Advancements in heavy duty suspension technologies

Feb. 4, 2019
Improve your fleet’s ride, maintenance and longevity with smart technologies and sensors.

Suspension technologies in heavy duty vehicles have come a long way since their inception; from rudimentary mechanical springs to modern air suspensions with hydraulic shocks. Today, manufacturers are taking the next steps in improving suspensions, with advancements such as air dampers, continuous damping control, air spring ride height sensors, body roll control monitors and automatically deploying auxiliary suspensions. These next-generation suspension technologies are here, or to be soon, and can help extend maintenance intervals and vehicle lifecycles, improve tire wear, increase safety and handling and even help with driver recruiting and retention. So, which of these enhancements are right for your fleet?

Suspensions 101

At its most basic, “the suspension of a vehicle is crucial in maintaining contact with the road surface,” says Holger Bublies, director of chassis systems engineering for ZF, a global designer and manufacturer of driveline and chassis technologies. The suspension also protects the vehicle, its occupants and its cargo from irregularities on the road surface.

There are two major components used to achieve this: springs and shock absorbers. Springs allow the wheels and body of the vehicle to move independently from one another, and shock absorbers act as dampers to control the motion caused by the springs.

When the vehicle is placed under load, the suspension is forced to respond.

“When using a mechanical suspension, ride height will change between a loaded and unloaded condition,” says Bryan Redeker, powered vehicle systems product manager for commercial vehicle axle and suspension systems manufacturer SAF-Holland. “This change in ride height may alter the optimized suspension geometry and change the ride quality.”

On the other hand, Redeker says the use of an air-ride suspension allows for the air spring pressure to be changed to maintain optimal ride height, geometry and ride comfort both loaded and unloaded. Therefore, air ride suspensions provide more consistent performance between loaded and unloaded conditions.

Improving on air ride

While air ride suspensions are a vast improvement for heavy duty vehicles over traditional mechanical suspensions, there is still room for improvement. For instance, shock absorbers in current air suspension technology are typically manually adjusted, meaning they can only be set for one particular condition.

Air damping

“When suspensions are tuned, the dampers are set for fully loaded conditions,” says Christine Reagan, product manager for Firestone Industrial Products (FSIP), a manufacturer of air springs and related products for commercial trucks and trailers.

Reagan adds that in the commercial vehicle industry, trucks and trailers only operate fully loaded about 30 percent of the time. When a vehicle is operating below its fully loaded weight, it is being over-damped, causing excessive wear on tires and other components as well as a rougher ride for the driver and cargo. This can eventually lead to increased maintenance and potential premature replacement of components on the vehicle; plus driver fatigue and a risk of damaged cargo.

Air damping, in which a damper is built into the air spring, is a solution to this problem that also alleviates other concerns. According to Reagan, air damping is automatically proportional to the load, so when the vehicle is unloaded it is damped less, and loaded it is damped more. “It automatically adjusts to make sure that the tire stays in contact with the road and there’s less movement in the vehicle,” she says.

Jim Rushe, senior program manager of trailer vehicle systems for Hendrickson, explains that air damping “reduces the number of maintenance items by removing the shock absorbers and designing the suspension damping function into the air spring.” Hendrickson is a manufacturer of truck and trailer suspensions and heavy duty springs. Rushe adds that the reduction in maintenance requirements can also reduce a fleet’s operating costs.

Shock absorbers should be replaced every two years, Reagan says, noting that fleets don’t usually follow this recommendation, only replacing the important components when lubricant or fluid is noticed to be leaking from them. These fluids can contact other parts in the suspension, leading to the potential for additional required maintenance or repairs.

Removing traditional shock absorbers from the equation also correlates to weight savings, which is increasingly important in today’s vehicles, where optimal efficiency is key.

Taking air damping one step further, technologies such as ZF’s continuous damping control (CDC) “allow the suspension system to receive data from sensors and the CAN bus, adjusting the damping levels within milliseconds,” says ZF’s Bublies. This presents obvious advantages when it comes to ride quality and control, increasing driver comfort and safety.

Altogether, air damping leads to a better ride under any load with no manual adjustment necessary, less wear on tires and other components, less driver fatigue and risk of cargo damage, additional weight savings and reduced suspension maintenance.

Ride height sensors

Though air suspensions have decreased ride height issues relative to traditional mechanical suspensions, fleets may still run into problems in this area.

“Today, a lot of [suspensions] have a mechanical height control – pretty much a lever arm that regulates a valve, letting air in or out,” FSIP’s Reagan says. “There [are] a lot of issues that we see in the field that we hear from fleets … drivers could tamper with it, there’s external influences ... debris on the road [kicking up] can influence how the mechanical system works and how it lets air in or out. That’s using a lot of your compressor time, causing a lot of fuel deficiencies, using more fuel because you’re running your compressor.”

Improper ride height can be especially detrimental when it comes to driveline angle. If the ride height is too far out of specification, either due to improper setting or external sources, improper alignment of the driveline can lead to “durability issues down to the transmission and driveshaft - all things that ring up a lot of dollars and can get expensive,” Reagan says. Therefore, it is imperative that ride height be within specification at all times, making ride height sensors an important component for vehicle longevity.

While ride height sensors aren’t exactly the latest technology, FSIP has developed a sensor that is built into the air spring, eliminating an additional mechanical component and, therefore, another potential maintenance item. Reagan notes that the sensor “should last the life of the air spring.”

She also says this smart component has additional capabilities beyond measuring ride height. It can measure pressure and temperature, both of which are very useful from a maintenance standpoint, as well as acceleration. All of this data can be communicated to the driver if immediate action is required, and to the fleet via a telematics system for predictive maintenance purposes.

From pressure measurements, the load on each axle can be calculated, helping avoid CSA overload infractions. Pressure measurements can also be useful from a maintenance standpoint: if a large amount of pressure is being lost over time, there may be an issue with an air spring. The fleet can flag the vehicle for inspection next time it is in for service, replace faulty components and avoid a potential costly breakdown on the road.

Temperature measurements are especially useful when it comes to predictive maintenance. If the temperature of an axle or wheel hub begins to increase, it can indicate an issue with a seal or bearing that otherwise may go undetected until a catastrophic failure occurs. If the temperature is noted to rise significantly in a short period of time, it can be a signal to the driver that the vehicle needs to stop immediately in order to avoid a thermal event.

Acceleration, the final measurement Reagan mentions the sensor is capable of, can be useful to fleets from a driver and vehicle behavior standpoint.

“There’s a lot of benefit if you’re looking at accident control and maintenance: who was really at fault, what happened at [a certain] time,” Reagan says.

Fleets can also use the data to incentivize drivers with safe driving habits.

Body roll control

Another issue with air suspensions is the potential for body roll. If a single height control is used on the same axle or multiple axles, air is able to move from air springs that are under load to those that are not, says John Hinz, vice president of engineering for Reyco Granning, a company that designs and manufactures suspension solutions for the transportation industry. As loads increase, body roll can become an issue.

“In newer systems, multiple sensors can be utilized in conjunction with data collection to improve situations like excessive body roll,” Hinz says. “Most systems would have a ride valve on each side that would start out closed. With data from right and left sensors, the system might decide that because the ride height on the right is going down and on the left it is increasing over a predetermined time period, that the right valve would open to add air to compensate.”

If the time requirement was not met, no change would be made, saving air, he continues. The same process might be applied to reactive air suspensions to decrease air usage. Changes in ride height might be taken over a 14-second timeframe, for example, so that large movements can be filtered out. Changes would only be made if the average ride height had actually changed.

Automatically deploying auxiliary suspension

In order to increase load carrying capacity, some fleets choose to specify vehicles with auxiliary lift axles, such as a pusher axle (located in front of the tandem drive axle) or a tag axle (located behind the drive axle). While these are especially common on straight trucks, they can also be found on trailers.

Auxiliary axles have their own suspension, and work to support the vehicle’s main suspension and frame by distributing the load more evenly. Distributing the weight between more wheels also helps to protect road surfaces, highways and bridges, and can help fleets meet local or state axle weight laws.

“Common auxiliary suspensions today are deployed by the operator operating a switch,” says Michael Hof, vice president of new business development at Link Manufacturing, a company that develops and manufactures suspension systems for commercial vehicles and equipment. “It’s either up or down, it’s kind of binary, either on or off.”

While this system works for the most part, it is dependent on operator training, knowledge and use of best judgement. This leaves auxiliary axle operation open to human error.

“If it’s not deployed properly, you’re going to get an increase in tire wear, your [fuel] mileage will go down and you could tear up the auxiliary suspension,” Hof says.

As part of its Road Optimized Intelligence (ROI) program, Link has developed a smart technology that allows the auxiliary suspension to be deployed automatically when needed. Additionally, the technology allows the auxiliary suspension to be deployed partially dependent on need, rather than the current all-or-nothing approach.

“It’s load-dependent rather than operator-dependent,” Hof says. “As the load increases on the primary suspension, [the ROI technology is] going to optimally lower that auxiliary suspension to meet the load requirements.”

This new technology takes the guesswork out of using an auxiliary suspension. Hof adds that it can even raise the auxiliary unit when backing up, and incrementally increase or decrease the degree to which the unit is deployed dependent on load. This would be particularly useful for bulk carriers who partially unload a load across multiple stops.

Taking driver knowledge out of the equation can make a difference in a couple of ways. For one, it reduces stress on drivers, giving them one less item to worry about and focus on. For another, it allows fleets to hire from a wider array of candidates since knowledge, training and experience with auxiliary axles would no longer be a requirement. Hof notes that this could help fleets alleviate driver shortage issues by allowing hiring from a wider pool.

The future of heavy duty truck suspensions

One common trend noted almost unanimously among manufacturers for the future of vehicle suspensions is lightweighting. As fuel prices continue to fluctuate and emissions standards become more stringent, OEMs will continue to look for ways to reduce the weight of commercial vehicles as a way to improve fuel economy and allow fleets to haul more cargo.

“As more equipment is added to the truck or trailer to meet new regulations, the overall vehicle weight continues to be a prominent factor, which drives us toward increasingly lighter designs that do not sacrifice durability,” says Hendrickson’s Rushe. “Fleets are also looking for ways to reduce operating costs, which drives us to develop new ways to extend component life and reduce maintenance requirements.”

Eliminating typical maintenance components such as shock absorbers is a win-win in that it reduces both price and complexity, and therefore operating cost. Any time a manufacturer can simplify a component or system, it is typically beneficial to fleets.

Extended component life and maintenance cycles are also important when talking about another commonly discussed future trend: vehicle autonomy. While many agree that full autonomy is far in the future, manufacturers are beginning to prepare for it now.

“They’re saying within the next 10 years we’ll have autonomous vehicles, but it’s all about reliability, redundancies in system and communication of data,” says FSIP’s Reagan.

If future autonomous vehicles don’t have drivers onboard to monitor systems, vehicles will need longer component life with less maintenance, and they will have to monitor their own systems. That’s where self-diagnosing smart technologies such as FSIP’s ride height sensors become even more important.


Even without autonomous vehicles roaming the roads, these technologies are already important to fleets looking to reduce operating costs, increase efficiencies and improve maintenance practices. From humble mechanical beginnings, suspensions have progressed to the point of implementing smart, self-monitoring technologies. Every fleet can benefit from these new advancements in heavy duty vehicle suspension technology.

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