Winnipeg Free Press - PRINT EDITION

AWD gains traction across all brands

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THE popularity of all-wheel drive (AWD) is growing, as is witnessed by the fact there are more AWD models available today than ever before.

The better news is that more and more of the systems being offered are shifting away from the old slip-first, grip-later units that really were very slow to react to a traction loss. As such, there is now a very real all-weather benefit.

Before looking at some of the better systems, it helps to know the subtle differences between the various types. For example, the four-wheel-drive systems employed on many trucks and truck-based SUVs are of the part-time variety. These things are really only of any value when off-roading or on a uniformly slippery surface -- engaging the system on a dry road will damage the drivetrain. Thankfully, these archaic systems are becoming obsolete.

The all-wheel-drive thing should be pretty straightforward; after all, the name is supposed to tell a potential purchaser that it drives all of the wheels. Unfortunately, this is not always the case. Some systems drive all of the wheels all of the time, while most only power up the all-wheel-drive side when the primary drive wheels (usually the front wheels) have started to slip.

The problem with this strategy is timing. Rather than coming to the rescue and gathering things up seamlessly, the sudden transfer of torque actually upsets the vehicle's balance. Powering up the rear wheels when the front wheels are in the throes of understeering pushes the car further into understeer before doing what it was designed to do, which is bring stability. In the end, the cavalry arrives a little too late to save the day.

The better systems use the all-wheel advantage all the time. Yes, this does consume a little extra gas, but the increase is minimal considering the inherent stability these systems add to any vehicle.

Audi's quattro (the true version, not the Haldex-based system that is making inroads) is a very good system that uses a mechanical connection -- a Torsen limited-slip centre differential -- to reduce the inherent instability that comes when the wheels begin to slip. The Torsen-based system, found in the likes of the S6 and S8, biases the power split to the rear -- 40 per cent going forward and 60 per cent to the rear. The system used in the RS5 goes one further as it has the ability to send as much as 70 per cent of the power forward or up to 85 per cent rearward depending upon the demand. It is an enviable system many are still trying to match.

BMW has its xDrive. It, too, is a sound system that preserves the rear-drive feel without giving up the ability to salvage a wayward situation by redistributing the power so seamlessly that the driver remains unaware of its action -- except for the fact that the car stayed on the intended line rather than sliding into the beckoning ditch.

The Mercedes-Benz 4Matic system is another good one. It permanently splits the power, sending 45 per cent to the front wheels and 55 per cent to the rear while retaining the ability to lock the central coupling, which splits the power evenly (50/50). If there is a chance of wheelspin, the traction control system brakes the appropriate wheel(s), which sends the power to the wheel(s) with grip.

Now, not to be left out of the all-wheel push, Jaguar is set to launch all-wheel-drive versions of the XJ and XF sedans. In both applications, the system has been integrated into a sedan that was traditionally rear-drive. This involved modifying the front crossmember, exhaust system and reworking the engine oil pan so that the front driveshaft can run through it. This was done to keep the car's centre of gravity as low as possible. Having to perch the engine over the shaft would leave it sitting too high, and it would have destroyed the sleek hoodline!

The system is advanced and features a 3.0-litre supercharged V6 that's married to an eight-speed automatic transmission. The key add-on is a transfer case that hooks up to the back of the transmission. It allows the drive to be directed to the front and rear.

To ensure the system imparts Jaguar's traditional rear-drive feel, the system is set up such that the vast majority of the power is relayed to the road through the rear wheels (around 90 per cent). However, by monitoring wheel speed along with steering angle and throttle inputs, it alters the split before the wheels are allowed to spin. Indeed, it's smart enough to recognize a takeoff situation so it automatically splits the power 50/50 front to rear, which minimizes the risk of unwanted wheelspin.

What separates all of the above systems is one fundamental benefit -- they are all proactive. This means that, when the system is deemed to be needed, the corrective action has already been initiated -- and before the driver has recognized the potential problem. Obviously, these systems are typically the domain of expensive cars, but the technology is trickling down to the everyday car.

The fuel-consumption penalty for driving all of the wheels all of the time? In the case of the Jaguar XF, it is just 0.4 litres per 100 kilometres. Given the inherent stability and the fact it transforms a fair-weather kitty to an all-weather cat with real bite, it has to be viewed as a bargain.

-- Postmedia News

Republished from the Winnipeg Free Press print edition October 26, 2012 F2

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