Technology is a double-edged sword. While it possesses the potential to greatly benefit what and how we drive, it also has the capacity to confuse us as to which features most benefit our particular driving needs and style.
For example, all-wheel drive is often confused with four-wheel drive because they are somewhat similar in design. A second reason for our misunderstanding is that four-wheel drive has been available in the market for much longer, so we naturally associate the two when discussing this technology.
Simplistically speaking, all-wheel drive vehicles are designed to operate using all four wheels to propel the vehicle in the desired direction under all driving conditions. But, just when you think things are starting to make sense, the auto industry throws a curve. There are also many part-time all-wheel drive vehicles available in the market. Two current examples of this are the 2008 Honda CR-V and Volvo S80.
The all-wheel drive systems found on these vehicles work seamlessly when activated, but require slippage in order to engage. Because of this, part-time all-wheel drive can leave the driver feeling somewhat powerless. Just when you need grip the most, your request may take a few seconds to register with the system. This lag time can be a serious determent to getting unstuck or gaining control as quickly as possible.
To best utilize a part-time all-wheel drive system, it requires the driver to be more aggressive than usual. Think of it as spurring a horse to move forward. Fortunately, electronic stability and traction control are generally available to assist, allowing the vehicle to stay pointed in the right direction while the system catches up and delivers power where it is needed most.
Four-wheel drive, on the other hand, is best suited for poor traction surfaces such as off-road driving. That said, four-wheel drive is limited in ways many drivers simply don’t anticipate. Surprisingly, should one axle lose traction on hard road surfaces such as ice or packed snow, it is easy to suddenly find your vehicle pointed in the wrong direction.
Engaging four-wheel drive on a continuous basis has the potential to cause serious wear and tear on tires, drive axles and possibly even the transfer case. Parking can also be a chore. Designed to lock the front and rear axles together, four-wheel drive has the effect of widening the turning radius. Consequently, it requires a great deal of effort and patience to park in a tight space or maneuver through the taut confines of a mall parking lot.
By comparison, all-wheel drive allows the front and rear wheels to rotate at different speeds. In this way, traction can be maintained while vehicle handling is improved.
Most all-wheel drive systems are designed to split the amount of available traction; 60% to the rear wheels, 40% at the front. Under normal driving conditions, this set-up works fine. On a slippery road surface, however, it becomes a job for the viscous coupling unit to split the amount of traction based on specific wheel needs.
A viscous coupling unit is a particular kind of fluid coupling in which the input and output shafts mate with thin alternately spaced discs in a cylindrical chamber. The chamber is filled with a viscous fluid that tends to cling to the discs, thereby resisting speed differences between the two shafts. Viscous couplings are used to limit the speed difference between the two outputs of a differential, or between the two axles of a vehicle.
In simple terms, if one wheel starts to slip more than the others, traction is restored to that wheel via the viscous coupling unit.
New technologies and materials have made all-wheel drive units smaller, lighter, and more responsive. Subaru and Audi may have the most experience with all-wheel drive, but manufacturers such as Porsche, General Motors, BMW, and Mercedes are showing a full range of all-wheel drive vehicles to suit every price range.
The Suzuki SX4 Crossover is a great example of superb all-wheel drive technology at an affordable price. Suzuki’s 3-mode Intelligent All-Wheel-Drive, (i-AWD) provides excellent control over the traction characteristics of the vehicle. On dry paved roads, two-wheel drive operation is the standard setup. Switching to i-AWD on snow and ice allows power to be delivered to the wheels in need of traction. An additional feature allows for locking the i-AWD system on extra slick road conditions.
All-wheel drive is a technology well suited for the vehicles of today. Consider it the next time you purchase a new or used vehicle. Chances are excellent you’ll be pleasantly surprised and wonder why you waited so long.