Don't Adjust the Ride

Getting drivers take matters into their own hands by adjusting ride height on an air bag suspension can be hazardous to your rolling stock's health. Whether it's a power unit or a trailer, the signs of a poorly adjusted air suspension can be detected from inside the cab by looking at your trailer "hop." It might just be apparent by watching a rental trailer return to the yard with an inch or two of extra height.

Drivers and some owners incorrectly believe that manually setting the ride height for their own convenience won't adversely affect a tractor or straight truck. This misconception usually results in a broken bag, an especially painful event if the vehicle is operating under loaded conditions and many miles away from the nearest repair facility.

"Many times tractors and straight trucks get broken bags simply as a result of drivers not understanding that too much air is as bad for the system as no air," says Bill Keen, director of leasing at Performance Leasing in Houston, Texas. "Overfilling the bags usually comes from the drivers wanting to bring up a fifth-wheel height or to level up to a dock." Ride Height Sensor Valve

Drivers are able to manually set the ride height by manipulating the ride height sensor valve. This valve inflates or deflates the suspension's air bags. Some cases have been reported where the valves have been permanently "tuned" by drivers, who believe that they have improved the ride.

The reality of over inflated air bags is anything but ride enhancing, and it can be mechanically devastating. Many maintenance personnel and operators believe that if an overinflated bag breaks, that will be the end of it, says Keen. "It's not the only possible damage for a tractor or a straight truck. There could be driveline damage if the condition has been prevalent for a long time, or really taken to an excessive level."

When the rear of the power unit sits at an elevated height, it throws the driveline into a condition known as "bind." An overfilled air bag spreads the axles from the frame. In conditions of extreme amplification of driveline angles, the U-joints, yokes, and driveshaft splines can produce the severe locking-up effect of bind. In actual practice, drivelines have a very limited angular alignment window. Going beyond that window limits the rotational ability of the actual driveline, because the universal joints can't make the rotational pivoting required to transfer power from the transmission to the axles.

Angular Deviation What kind of circumstance could cause that much of an angular deviation? Notice the movement of a power unit while it crosses a rural railroad crossing. A high-percentage grade will cause a split between the bogie and frame rail assembly.

Operating beyond the window of angular alignment usually does the most damage when the power unit is operating without a load.

Trailers are frequently damaged by overinflated air suspensions. When a trailer's air suspension is set too high, a bucking motion can take place at highway speeds. In an unloaded state, "hop" is noticeable if the air suspension is overfilled to an extreme degree. In both cases, noticeable jaw wear will be found if the trailer is dedicated to a tractor, and the overfilling condition is allowed to continue.

"A situation can develop where dock workers will raise the back of the trailer by using the air suspension," explains Keen. "Then an inexperienced driver returns the rented tractor and trailer to us. That's when we may find fifthwheel damage caused by the over inflated air suspension."

An air bag suspension has only one correct setting, says Keen. "Let the suspension level itself out. It will do that without the need to play with it." Today many manufacturers make a near zero or zero delay air suspension control valve. These valves will automatically set the correct ride height for the power unit and/or the trailer.

Heavy-duty truck manufacturers also have started to install the actual control valve in a less accessible location of the frame/bogie area. Trailer manufacturers are placing the control valve in less accessible locations. "They are making it pretty clear that they don't want over-the-road adjustments made to the equipment," Keen says.

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