Which compact SUV has the best all-wheel-drive system for snow and ice?

Among mid-priced cars, Subaru’s Symmetrical All-Wheel-Drive (drivetrain image above) is an exception to the part-time AWD design philosophy. It’s always delivering torque front and rear, more or less equally split, and can deliver the majority of torque to whichever end has better grip. A center differential allows front and rear wheels to turn at different speeds. At least theoretically, a drivetrain with powertrain always working both front and rear is harder on fuel economy, and the engine and transmission packaging may push the engine farther forward relative to the front axle. You don’t want heavy parts of the car at the far ends of the vehicle.

Still, Subaru has been considered the champ of winter driving among under $50,000 vehicles. Go to YouTube and you’ll see Subarus beating up on the competition in rollers-on-ramps tests where three of four wheels are slipping, an extreme but not rare event.

Four-wheel-drive, or 4WD, vehicles are typically trucks and big SUVs with big, heavy drivetrains. A 4WD vehicle has a transfer case splitting off from the driveshaft to send power back to the front wheels, and differentials front and rear. Some are part-time 4WD, where the driver selects when to engage the system and then chooses a low or high gear range. Some allow the differentials to be locked so the wheels turn at the same speed. This works at low speeds only and is not for highway driving.

Mazda’s i-Activ predictive AWD

Mazda has introduced a predictive form of all-wheel-drive on its subcompact (CX-3) and compact (CX-5) crossover SUVs. It will also be on the next generation full-size Mazda CX-9 crossover later this year. Normally the i-Activ system delivers 98% of the power to the front wheels. An array of sensors looks for impending tire slip and begins to shift power to the rear wheels before the driver notices.

According to Dave Coleman, Mazda development engineer, in bad weather there’s a time of limited traction where the tires are just beginning to slip that can be noticed by the car’s sensors (but not yet by the driver), and acted upon, so that power quickly begins flowing to all wheels. Mazda calls this golden moment the “control area” (illustration above) where the car can intervene.

The predictive part comes from all the sensors working together, not just wheel-spin measurements. Mazda says the car reads 27 channels of sensor data 200 times a second to determine when to begin powering the rear wheels. The sensors include inside and outside temperature, wipers on/off, road incline, yaw (off-center movement left or right), steering wheel effort vs. angle, individual wheel speeds, position of the gas pedal, brake fluid pressure, transmission gear, and dynamic stability control activity. The only sensor Mazda added for i-Activ was oil temperature.

The “condition biasing” sensors alert the car to obvious possibilities: Cold weather means it could be slippery, wipers on mean it’s probably raining and maybe snowing, and an uphill incline means weight has transferred from front to back wheels with possible loss of traction. The amount of steering effort relative to the angle of the steering wheel (image right) correlates to slippery conditions; it’s easier to turn the wheel on a snowy road. That would bias the computers to order up more traction in back.

In order to quickly share power with the rear wheels, Mazda employs an electromagnetic activated coupling between front and rear wheels. To avoid a jerky switchover, Mazda runs a small “pre-load” of power, 2% of total, to the rear wheel.

Other automakers’ AWD designs

Most all-wheel-drive systems power the front wheels and shift power to the rear wheels after the car starts to lose traction. But it might take a full second before power shifts, not milliseconds. This can be an eternity if you’re trying to maintain traction uphill, or get going through an intersection with traffic bearing down on you. If only one of the two wheels on the axle has traction, the vehicle’s traction control will brake the spinning wheel in an attempt to shift power to the other side.

BMW, some Lexus, and Mercedes-Benz AWD vehicles are based on rear-drive platforms that can split the torque (power) up to 50/50 front-rear, but more typically are set to 40/60 or 30/70. At most the front wheels get 50% of the power.

Audi’s Quattro system employs three differentials: front, center, and rear. A differential allows wheels to get power when turning at different speeds, as when you’re going around a corner. The center differential (it actually may be in the gearbox, not in the center of the driveshaft) separates power going to the front and rear. (Smaller Audis such as the A3 with a transverse, or sideways-mount, engine use a slightly different system.)

Acura’s Super-Handling All-Wheel-Drive system employs an electromagnetic clutch to send as much as 70% of power to either the front or rear wheels. As much as 100% of rear axle power can transfer to the non-slipping rear wheel via a second electromagnetic clutch. When cornering in dry or wet, extra power is sent to the outside rear wheel to help power it through a turn; this is called torque vectoring. Other high-end cars have their own mechanical torque vectoring systems (heavy, costly, and good) and some automakers employ a simpler system that brakes the inside wheel, so the outside wheel runs faster around a corner.

Some hybrid crossovers have the combustion engine power the front wheels (along with an electric motor for battery-only driving), then add a second motor to power the rear wheels. There’s no driveshaft and no transmission hump to steal cockpit space. As long as there’s power in the hybrid battery or the ability to generate electricity on the fly, the car can be all-wheel-drive. Lexus, Toyota, and Ford hybrid crossovers use this method.

Virtually all vehicles with all-wheel-drive have predictive technologies. Mazda stands out for the number of sensors looking for slip and the ability, in mid-priced vehicles, to combine the sensor data to sense slip sooner.

Testing Honda, Mazda, Subaru on snow, ice

In the past, I’ve driven medium-large SUVs such as Acura MDX at a special snow and ice course in Canada, and high-end sedans/SUVs at a hockey rink in Indiana (where Tire Rack is). This time it was in Crested Butte, Colorado, on a snow- and ice-driving course created by Aston Martin and now being time-shared by Mazda as the Mazda Ice Academy. Ten inches of snow fell the night before, making for perfect testing-and-compare sessions if you didn’t mind below-zero temperatures.

Mazda, seeking with i-Activ to match or surpass the longstanding reputation of Subaru Symmetrical All-Wheel-Drive, set up several test courses: a hill climb and right turn coming off the hill; uphill and downhill slaloms ending with hard braking on packed snow; a sweeping right hand right-hand corner on snow at city-suburban driving speeds; a drive on local roads (mostly snow-covered); and just for the heck of it, a handling course on bare ice with Mazda Miatas. All were fitted with the same tires, Bridgestone Blizzak winter tires, plus for comparison a front-drive and an all-wheel-drive Mazda CX-3 fitted with all-season Yokohama tires that come as original equipment.

No surprise: Everyone looks pretty good on winter tires

Weaving through cones going slightly uphill and downhill, then braking hard at the end, the results were pretty much what you’d expect from a trio of subcompact CX-3s: The all-wheel-drive car with winter tires was head-and-shoulders above the rest accelerating, turning, and braking. There was a sense you were in control of the car and that the car was stopping willingly, especially going downhill. (This on lightly packed snow, no ice, at 0 degrees. The colder the weather, the less slippery the surface.)

The AWD car with all-season tires did better accelerating and slaloming through the cones than the front-drive car. Since AWD plays essentially no role in stopping distance — it’s the tires and ABS — the two with all-season tires were about equal in that category. You could see where winter tires make a big difference on packed snow — for instance, at a stop sign to keep you from sliding into the intersection, when you or an inexperienced driver might misjudge when to start braking.

Driving around a big, arcing right-hand curve, on somewhat packed snow atop more snow in a comparison among three compact SUVs with AWD — Subaru Forester (the reigning winter performance champ in most eyes), Mazda CX-5 (the upstart winter performance challenger), and Honda CR-V (best-selling compact SUV) — all got through going 30 to 40 mph with help from AWD, stability control, and traction control. The Mazda seemed comfortable going a couple mph faster (closer to 40 mph) than the Subaru (high 30s), but there wasn’t enough time to measure exact speed differences. The Honda was a bit behind, with more intervention from stability control. Pushed too hard and beyond their limits, all three would plow ahead (understeer) but wouldn’t spin. A lot of that comes from stability control. It’s easy to be daring and drive fast when no other vehicle is near you and the side of the handling course is big piles of snow and not guard rail.

Driving a semi-rural loop on hilly public roads, the Honda-Mazda-Subaru mix was fleshed out with two other best-selling compact SUVs, the Toyota RAV4 (No. 2 in sales behind CR-V) and the Nissan Rogue (No. 4, behind Ford Escape, not tested), as well as the subcompact Honda HR-V. All were on Bridgestone Blizzak winter tires. My conclusion: They all would be competent on public roads that had been plowed, or even with a couple inches of snow still on the ground. For clawing through, say, a half-foot of snow or on hilly terrain (or a steep driveway), it would be preferable to have the Mazda or the Subaru.

Hill climb: the clearest difference among compact crossovers

A standing-start climb up a short hill, a stop at the top, and a slight right turn to descend, was the most dramatic. It was meant to mimic coming up a steep suburban driveway and making a right turn onto the public road. Starting from a level surface with the steering wheel centered, all three compact SUVs got up the hill. The Honda then slipped back a bit with front tires spinning and back tires not initially engaging; the rears eventually caught and the CR-V it made it over the crest and down, at least for me.

The Subaru, unexpectedly, had trouble starting up at the crest, because the front wheels were turned to the right rather than straight ahead when power was applied. Subaru’s AWD may not transfer power smoothly between front and rear with the wheel cocked, Mazda helpfully pointed out. You could see the front wheels turning but the rears weren’t, initially, providing much traction.

The Mazda’s two dozen sensors, sensing low-grip, low temperature, and the incline, immediately added more power to the rear wheels. This test was the most dramatic in showing differences among AWD systems. It suggests other automakers could improve their systems as Mazda did by using additional sensor data and more sophisticated algorithms. It suggests also the automaker that sets up the tests has something of an advantage.

It would have been interesting to try some of the tests in a front-drive crossover with winter tires. My sense is that the front-drive vehicle with winter tires would have acquitted itself well (except on the hill-climb). But then people who regularly find themselves in six or more inches of snow are probably going to be getting an all-wheel-drive vehicle.

Even a Miata can handle well on snow

One final exercise of the test day was driving a Mazda Miata (“because why not?” Mazda said) with the top down (because when will you get a chance again?) — with winter tires on a huge icy course, covered with a couple inches of loose snow, and with traction control and stability control turned off. With no electronic technology to help out, the car would sometimes plough straight ahead on turns (understeer), and other times the back end would kick out and you’d slide, sometimes even spin. All this at 10 to 15 mph.With traction control back on, the Miata wouldn’t slip or spin as much, but you couldn’t go very fast, either. It does show — no surprise again — how little traction there is on snow and less on ice, and how much electronic aids help safety.

Later, rally pros drove us around a bigger snowpacked course with the tail end stuck out in corners, proving that the average driver isn’t very good compared with people who do this for a living.

Other conclusions about snow, ice, and tires

Every vehicle with winter tires does markedly better in snow, even light-in-the-rear vehicles such as a Mustang or Camaro. Even in snow belt states, only about a quarter of drivers use winter or snow tires. They’ve had reasonably good luck in the past, especially if the car is front- or all-wheel-drive, with all-season tires. Or they’re in jobs where work gets called off for a day until the plows get through.

Winter tires commonly sold include Bridgestone, Continental, Dunlop, Goodyear, Michelin, Nokian, Toyo, and Yokohama.

All-wheel-drive, especially when done well, as Mazda and Subaru have shown among compact / midsize vehicles, is a blessing in bad weather. The cost of a good set of winter tires in a popular size is about $500 to $750 for a set of four, or about what it would cost you if you ding the car in a winter accident and have to pay a deductible. They are called winter tires, at least by tire makers, to give you the idea they’re fine in cold weather, not just when it’s snowing. Most people still call them snow tires.

If you drive in icy conditions, you might consider studded tires, which embed steel (sometimes hard rubber) studs in the tires. They help bite into ice on the pavement for better traction when starting and stopping. They don’t do a whole lot in snow, they’re hard on public roads, and some states ban them outright.

If you have a car with summer tires (sometimes called summer performance tires), you cannot safely leave them on year-round unless you live in a state that grows citrus fruit. Summer performance tires have tread compounds that get hard and lose grip as the temperature falls below 40 degrees. In freezing weather with no snow or ice, your safety is compromised. At the least, you need to have a set of all-season tires.

Conversely, winter tires wear quickly in warm weather, which is why you don’t want them on once you’re in to April. Should you keep your car a long time (a decade or longer), the rubber starts to decay from exposure to the atmosphere and should be replaced — even if there’s tread left — after 6-7 years. Certainly within 10 years.

If you get winter tires, get a second set of wheels. Steel wheels are light enough to work fine in winter, and alloy wheels — some, at least — are at prices closing in on the cost of steel, less than $50 a wheel. You don’t want to be switching two sets of tires between a single set of wheels. The unmount/mount/rebalance costs add up. If you have low-profile 20-inch tires, you may be able to order 19- or 18-inch wheels and winter tires with the same overall tire circumference as the originals and be less susceptible to pothole damage. Just get assurance from the people selling and mounting the tires that the package fits and clears the brake calipers.

If you get winter tires, you need to get four. The car loses stability with widely disparate tires front and rear. In previous tests on dry pavement at a small race course set up by Tire Rack, cars with mismatched tires felt unstable going through sharp corners.

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