Subaru Forester.
Drivers held the throttles at a steady position thus allowing the systems to take over control of traction.
As you can see there is a lot of difference in performance!
Now in case you think the test in the video above is “too simulated,” the video below shows a similar result with vehicles climbing an actual off-road incline.
Let know what you think!