Trailer creep

Feb. 1, 2006
MOST OF the time, tank truck carriers want those wheels moving in order to generate revenue and keep the bottom line in the black. But there are times

MOST OF the time, tank truck carriers want those wheels moving in order to generate revenue and keep the bottom line in the black. But there are times when movement isn't in order.

Jeff Miller of Hendrickson discussed unintentional trailer creep during loading and unloading and when the vehicle is parked — and ways to correct the problem.

He pointed out that leaf-spring suspensions present a gradual downward movement when the trailer is being loaded, but there is little or no horizontal movement.

With air suspensions, though, the movement is vertical and horizontal during loading, or as suspension air loss occurs while a trailer is parked.

Air suspensions move for several reasons. Like a tire, air suspensions must have enough air pressure to carry the load. They also have low spring rates. Pressure requirements can range from 10 psi for an empty trailer to 60-100 psi for a loaded trailer.

“Sudden load increases require a sudden increase in air pressure,” Miller said.

The air system response comes from the system airflow, the height control valve operation, system pressure, and suspension jounce or “up travel.”

When there is no air in the reservoir, the suspension is prevented from raising. Pressure below 70 psi will prevent airflow to the suspension, and low pressure will allow the reservoir to drop below 70 psi. Once the 70 psi is reached, the control valves kick in and the trailer then can be moved.

With the trailer brakes released, tires rotate independently of the axle. The axle moves rearward and the trailer only moves vertically. In this instance, creeping does not occur.

However, when trailer brakes are locked, tires rotate with the axle. Tire rotation pushes the trailer forward as the trailer drops and creep occurs.

Movement can be limited through various means. Manual suspension dump valves can be employed, but they depend on the operator to be used properly. They can limit horizontal movement but will not eliminate vertical movement.

Automatic suspension dump valves eliminate operator involvement, and some designs can limit horizontal movement, but they do not limit vertical movement.

A ride height lock is an automatic device that eliminates operator involvement, as well as horizontal and vertical movement.

One good way to prevent unwanted vehicle movement is through regularly scheduled suspension inspections.

The ride height inspection should include a review of supplier instructions for proper measurement techniques, key design element, and key operating element. Miller said the definition of ride height is the distance between the suspension mounting surface and the center of the axle.

Improper settings can cause air spring, shock, and trailer damage, as well as providing poor ride quality and handling, and capacity problems.

Suspension inspections should include a visual inspection at service intervals, including the bushing tube spacers.

The suspension pivot hardware should receive a visual inspection at service intervals. Mechanics should look for problems evidenced by movement and rust trailing down the frame bracket from underneath collars.

Visual inspections of shock absorbers also are important. If a shock is misting fluid, it may be OK, but if it is leaking, it must be replaced. If it is leaking, the oil streams down the barrel of the shock. Misting is a light mist of oil that can flow down the shock barrel. Test the shock by running the vehicle for 20 minutes and see how hot the shock is. The shock should be warm or hot to the touch. If the shock is not warm or hot, then it needs to be replaced.

Air springs should be mounted properly with no external abrasions and not folded or pinched when inflated. There should be no air leaks.

Miller pointed out that original inspections of suspensions on a recently acquired trailer should note that the trailer is level, all welds are of acceptable quality, all bolts are in place and secure, the pivot-connection nut is tack-welded to bolt threads, and no component interferences exist.

Daily inspections should include a quick look to verify that a level trailer is riding at the correct height.

At 30 days, the inspection should include clearances around air springs, tires, shock absorbers, and all other moving parts. Evidence of component interference requires immediate attention by a qualified mechanic.

The 30-day inspection should insure that bolts are secure and axle connections are tight. Mechanics should look for any sign of wear.

At 90 days, all items should be thoroughly checked that were inspected at 30, and welded connections should be examined for deterioration. Mechanics should check for any problems with frame attachment joints, crossmember structures, and pivoting and clamping connections.