It’s All In the Springs

May 8, 1998
Getting good ride and handling out of a truck is a matter of matching the right spring to the job

Engineers designing truck suspensions have their work cut out for them. Fleet owners and operators want to save money so they want suspensions that are inexpensive, lightweight, and reliable. Highway officials want suspensions that protect the roads and bridges. Customers want them to protect what they’re carrying. And drivers want them safe, comfortable, and easy to maintain. Engineers also have to take into account the fact that the truck can be full or empty — a difference of more than 60,000 lb for a dump truck. One day the truck might carry steel coils that concentrate the load in a small area, and carry cartons of computers that spread the load evenly over the truck bed the next day. And while the truck might carry its load along on highways at 55 mph most of the time, it might also go offroad onto construction or logging sites.

To cushion the load, and the driver, truck suspensions make good use of a variety of springs — leaf springs, air springs, and rubber springs — each with its own advantages and limitations.

Steel leaf springs have been around the longest and for good reason. They’re simple, durable, and do a good job when the springs are matched to the load. One of the most advanced leaf-spring designs is the Tuftrac tandem-axle rear suspension built for heavy-duty vocational trucks from Freightliner and Sterling Truck Corp., Willoughby, Ohio. Vocational trucks go on and off highway and need a suspension that will give maximum traction and control in difficult and differing conditions. The suspension will also handle more stress than those designed strictly for highway travel.

Tuftrac is a six-rod suspension, with a wishbone at the top and two control links at the bottom of each axle. These rods hold the axles in position and take up the driving and braking forces, while the leaf springs support the load. “This lets the rods act independently of the springs so our engineers can optimize each component for the job it has to do,” says Dan Fuchs, manager of chassis engineering for Freightliner and Sterling. “Unlike other designs, it lets the axles follow the terrain without generating large internal forces and maintains an even load on all four wheels.” The even wheel loading helps prevent wheel slip and maintains traction on uneven ground.

The leaf springs are composed of a series of parabolic or tapered steel leaves, which lets engineers put the steel where it’s needed and keep weight to a minimum. The more leaves in the spring the more weight it will carry. Engineers also try to minimize the friction between the leaves. “As the spring works, the leaves slide against each other. If there’s too much friction, the spring won’t deflect and ends up acting as solid beam,” notes Fuchs. “You can really influence ride by managing the friction in the spring.”

On the Tuftrac, the spring connects both axles and pivots on a large, elastomer rubber bushing shaped to let it pivot without lubrication or other maintenance. Most other leaf-spring designs use a heavy trunion which needs lubing to support the springs. To make the entire suspension lubrication-free, all of the Tuftrac suspension components are held in place with rubber connectors. The connectors don’t slide against one another, so there’s no need for periodic lube jobs. Using rubber also makes the suspension quieter, a major issue for trucks that frequent residential neighborhoods.

The Tuftrac suspension also houses the brake in front of the axle, as opposed to behind as on other suspensions. Tucking the brake components there protects them when backing up on rough ground or around tree stumps.

Air springs use a driver-adjustable bladder to supply the spring force, supporting the load on a column of air. Similar springs serve as vibration isolators supporting heavy equipment in plants. On some trucks, they’re even used to mount the cab and seats, cushioning the ride even more.

The springs are made of several layers of rubber and fabric, and are filled from the truck’s compressor to between 60 and 90 psi, depending on the load. The base or piston of the spring was once made of steel or aluminum. Engineers at Firestone Industrial Products, Carmel, Ind., however, have developed a glass-filled nylon as a lightweight substitute.

Since the springs are adjustable, the suspension can be optimized for a wide variety of loads. “The system is flexible,” says Paul Gibson, product manager of Firestone’s Ride Rite Group for Lighter Trucks. “It lets drivers tailor the ride to the exact load, giving a good ride when the truck is fully loaded and when it’s empty.”

A height sensor, which can be electronic or mechanical, controls an air valve in the spring and helps maintain ride height. Ride height can also be adjusted, to match that of a loading dock, for example. And the automatic leveling keeps the headlights and suspension at an optimum height.

If there’s an accident and a spring is punctured, a shut-off valve stops air from bleeding off and the suspension will come down to rest on a hard rubber bumper. “It gives a much rougher ride, but should last until the driver can get it fixed,” says Gibson. “But air springs are durable, with some going a million miles and most lasting four times longer than leaf springs, but some applications, especially off-road, can cause problems.”

Still, the advantages of air springs are making them very popular, with 80% of new commercial trucks using them on the rear axles and 70% of the trailers being built with them. But there are some drawbacks. Although the air springs themselves don’t need maintenance, the overall system is more complicated than a simple leaf spring and it requires more space. Air springs are rarely found on front suspensions because of the space limitations there and the fact that the load on the front axles remains fairly constant. A constant load eliminates the need for air spring’s adjustability and lets engineers optimize a leaf spring for the front end.

Rubber springs, often used to beef up a truck’s leafspring suspension, is a hollow, injection-molded rubber component. It has a progressive spring rate, so the farther it’s compressed, the more it resists. Engineers control the inner and outer profiles to tailor the spring’s characteristics, putting more rubber in the cross section to add stiffness.

Unlike air springs, rubber springs are vented to the atmosphere and don’t compress air. All the spring force is supplied by the rubber.

“We use natural rubber, no steel plies, because it’s the most durable and flexible material,” says Ed Sanders manager at Timbren Industries, Niagara Falls, N.Y. “And we stringently control the rubber composition to get the ride effect we’re looking for.”

In a well-designed rubberspring suspension, the springs are mounted between the leaf springs and the axle, supporting the body and unloaded chassis. The leaf springs kick in when the truck is carrying a load. Being softer than the steel leaf springs, the rubber springs absorb much of the road shock and extend the life of the leaf springs. They also quickly press down on the axle when a wheel tries to go airborne, preventing axle hop.

Installation is simple since the spring is only attached at the top with a single bolt. The rubber is strong enough to resists tearing at the bolt hole and is thick enough that deterioration due to sun, oil, or salt doesn’t affect performance. And while they’re not drive adjustable, they do give more roll stability the farther out on the axle they are mounted.

Engineers at Timbren are developing rubber springs that will be integral components on front suspensions to give trucks a better ride with a more durable and lighter-weight suspension. They’ve also built springs that would work with air springs for trucks with high centers of gravity to reduce roll and sway.

The next-generation truck suspensions will also see more electronics, with air spring systems hooked into engine and transmission performance, as well as load and leveling sensors. Air-spring components will also be shrunk so they fit on truck front ends and on more consumer vehicles. Several cars and sport utility vehicles, including the Lincoln Navigator and Ford Crown Victoria, already carry air-spring suspensions.

Trucks traveling the roads must comply with federal and local regulations, and one type of law that affects them directly are the bridge laws. They limit the weight a vehicle’s axles may carry. For some trucks and tractor/trailer rigs, especially dump trucks and those carrying other heavy loads, this means that although they’re designed to carry a full load on their front axle and rear tandem axles, they must add an axle or two, and sometimes three, to distribute the load among more axles to comply with the laws. When these kinds of trucks reach an off-road destination, such as a logging or construction, site, the lift axles are pulled up and out of the way for maximum maneuverability. In other cases, trucks need the axles full time to handle the load.

These axles, called lift or tag axles, are positioned by the driver using a cab-mounted switch which inflates or deflates a rubber bladder that lifts or lowers the axle. In most cases, air springs rather than leaf springs provide the resistance in the extra axle and the spring’s pressure is also controlled from the cab. Using air springs lets engineers design compact lift axles, a design goal since space is limited on some trucks. At Meritor Heavy Vehicle Systems, Canal Fulton, Ohio, engineers try to design lift axles so that if a wheel fits under the frame, their lift axles fit within that profile as well. To cut down on the distance between the axle centerline to the mounting bracket was minimized by angling the racket rearward.

There are two types of lift axles, steerable and nonsteerable. Steerable versions use a parallelogram design with upper and lower control arms connected via pivot points. A positive caster angle makes them steerable, letting the wheel track with the front wheels. (Caster angle is the angle, in side view, between the steering axis and the vertical. It is considered positive when the steering axis is inclined rearwards in the upward direction, and negative when the axis is inclined forward.)

Steerables cut down on tire scuffing, a problem when there’s too much space between axles. When a truck with two rear axles makes a turn, the wheels on the front axle turn a tighter radius than the rear wheels, causing the front wheels to scrape across the pavement. This is hard on the tires and the road. Nonsteerable versions are less complicated, but add roll stability to a truck while distributing the load. Both include brakes, so they increase a vehicle’s available braking power as well.

One drawback to steerable lift axles is that they become a hindrance when backing up. In reverse, the positive caster angle makes the wheel cut to the left or right and drag, much like pushing a grocery cart from the front. There are two solutions. Some lift axles automatically rise up and out of the way when the truck is put in reverse. Although this solves the problem, it increase the load carried by the other axles. Others, such as those designed at Meritor, use a mechanical lockout. With a truck outfitted with this type of lift axle, putting it in reverse forces pushrods against the tierod arms of the axle until the wheel is straightened and the caster angle becomes zero. “This can be done even if the lift axle’s wheels are turned, so the truck doesn’t have to be repositioned to straighten all the wheels before going into reverse” says David Croston, a manager at Meritor.

© 2010 Penton Media, Inc.

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