The All-Wheel-Drive Diagonal Test Shows a System’s True Capability
This is actually based more in reality than you might think.
Summer has officially ended here in the northern hemisphere, which means we’re fastly rocketing toward winter. Soon enough, city streets will be covered in a blanket of snow, and with everything else equal, a two-wheel-drive car generally won’t fare as well compared to an all-wheel-drive (AWD) car. Still, that doesn’t mean that every all-wheel-drive system is created equal. To separate the wheat from the chaff in the world of all-wheel drive, there’s no quicker method than a diagonal test.
In an all-wheel-drive diagonal test, a vehicle is deliberately placed on a surface (or incline) where two wheels diagonal to each other have limited traction. The test is significantly more complicated than it sounds. Look here:
Crazy, right? The diagonal exercise is a very easy way to test out and examine a vehicle’s torque and traction management systems, whether that is software or actual mechanical hardware. In vehicles with open differentials, the power will generally take the path of least resistance, meaning power will mostly go to the easiest wheels to spin. In this case, that’s the wheels with the least traction. From there, the vehicle has to figure out how to get the power to the wheels that do have traction. Ideally, some form of locking differential(s) would be the most ideal way to get traction around, but those are often omitted due to price and complexity. Instead, manufacturers rely on computerized traction management systems like brake vectoring to achieve the desired result. Does it work? Not always.
The people who operate Montreal-based YouTube channel Car Question do a diagonal test on nearly every AWD-equipped vehicle they can get their hands on, and some vehicles perform better than others. Watch as the previous-generation Toyota Highlander basically fails the diagonal test. Even after engaging the “AWD lock,” which should lock the AWD system’s wheel speeds together, the Highlander can’t dole out power to the correct wheel with traction.
It’s not a power issue or a Toyota-specific issue either. The Lexus UX250h is a hybrid, which means the rear axle is powered by an electric motor like most AWD Toyota hybrids. There are no physical drive linkages from the rear axle to the main powertrain. The UX’s rear motor is tiny, only rated for a whopping seven horsepower (55 ft/lbs of torque). Yet, its superior torque management meant the UX passed the diagonal test with relative ease.
You might ask yourself, “When the hell am I ever going to drive up an incline that wonky and steep?” Trust me, this test is based more in real life than you’d think. Think about it like this: hat if you were stopped on a snowy road with a lot of ice and random clean contact patches? A good AWD system would be able to get you going despite the surface, but a bad one might not be much more usable than a simple two-wheel-drive setup. Remember, not all AWD is created equal!
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