Trucks with Off-Center Loads:More to Consider Than Just Weight

May 1, 2000
Solving the problem of off-center loads in trucks involves more than adding another spring to the suspension, according to Tom Gillespie with the University

Solving the problem of off-center loads in trucks involves more than adding another spring to the suspension, according to Tom Gillespie with the University of Michigan Transportation Research Institute.

Even when the suspension is strengthened on the heavily loaded side of the truck, the asymmetrical load can adversely affect the ride, performance, and durability of the vehicle, Gillespie told those attending his engineering session at the NTEA convention in St Louis.

No truck is perfectly symmetrical, Gillespie said, because of the presence of such factory-supplied offsets as fuel tanks, batteries, and other equipment. Customers also can cause off-center conditions by the way they load the vehicle with tools, equipment, and supplies.

However, truck equipment distributors who install corner-mounted cranes or other heavy loads can convert slight asymmetrical loads into conditions that can adversely affect the chassis.

Although adding another leaf can provide the suspension with the strength it needs to carry the additional weight, this is only a partial solution, Gillespie said. Off-center loads can cause other problems that beefing up the suspension beneath the load does not address. Gillespie listed several, including:

* The feel of the truck as it drives down the road. * Tire wear. * Braking performance * Its ability to turn properly. * Roll behavior.

The feel of the truck when being driven is the result of an imbalance of the load on the front axle, Gillespie explained. Vertical loads produce a toe-in reaction in the steering geometry. Any difference in the loads placed on the two front wheels causes the vehicle to steer in the direction of the light side.

Affecting Vehicle Performance Off-center loads cause tires to wear faster, Gillespie said. To minimize tire wear:

* Keep tires vertical with respect to the road. * Keep tires on an axle pointed in the same direction. "Tires are designed to run flat on a surface," Gillespie said. "If you camber them, the load is not uniformly distributed across the tread where the tire contacts the road, resulting in uneven wear."

Gillespie said keeping the tires pointed in the same direction is common sense, but the objective is not always easy to achieve. Axle alignment is load sensitive. Even slight misalignment causes tires to slip sideways along the road, he said. A one-tenth degree change in alignment produces 10 feet of side-slip per mile.

"Your goal as a final-stage manufacturer should be to not affect the alignment of the chassis manufacturer," Gillespie said. Braking performance also is affected by off-center loads, Gillespie said.

"Off-center loads generally will cause a brake force imbalance around the center of gravity," he said. "When you apply the brakes on a truck, ideally the same brake force is generated on all wheels. But that doesn't happen. It's not uncommon to have a 15% range between tires. But if the load is not centered, its brake force is not balanced between the two sides. When a truck is loaded unequally, hard braking will cause the vehicle to steer toward the light side."

The ability of the truck to travel through turns also is affected. Vehicles corner best when tire loads are as equal as possible, Gillespie said. When loads are substantially off center, the truck will react differently when turning, and the reaction will differ according to the direction the truck is turning.

"You can reduce the transfer of tire loads by lowering the center of gravity of the vehicle," Gillespie said. "This is not always possible with trucks, though. The center of gravity typically is six feet above tires that are six feet apart. This really shifts loads, and offset loads add to that in dramatic fashion."

Off-center loads also affect the truck's ability to resist rollover. The truck will have different rollover limits, depending upon whether the turn is to the left or the right.

"Put in suspensions that are as stiff as possible," Gillespie advised. "Make sure there is no lash in the suspension."

Keeping in Balance Gillespie advised against solving off-center load problems by simply adding a leaf to the suspension.

"Don't do that," he said. "You will cause some type of deterioration in the performance of the vehicle, perhaps in ways that you don't know. While you may rely on the purchaser of the truck to have reliable drivers, the reality is that the world is getting very litigious. If a truck is involved in an accident, the final-stage manufacturer of that vehicle undoubtedly will be placed in the loop."

If forced to equip a chassis with offset loads, Gillespie recommended adding the leaf so that the vehicle sits level. He did not recommend using an air leveling system because of softness that allows the chassis to roll.

"You really need some auxiliary roll stiffness-a roll bar or a stabilizer bar," he said. "These are beginning to show up on truck suspensions as chassis manufacturers get more sophisticated."

Gillespie suggested that distributors do what they can to offset an imbalanced load with an offsetting load on the other side.

"If you put a corner-mounted crane on one side of the truck, try to put your load boxes on the other," he said. "That may not always be possible, but it is important to be aware of off-center loads and to do as much as possible to keep the truck in balance."