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Off the Beaten Path
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| Drive Systems: Know the Difference |
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| Updated |
Jun 10, 2004 22:01:25 |
| Rating |
 79 ( -12 -15.18% ) | | Author | Sue Mead |
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Description:
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In 1988, when I began my automotive career as a test driver for Four Wheeler Magazine, I knew very little about the mechanical systems of vehicles. I did know, however, that I liked sport utility vehicles for their safety and size, high point of vantage when driving, and for the four-wheel-drive traction capabilities they offered. When it came to choosing a vehicle for our family, after my daughter was born, I had a strong voice in our selection of an SUV, with 4WD. After all, we lived in the snow belt and, in our spare time, I enjoyed four-wheeling in the backcountry.
Today, as a full-time automotive journalist, with a specialty in 4WD and the light truck market, I have learned a great deal about mechanical systems, and have had the good fortune of testing hundreds of trucks and SUVs on the highways, byways and backcountry trails of America, as well as around the world. In fact, over the past decade, my work with automobiles has taken me to over fifty countries and the miles I have traveled off-highway would likely span the width of the globe. During that time, I've witnessed a significant change in the automobile market of America. When I joined the staff at Four Wheeler, there were a dozen or so new or upgraded models of SUVs and trucks only, and, as a short female driver, they weren't built for me!
It's no secret that increasing numbers of buyers are choosing sport utility
vehicles (SUVs) and trucks as their primary or secondary automobile. Now, the majority of these vehicles are sold as four-wheel-drive (4WD) or all-wheel-drive (AWD) models, especially in areas of the country where the going can get nasty with mud and snow. In fact, light trucks and 4WD/AWD offerings are the fastest-growing segment of the automobile market in America. Most owners of these vehicles pinpoint safety and security features, as well as versatility and utility, as the reasons for their purchase.
It's also clear that the status of ownership is an appealing attribute
since "sport utes" and light trucks are now cache for errands at the grocery store, as well as dinners at the country club. Today, these vehicles are often purchased as a replacement for more traditionally-upscale offerings, such as luxury sedans and sports cars.
Off the Beaten Path
Drive Systems: Know the Difference
---Sue Mead
There are now over sixty models of SUVs available in the U.S. market, and increasing numbers of light-duty trucks, that range from minis such as the Jeep Wrangler, Toyota RAV4 and Kia Sportage to full-sized versions such as the Chevy Suburban, Ford Excursion and Lexus LX470.
Likewise light trucks, once relegated to farms and construction sites,
have gained popularity with more mainstream buyers, who are
selecting 4WD versions for their safety attributes, along with the "truck
tough" and outdoor-adventurer image. In fact, trucks have been the
best-selling vehicle in America for over 12 years.
Surveys reveal, however, that only a small number of owners (ten
percent, perhaps) actually drive their vehicle off-highway, although
certain manufacturers, such as Land Rover and Daimler Chrysler's
Jeep, report much higher statistics. While some drivers use their
SUV's or truck's four-wheel-drive traction for slippery surfaces, such
as snow and ice, the vast majority of drivers use their vehicles for
commuting to work. Some drivers consider "off-roading" travel along any dirt or gravel road, even in two-wheel drive. Very few really leave the pavement behind and shift into 4-high or 4-low to explore the lands beyond and learn about the true performance and fun of 4WD.
Regardless of whether they intend to use four-wheel drive or
all-wheel drive as security on ice and snow, or plan an all-out assault on
the dirt roads of America, truck and sport-utility shoppers should
understand the types of systems available on popular vehicles. Here is a primer on the basic differences between two-wheel drive, four-wheel drive, all-wheel drive, as well as traction control systems and
locking differentials, which can provide some of the benefits using
different technology.
*2WD or 4 x 2: These are terms for two-wheel drive. In front- or
rear-wheel-drive vehicles, a differential along the axle drives this pair
of wheels and adjusts the speed at which these wheels turn while
cornering. Basically, the other set of wheels goes along for the ride. The
majority of today's cars have front-wheel drive, while many sports
cars and luxury models have rear-wheel drive.
*4WD or 4 x 4: A general term used to describe a vehicle with power
that goes to all four wheels. There are many different types of
four-wheel drive. The basic systems are full-time, part-time, and
permanent.
Full-time- These systems offer both 2WD and 4WD modes with
some type of center differential that allows 4WD to be engaged on dry
pavement, for highway driving in the snow, for instance. These versions typically come with 2WD, 4WD "auto" and 4-Low- a low-range sets of gears for slow-speed off-road driving. Ford's Expedition is an example of a vehicle with full-time 4WD. The "auto" setting is basically 2WD on dry pavement.
Part-time-This traditional 4WD system comes with a two-speed transfer gearbox where 2WD operates all of the time on pavement and 4WD can only be used off-road or when the pavement is very slippery because the front and rear wheels travel at the same rate of speed. In other words, traveling at highway speeds in 4WD for lengthy periods would damage the gearbox because it is designed for slower driving. The familiar Jeep Wrangler uses this system.
Permanent- Systems without a 2WD mode that send the power to all
four wheels "permanently" on pavement or off. These usually include
4-High and 4-Low and sometimes have a locking center differential. The Land Rover Discovery is an example of a vehicle with permanent four-wheel drive.
*All-wheel drive or AWD:These systems power all four wheels-usually
full-time- giving improved traction and handling. There is no
two-speed transfer gearbox to provide a low range set of gears. In the
past, these systems were a passenger-car based four-wheel-drive
system, but AWD is now crossing over into sport utility vehicles. Subarus use this system.
Manufacturers also offer a couple of traction enhancers that don¹t
involve four-wheel drive, or can be used along with four-wheel drive:
*Traction control-Available on most cars priced above $20,000 or
so, improves their grip in situations where traction is compromised. It
works, and works quite well usually, by either briefly cutting engine power or pulsing the brake on a spinning wheel (or both) to help send power across the axle to the wheel with the most grip. Mercedes-Benz M-Class sport-utilities use traction control in addition to a full-time 4wd system.
*Locking differential:This mechanical device "locks" the
wheels along the axle or, as in the case of a center differential, locks
the front and rear driveshafts together. A conventional differential sends
the power to the wheel or axle with the least amount of traction, which
works well when driving on dry pavement. A locking differential sends an equal amount of power to each wheel or axle, allowing the wheel with the most traction to pull the car out of the slick, or traction-compromised situation, and should be reserved for limited traction surfaces and slow speeds. They Toyota RAV4 has an available locking differential.
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About Sue Mead
Sue Mead works as a photojournalist and features writer for more than two dozen publications. She has written for Parade, Popular Science, Open Road, the Road & Track Buyers Guide, Men's Journal, Popular Mechanics and Diversion and she has worked as an auto editor for CNN/fn. Sue has co-driven in two of the world's toughest off-road races, the Baja 1000 and the Paris-Dakar Raid. She attended four Camel Trophy adventures for Land Rover North America and has participated in several long-distance adventure drives. She recently completed her first book, "Monster Trucks and Tractors" published by Chelsea House. Sue lives in New England. |
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