NMA Article: Is Your 4WD Vehicle Really 4WD?
By Eric Peters, Automotive Columnist
It says “4WD” on the tailgate — and there it is again on the list of standard or optionally available equipment. But is it really four-wheel-drive? Or just all-wheel-drive pretending to be something more than it really is?
Increasingly, it’s the latter — most notably when it comes to the ever-growing ranks of so-called “crossover” vehicles and compact SUVs. Examples of this category of vehicle include models like the Ford Escape, Volvo XC90 and Honda Element.
These popular vehicles combine the appearance and stance (including ride height) of an SUV but handle and drive more like cars — because they are typically built around passenger vehicle platforms. They usually come with front-wheel-drive standard and some sort of all-wheel-drive system available as an extra-cost option.
But AWD is not the same thing as 4WD — and it’s arguably false advertising to lead people to believe otherwise.
Each system works differently and offers different levels of capability.
4WD used to be synonymous with a drive system in which the engine’s power is (or can be) transmitted to all four wheels through a two-speed transfer case and drive axles. The transfer case’s main function is to provide a gear reduction feature and the ability to shift the vehicle into 4WD Low range. 4WD Low range is designed for severe conditions such as pulling through heavy, unplowed snow or mud, or slowly climbing up (or inching down) a backwoods hunting trail.
An AWD system also transmits engine power to all four wheels — and will also give the vehicle a better grip on the road in inclement weather than a front-wheel-drive (or rear-wheel-drive only) vehicle. But an AWD-equipped vehicle lacks the two-speed transfer case that is the defining feature of a heavy-duty 4WD system. This means there is no 4WD Low range for deep snow/mud and uneven terrain.
And that means you should probably keep the vehicle on paved roads — no matter how much it may look like a burly SUV on the outside.
Another key difference between 4WD and the AWD systems used in many of today’s “crossovers” is that the AWD systems are heavily biased toward the front wheels.
Under normal driving, usually upwards of 90 pecent of the engine’s power goes to the ground through the front wheels — so most of the time, you are driving, in effect, a front-wheel-drive vehicle. When the front wheels begin to slip, some engine power then flows to the rear wheels via a device known as a viscous coupling. But the “default setting” in the typical AWD system is front-wheel-drive — just like the front-drive passenger cars they’re descended from.
In contrast, a 4WD system is typically based on a RWD layout and usually transmits almost 100 percent of the engine’s power to the rear wheels (the “default mode”) until and unless the setting is changed from 2WD High to 4WD High (or 4WD Low). When 4WD (High or Low) is engaged, the power split between front and rear wheels is also closer to 50-50, as opposed to the (typically) front-dominant AWD system, which usually sends most of the engine’s power to the front wheels.
These functional and design differences betray the different origins of vehicles equipped with 4WD — which are almost always descended from pick-up trucks built on rear-wheel-drive platforms — vs. the trucky looking but lighter-duty crossovers — which are almost always descended from light-duty, front-wheel-drive passenger cars.
Yet despite the clear differences in design and capability between 4WD and AWD, several automakers brazenly conflate the two as a way of bulking-up the perceived capability of their light-duty, car-based “crossovers” to make them appear less like the passenger vehicles they’re typically descended from.
But don’t be fooled.
And don’t expect a car-based crossover vehicle with a car-based AWD system to be able to do things a true 4WD-equipped vehicle would be able to do.
Is Your 4WD Vehicle Really 4WD?
Further Reading: © 2008 NMA