The YouTube channel Car Question tests the all-wheel drive systems in various soft-roaders with a “diagonal test,” which involves taking each car up a ramp at an angle. Some of these car-based SUVs and minivans were clearly not designed to go off-road, but that just makes watching them flex out and spin their wheels that much more entertaining.
The point of Car Question’s test, presumably, is to demonstrate how well a vehicle can traverse uneven terrain, which tends to lift wheels up off the ground. To continue on with a front wheel off the ground, many of these front-drive-based crossovers have to effectively transfer torque to the rear wheels via clutches in their power transfer units and/or rear drive modules.
But when two wheels come up off the ground—one from each axle—things get more difficult, as power will then tend to want to go to the two wheels without traction (such is the nature of open differentials). Some crossovers use their brakes to slow down the spinning wheel, which then sends torque to the wheel on the ground, and gets the car moving. But other vehicles, like the Mazda CX-9 below, just flounder as their tires spin and the vehicle goes nowhere:
Speaking of Mazdas, the CX-5 isn’t particularly graceful as it tries to ascend this small gravel slope:
Here’s a look at the Toyota Sienna minivan struggling to get up the grade, even rolling back as its rear passenger’s tire and front driver’s side tire desperately claw at the air for grip:
Eventually the driver backs up, and comes at the slope with a bit of momentum (momentum is key when it comes to off-roading with open differentials), and the Sienna reaches the summit triumphantly.
Another car that was clearly not meant to ever go off-road is the Mini John Cooper Works Clubman but it seems to have little trouble going up the ramp multiple times (I assume Car Question has the cars go up the slope a number of times to test how quickly their all-wheel drive systems overheat).
Then there’s Acura’s Super Handling All-Wheel Drive, which the brand says shifts power to the rear wheels when the fronts slip, and then apportions power left and right to the wheel with most traction. As you can see in the image below, the all-wheel drive system does just that, though it seems to take a bit of wheel-spin to get that lateral torque transfer going.
Staying in the Honda family, the CR-V manages to do fairly well, which is good, because a few years ago, Honda’s all-wheel drive system came under scrutiny for failing tests like this one.
The Ridgeline also passes, though there’s quite a bit of wheel-spin and not a lot of articulation from that suspension:
The Subaru Crosstrek is interesting because the difference between making it up the slope and failing the test is simply the transmission’s shifter position—it fails in manual, but does fine in drive.
The little Impreza, with its “multi-plate transfer clutch that maintains a 60/40 front/rear torque split and uses a range of sensors to automatically adjust this split on-the-fly as conditions warrant” also made it up with the help of a bit of pressure on the skinny pedal.
On the other end of the spectrum is the Nissan Titan XD, which—with its enormous wheelbase, flexy rear axle, torquey diesel engine and locking diff— can make it up the grade without any wheel-spin whatsoever. It even makes it up in two-wheel drive!:
And then, of course, there’s the Ford Raptor, which needs no introduction:
What do these tests accomplish that your typical driver should care about? Not much. But with SUVs having ditched solid axles that used to be so prominent, articulation is limited, and clever four-wheel drive systems are that much more important to drivers hoping to traverse uneven terrain. And, as you can see from the videos, not all of these systems perform equally.
h/t: Zafir