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Suspension improves off road stability

Suspension improves off road stability

Tucson, AZ--During serious off-road driving, it's not unusual for one of a vehicle's wheels to get hung up in the air. When that happens, the vehicle suddenly loses stability as it teeters precariously on three wheels.

To solve that problem, one engineer has developed a three-link suspension that makes it almost impossible for any wheel to lose contact with the terrain. The new suspension accomplishes that by allowing for an extraordinary 21/2 ft of vertical wheel travel--about five times as much as a conventional sport utility vehicle. "We knew there had to be some way to make these vehicles more stable," notes David Burke, the designer of the three-link suspension. "The best way was to keep the wheels in contact with the terrain, so that you're never spinning your wheels."

Developed as part of a design project at the University of Arizona, the three-link suspension departs radically from conventional designs. Unlike virtually every system on the market today, the three-link allows the vehicle's axle to separate completely from the suspension. That's critical, because it not only lets one wheel droop more when climbing over rough terrain, but it also keeps the center of gravity low, thus providing a measure of resistance against accidental rollover.

Burke's system consists of four main parts: the three-link suspension; quarter-elliptical springs; variable dampening shocks; and nylon limiting straps.

The key to the system is the three-link suspension. It includes two truss-like lower links, which connect the axle to the vehicle's frame on the left and right side, and an A-shaped upper link, which runs from the top of the differential to points on both sides of the frame. Together, the three links absorb the same forces as any conventional suspension. The lower links prevent fore-and-aft movement and yawing of the vehicle, while the upper link absorbs lateral forces and prevents rolling.

That's critical, because the system's quarter-elliptical springs arenot rigidly attached to the axle. Instead, they sit loosely on rollers atop the axle, and are kept in place only by the weight of the vehicle.

In that sense, the design departs from conventional leaf or coil spring configurations, in which the spring is constrained in the vertical and horizontal directions by an attachment to the axle. That constraint, Burke says, is responsible for the lack of wheel travel in conventional suspensions. Because those axles are mechanically attached to the springs, their wheels can droop no more than about 6 inches.

The three-link, in contrast, allows huge wheel droop, while still maintaining stability against yaw, roll, lateral, and fore-and-aft forces. "The three-link always lets the axle stay connected to the ground," Burke says. "That's a big help in rock climbing, because if you have a high center of gravity and you get the vehicle up on top of a rock, you're going to tip over." Burke says that the additional stability has allowed his vehicle to roll as much as 35 degrees without tipping. Most conventional vehicles dare not roll more than about 25 degrees , he says.

Under normal driving conditions, quarter-elliptical springs maintain contact with the vehicle's axle. Under extreme conditions, the axle separates from the spring, allowing the wheel to droop. The three-link suspension keeps the axle stable with respect to the frame.

Burke's system does place some constraint on wheel travel through its use of variable dampening shocks and nylon limiting straps. Without the shocks, he says, wheel travel would be as much as 3 ft. The shocks, he says, are mainly for the higher speeds of on-road driving. Nylon straps, which extend from a truss atop the axle to the vehicle frame, have been added mainly as a means to prevent the shocks from overextending and pulling apart.

Burke's design has won awards at the University of Arizona and in a contest sponsored by B.F. Goodrich. The suspension has not yet been offered to the public, however.

Burke believes that the three-link suspension offers safety advantages for serious off-roaders, however, because of its ability to keep all four wheels on the ground. "It's virtually impossible for one of my wheels to lose contact with the terrain," he says. "The mass of the tires, wheels, and axle give me enough down force on the terrain to provide good traction, so I never spin my wheels."

Additional details...Contact David Burke, 5652 E. Whittier St., Tucson, AZ 85711, (520) 747-8702.

Other Application

- Any solid axlefour-wheel drive

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