All wheel drive vs 4 wheel drive vs Mother Nature

Posted Jan 3, 2013 By Brian Turner

EMC Lifestyle - Happy New Year to all and here's to a year without motoring mishaps!

With the first major winter storm of the season behind us, more than a few drivers may have been disappointed at the snow-defeating performance of their AWD or 4WD vehicles, and many of these sport-ute and crossover fans were surprised by various warning lights appearing on their instrument panels during the worst of the challenges they faced getting out of their driveways and down unplowed roads and streets. But first things first...

All Wheel Drive (AWD) is the term usually reserved for the drive systems on vehicles that use their front wheels as the main method of propulsion and automatically activate the rear wheels when traction is poor due to road and weather conditions. Depending on the make and model, there may be manual override switches to permanently engage the rear wheels, but often no driver interaction is required. Four Wheel Drive (4WD) is most often used to describe truck-based SUV drivelines where a separate transfer case is used to provide power to the front wheels when needed and the rear wheels do the driving the rest of the time. Most of these systems have control switches or levers and many have a dual range transfer case which can be helpful during off-road adventures when controlled traction, not speed is required.

No matter which system your vehicle uses and no matter what type of tires it has, Mother Nature can mix up a recipe of snow, freezing rain, winds, and temperatures that can defeat the best designed, engineered, and built systems leaving us literally spinning our wheels.

AWD and 4WD systems both have their benefits and downfalls, but when navigating through thick wet heavy snow, it's the AWD that can frustrate more drivers. The key to most AWD systems is a unit called a viscous coupler. This device is located on the driveshaft going to the rear axle, or built onto the transmission, or on the rear axle itself on a primarily front-wheel drive vehicle and as its name suggests it connects or couples the front and rear axles so that in very poor traction conditions they both power the wheels.

On dry roads with good traction the coupler is disengaged providing no traction to the rear wheels, but when the going gets slippery, the shaft leading into the coupler from the vehicle's transmission will spin relatively faster than the wheels. This will heat up a special liquid inside the coupler causing it to expand and lock the unit's internal clutches together transferring power to the rear wheels. With late model vehicles that use this system, an on-board computer will constantly monitor the AWD system and if any preset threshold relating to the risk of overheating or otherwise damaging the system is exceeded, the AWD will be shut down and a warning light will appear on the instrument panel. For vehicles without AWD but with any type of traction control, a similar failsafe system exists that has a similar effect to temporarily shut down the system to give it a chance to cool off so to speak.

The key to avoid AWD system shut-downs and possibly a trip to your local service provider to check out your dash warning light(s) is speed (or rather the lack of it). When faced with a foot or two of thick white stuff covering your driveway or residential street, take it easy. Plowing through that slop at breakneck speed may seem like fun, but the constant wheel spins it creates can stress even the best AWD system or leave you in the ditch or both.

The other common result of a sudden onslaught of heavy wet snow is windshield wiper failure.

Sometimes caused by drivers who flip on the wipers without first clearing the windshield with a brush (or checking to ensure the wiper blades aren't frozen to the glass) but more often caused by those who neglect to clear the snow off the roof of their vehicle and then at the first stop sign of the day 50 lbs or more of slushy snow and ice come sliding off down the windshield to twist and bend the wiper arms beyond recognition and damage their drive linkage if the wipers happen to be turned on at the time.

Replacement wiper-arms can range in price from $50-$150 each and the linkage system can often exceed $400 (not to mention labour or taxes). A few minutes with a snow-brush can save a lot of grief.

Email me at emc@perfprint.ca or bjoeturner@hotmail.com listing 'Question for the Car Counselor' on the subject line or by post to Record News Communications, 5 Lorne St., P.O. Box 158, Smiths Falls, Ont. K7A 4T1).

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