New Suspension Technology

Oct. 1, 2000
May Reduce Tank Rollover BULK liquid cartage may be the most challenging of all line haul operations for testing the capabilities of a truck or trailer's

May Reduce Tank Rollover BULK liquid cartage may be the most challenging of all line haul operations for testing the capabilities of a truck or trailer's suspension. The Maintenance Council's (TMC), Tanker Suspension Improvement Task Force (S-7) has been looking for ways to increase suspension stability on these vehicles.

S-7 members, mostly operations executives from the bulk hauling industry, are concerned about roll stability, structural durability, tire to road contact, and suspension walk. TMC members, and especially those involved in the operation of tankers, are concerned about educating the general public regarding tanker safety, as well as providing recommendations and guidelines to help equipment managers improve the overall operations of their equipment.

Tank trucks and tanker combinations operate under a principle of a diminishing load or in operations where the load is extinguished at one delivery location. The challenges that suspensions encounter remain the same for both operations.

All suspensions provide the same basic function for the vehicle. The suspension provides stability by counteracting internal and external forces applied to the vehicle from driving operations and while the vehicle is stationary.

These multi-directional forces are induced from turning, avoiding road obstacles, stopping/starting, and many other movements that encompass normal driving. Items such as wind, rain, and other weather factors can apply energy forces to a vehicle while it is stationary. Suspensions should compensate for all of these forces and return the vehicle platform to a state of equilibrium. The suspension provides the compensation factor by applying an equal but opposing reaction to the force being exerted on the vehicle.

Suspensions vary in their rate of counterbalance response. It is this latitude of counterbalance response in the suspension system that brings the vehicle into a state of equilibrium, ideally without overcompensating. A suspension that overcompensates or is too stiff to adequately respond to forces applied to the vehicle platform can cause controllability problems.

Diminishing and possibly shifting loads can provide the most challenge to the fleet operator when selecting a style of trailer suspension. The selected suspension has to offer the correct counterbalance response to not only the trailer's tare weight, but also the diminishing weight of the cargo. Other factors such as road perils and uneven surface conditions would be directly transmitted to the chassis of the vehicle without the compensation effect provided by the suspension.

TMC's S-7 Task Force recognizes that tanker suspensions must perform when the tanker is empty, has a minimum tare weight, and is fully loaded to its maximum capacity. Therefore, the suspension must be able to return counterbalance response force throughout a wide range of weight conditions. How quickly, efficiently, and how much latitude the suspension has in performing this operation can be a method of rating suspensions.

Tank operators and trailer manufacturers are familiar with the spring, air bag, air with spring, and walking beam suspensions, just to name a few. Most important to the tank trailer operator is how these suspensions perform. Each design has some benefit that might not be found in other designs.

Allowable Axles The S-7 Task Force also recognizes that the number of allowable axles plays an important role in the design of a truck or trailer's suspension. Federal and state regulations should be factored into the equation to determine the type of suspension used to spread load over a given span.

The Task Force recommends that every suspension design should include some basic focus points:

- Adequately transfer the longitudinal, lateral, vertical, and torsion forces in a way that brings the vehicle platform back into a state of equilibrium, whether loaded or unloaded.

- Provide for competitive weight and cost considerations.

- Provide a low maintenance nomenclature that supports a long life-cycle.

Operators of tanker combinations are looking to differentiate suspension mechanisms. They are questioning if certain types of suspensions will assist in keeping the total vehicle platform under control during normal and hazardous operating conditions.

Tank manufacturers can better serve these operators during their specification process by clearly defining the strengths of the suspension system that they are installing under the tank.

Suspension Improvements Members of the Task Force have identified several key stability factors that they believe are vital in understanding and developing newer improvements to suspensions.

- Lateral track between tires or dual-wheeled axle ends.

- Center of gravity height.

- Counterbalance (compliant) response to load input, such as tires; frame twist of truck, tractor, or trailer; suspension roll stiffness; and fifthwheel lash.

- Driving demands: speed, turning, general maneuvering, and road condition.

- Load shift.

Driver-related factors that are important for stability include speed, turning technique, driving tempo, and road conditions. Although declining in importance, improperly loading the tank can be a factor in rollover incidents. Today the human error that causes a catastrophic load-shift is predominantly speed.

Suspension systems that use new technology should help reduce the incidents of rollover. However, some drivers erroneously believe that they can tell when a tractor/trailer unit is about to become unstable during cornering in time to prevent it. That's not the case. The type of feedback they anticipate - such as tire slippage or squeal - is apparent only after the tractor or trailer becomes unstable.

Upcoming Trends Manufacturers of suspension systems are aggressively looking to incorporate newer technologies into the suspension systems to both alert the driver of and to compensate for situations that could lead to instability and cause a tanker rollover.

New technology that integrates advanced dumping or loading of suspensions with tractor/trailer instability warning monitors would greatly lower the rollover threshold of commercial vehicles. Today new shock absorber components can theoretically be manufactured with dynamic fluid-filled components that will react almost instantly to bring the vehicle platform back to a condition of equilibrium.

Shocks are now dampened by use of dynamic fluid-filled componentry that provides an almost instantaneous reaction to bring light vehicles into a state of equilibrium. New absorbers could be designed to adjust the counterbalance forces in advance of counter instability issues by using a system of onboard computer and radar technologies. These systems will be available in the future for combined tractor/trailer operations.

TMC members and industry officials are looking to these new technologies to help lower the rollover threshold. Nevertheless, TMC is quick to warn fleet operators that driving faster can negate greater stability and suspension improvements. Pushing a more stable suspension to the limit won't lower the rollover statistics, nor improve the public's perception of the tanker industry and tank trailer manufacturers.

About the Author

John Nahas