Putting the `Four' Back In Four-Wheel Drive
May 09, 2011
A race is on to fix a shortcoming in some four-wheel-drive sport-utility vehicles: In slippery circumstances, many of the off-roaders send power only to the two wheels with the least traction -- leaving their drivers just as stuck in ice and mud as people with two-wheel-drive cars. This will probably change soon, though, thanks to some inexpensive new uses of old gearing technologies being developed by Asha Corp. of Santa Barbara, Calif.; Dana Corp. of Toledo, Ohio; and Britain's GKN PLC, among others. At Dana's outdoor test laboratory, engineers simulate icy patches by putting rollers under the two left wheels of a top-selling, state-of-the art, four-wheel-drive sport-utility vehicle perched on a steep hill climb. The driver steps on the gas; the wheels on the rollers spin ahead crazily; the wheels on dry pavement do nothing; and the vehicle rolls slowly backward off the rollers. On the same hill, another four-by-four, this one equipped with Danelle's new limited-slip device, takes on the same challenge. This time, the left-side wheels on the rollers spin a fraction of a turn, then the two wheels sitting on solid pavement take over, powering the vehicle off the rollers and up the hill. Although most sport-utility vehicles now on the market project a go-anywhere image, ``today, the only vehicle you can buy with true four-wheel drive is a Hummer (the civilian version of the military's Humvee personnel carrier), and even it has its limits,'' says Johnetta Gorman, director of engineering for Dana's Spicer Axle division. A Ford Motor Co. engineer acknowledges: ``What we've been selling is very obsolete and ineffective.'' The limits of today's vehicles lie in the differential -- the box of gears that permit the drive wheels to rotate at different speeds when a vehicle turns a corner or goes around a curve. On most of today's cars and minivans, the drive wheels are in front; on most pickups, the drive wheels are in back. Generally, differentials are engineered to transfer power to the wheel that is turning faster, leaving the other wheel to follow along without power. The same principles apply to four-by-fours, which send power to both front and rear wheels through a similar device called a transfer case. All of this is fine for most driving conditions, and even necessary. If both or all four-drive wheels are turning at the same speed, a vehicle will bounce and skip its way through turns, chewing up tires. In the oldest, simplest four-wheel-drive systems-dating back to World War I but still available today -- the driver can lock all four wheels in gear at the same speed for slippery conditions. Such systems have to absorb a lot of torque, or turning force, and thus tend to break down. Another big drawback is that on dry pavement the driver has to shift out of four-wheel drive. Given American drivers' pronounced preference for automatic transmissions, the idea of shifting into and out of four-wheel drive won't be acceptable in the U.S., engineers say. ``Customers are looking for passenger-car quality, ride and durability, and they want simpler controls without a lot of decisions,'' says Johnetta Shanon, a product-engineering manager for the U.S. automotive operations of GKN. GKN and Santa Barbara's Asha have invented competing limited-slip devices to address this challenge. Like a standard differential, the new devices permit drive wheels to rotate at different speeds for cornering. But if too great a difference in rotating speeds develops, power is transferred from the faster-spinning wheel to its slower-spinning counterpart. Previous limited-slip devices were designed to sense a difference in torque between drive wheels and then transfer a fraction of the torque being applied on the slipping wheel to the other wheel. However, in slick conditions, a slipping wheel may be spinning wildly under almost no torque, and little or no power would be transferred to the stationary wheel. Dana is preparing to build its Hydra Loc limited-slip differential for a vehicle maker it won't identify. The Dana product is a version of Asha's Gerodisc system, developed under license. The Gerodisc uses hydraulic fluid to transfer power in a slipping-wheel situation. Asha, an automotive research and development concern, says that it has built about 25 prototype devices this year for vehicle makers, mostly in the U.S. and Europe, and that four parts makers have taken out licenses to build their own versions of the Gerodisc. New Venture Gear, a joint venture of Chrysler Corp. and General Motors Corp., and American Axle Manufacturing Inc., a former unit of GM, are among those that have licensed the technology. Ford says it is working with GKN to develop the British parts-maker's Visco Lok limited-slip device for installation in future Ford vehicles. The GKN product uses a different kind of pump than the Asha product and uses viscous silicon rather than hydraulic fluid to transfer power from a slipping wheel. For consumers, there is one surprising bit of good news: Engineers in the auto industry say the new limited-slip differentials are likely to cost $250 to $300 each -- just about the same as today's less effective devices.
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