All-wheel drive (AWD) and four-wheel drive are big sellers in the Canadian market, as crossovers replace sedans as the mainstream family choice, and luxury makers convert even their high-performance machines to all-wheel surefootedness. But as the days shorten and the weather turns grim, which is best?
Subaru says its AWD has a symmetrical advantage. Audi notes that its Quattro has been winning rallies since the 1980s. Jeep makes a trail-rated version of every machine it produces, and surely that’s good enough for the school commute on snowy streets?
The fact is, the rise of AWD as an expected feature in the average Canadian car has made things somewhat muddled. Here’s a brief breakdown on the technology to help you get a grip on things.
Even though AWD and four-wheel drive seem to be the same thing, they’re not. Four-wheel is the earlier technology, and the more ruggedly simple.
Properly, a four-wheel-drive vehicle should not just be able to transfer power to all four wheels, but should also have a low and high-range gearbox. Take the Jeep Wrangler, which has defined four-wheelin' since the days of the Second World War. In normal driving modes, it’s rear-wheel drive only. Select four-wheel-drive high range, and power is transferred to the front axle at street speeds. Select four-wheel-drive low range, and the Wrangler can now crawl up steep grades at slower speeds.
This is the type of four-wheel drive or four-by-four that you find on most true sport utility vehicles and pickup trucks, vehicles like the Toyota 4Runner or Ford F150. The drawbacks are that on dry pavement, putting a mechanical connection to all four wheels will cause the front wheels to scrub and skip in turns on higher traction surfaces, as all four wheels try to turn the same distance.
To combat the problem, most modern four-wheel-drive systems now have an intermediate, four-wheel-drive automatic setting. Jeep calls it “Selec-Trac,” and it allows for the front and rear axles to turn at different rates. Thus, you can keep your Wrangler in rear-drive on sunny days for best fuel economy, automatic four-wheel drive on a heavy rain day, four-wheel-drive high in the snow, and four-wheel-drive low when climbing hills in the backcountry.
Part-time all-wheel drive
The idea of having driver-selectable four-wheel-drive isn’t new, but most mainstream offerings claim to have AWD available all the time. In the Jeep, you pick what’s best for the terrain; in your average Honda CR-V or similar, the car does all the thinking for you.
Most mainstream crossovers are built to share parts with related sedans, so they have a front-wheel-drive bias. For on-road driving in slippery conditions, front-wheel-drive with good tires is already quite capable, so adding a bit of rear-drive torque as a helper is an easy advantage.
Computer-controlled clutches and hydraulics are key to these systems, which normally operate in full front-wheel-drive mode for maximum efficiency, but can send power rearward in the event sensors detect a tire spinning. Because it’s not as mechanically robust, some refer to these systems derisively as slip-and-grip.
However, the most current applications of part-time AWD are so quick to react that they are effective in almost any situation. Engineers have also programmed in the ability to transfer power rearward automatically at a stop, as well as putting power to the rear wheels in turns, for grippier cornering.
Full-time all-wheel drive
As programming has improved, the advantage of full-time AWD has shrunk a little. However, there are those who still swear by the mechanical attributes of something like Subaru’s AWD system, especially in deep snow.
In a full-time AWD system, power is constantly being split among all four wheels, albeit most often with a bit of a front-drive bias (except in some luxury marques). Having the power spread out constantly gives a more secure feeling to the drive, rather than waiting for a slippage event for the system to rout power around. However, as mentioned, the speed at which modern part-time AWD systems react makes the gap less detectable.
The system of something like a Subaru Forester has a noticeable advantage is in its simplicity. Subaru also uses sensors to slightly vary the torque split between the front and rear axles, but a symmetrical left-to-right layout keeps things balanced from the outset.
Performance all-wheel drive
Mercedes-AMG and BMW’s M-division have adopted AWD not just as a traction aid in poor conditions, but also as a way to put power down early out of the corners. However, we don’t need to go quite to the top of the food chain to find a more advanced AWD system.
The twin-turbo V6 version of the Genesis G70 is only available in AWD in Canada. The electronically-controlled system has a rear-drive bias as default, and in sport mode, will only send 30 per cent of available power to the front wheels. You can also opt to send 100 per cent of the power to the rear wheels, if snowy doughnuts are on the menu. In comfort mode, however, 40 per cent of the power is sent up front as default and even more can be shifted forward to maintain balance if the rear wheels slip.
Further, the G70 has brake-based torque vectoring, which is a simplified version of the next major advance in performance AWD. Using a brake to slow down a spinning wheel, or to brake the inside wheel in a corner to help a car turn-in, is readily available. True torque vectoring, where extra power is fed to the wheel with the most grip, or on the outside of the corner, is becoming more common.
With more cars incorporating some kind of electrification, AWD looks ready to take its next shift forward. Hybrid performance machines such as the Acura NSX can use its electrically-driven front wheels to help corner harder, as well as cope with all-weather performance.
Because electric motors can be used to either power or brake an individual wheel, our control over AWD will likely get even better in the near future. So, assuming you’ve also got a set of proper winter tires fitted, let the weather outside be frightful: AWD technology is here to make sure the drive’s still delightful.