Both the suspensions and tires of heavy duty commercial vehicles have a uniquely correlative dynamic. As the maintenance of the suspension impacts tire performance, so too does the maintenance of the tire affect the suspension’s performance. Such a correlation is a sweet spot for a fleet’s maintenance department, as an intimate understanding of one system can lend to diagnosing problems with the other; conversely, inadequate upkeep of one system will directly impact the wear and tear of the other.
Fleets can utilize simple monitoring, measuring, and maintenance practices of both the suspension and the tire in order to maximize the life of these systems and ensure better, longer performance.
Role call: Suspension
The suspension carries out many different roles required for proper vehicle performance.
“One of its primary functions is to control the position of the axle to make sure that things are in the right spot while the vehicle is moving, [and] when it is not moving,” says Mark Molitor, product manager – heavy vocational, Link Manufacturing.
Beyond positioning, shock absorption is another major duty required of the suspension.
“The vehicle is going up and down and the road surfaces are changing rapidly,” says Homer Hogg, director technical service, truck service, TravelCenters of America. “The suspension absorbs that impact and those vibrations to reduce both the frequency and harmonics up into the vehicle itself. Whether that be the cab, whether that be the driver, whether that be the freight, but it is to protect all of those from the impact that you would feel in that vehicle if it did not have a suspension engineered properly, and then maintained properly.”
The suspension system is also responsible for carrying the load, providing stability, transferring braking forces throughout the vehicle, and maintaining continuous contact to the driving surface through the tire, explains Tyler Bernens, senior product manager, suspension, Meritor.
“One that people don’t necessarily think of as often is that it transfers the braking and the driving force,” Bernens says about a commercial vehicle suspension. “For instance, the brakes apply on the wheel end, that force has to then get transferred out through the vehicle and it goes through the suspension… [Another] thing that the suspension does is it maintains tire contact with the road.”
Furthermore, the suspension maintains tire alignment and orientation.
“While the vehicle suspension has to have many capabilities that are required for safe, reliable field service, one of them is to maintain proper tire alignment and orientation to facilitate optimal tire life and performance,” says Bill Hicks, SAF product manager, SAF-Holland. “In addition, drive axles require optimized road contact for traction and steer axles must provide the vehicle’s directional change needs without sacrificing tire life. All suspensions are designed with important considerations to maintain factory settings for alignment, caster, camber, toe in/out, thrust angle, et cetera.”
Suspensions come in all shapes and sizes. This is not by chance, as the suspension serves a multitude of purposes and roles which vary not only between the tractor and the trailer, but also vocation and application.
“Ride quality is a consideration for both truck and trailer applications,” says Melanie Elliott, marketing manager, Hendrickson. “Truck suspensions often focus on driver comfort and reduced driver fatigue while also prolonging the life of the tractor. Trailer suspensions are primarily focused on cargo and trailer protection with some consideration towards driver comfort.”
“Commercial trucks can require a wide range of suspension parameters depending on the vocation (refuse, dump, concrete, et cetera),” adds Don Hester, senior program manager, Hendrickson. “Tractors designed to be part of an 80,000-pound combination vehicle also have unique suspension requirements. The intended application typically dictates the suspension properties such as ride height, capacity, and travel. One unique difference between tractor and trailer suspensions as it relates to tire performance is that the tractor suspension is attached to a driven axle whereas the trailer suspensions are attached to axles with free rolling tires. This distinction is often overlooked when discussing tire performance and diagnostics.”
While the truck and the trailer suspensions perform similar functions, the way they go about it is different, says Bernens. “The tractor suspension, because of where it is [on the vehicle] and the forces it is working with are greater, it has to perform more work. The tractor suspension is hooked up to the drive axle. That drive axle is taking all the same forces that a trailer suspension is because they are going over the same road, they are dealing with the same swaying, but the tractor suspension is also dealing with the torque that is happening on the drive axle. Whereas the trailer suspension is really just focused on the trailer and carrying the load that the trailer is rated for,” Bernens continues.
This difference in operational requirements is why trailer suspensions are lighter weight, and don’t typically require as much maintenance.
“You will notice if you were to look at the two, that the tractor suspensions are larger, they tend to be more robust,” adds Bernens.
Tractor alignment
Alignment procedures are essential when it comes to suspension and tire maintenance. The impact of the correlative nature between a suspension and the tires is most notably depicted through the vehicle’s alignment and its subsequent performance under its current alignment conditions.
“If your vehicle is out of alignment, the one thing that suffers the most is your tires,” Link’s Molitor states. “They are going to scrub on the edges throughout every day, all day long, every mile.”
Properly aligned vehicles will mitigate irregular tire wear. It is also important to note, proper alignment of both the tractor and the trailer is imperative.
“With the alignment, you are making sure all the axles are basically the same in relation to each other,” Meritor’s Bernens says. “Even though for a tractor-trailer, yes, they are independent vehicles, but misalignment between one vehicle can affect tire wear on the other. Say you have a trailer that is out of alignment. It is going to be pulling that tractor, constantly, in one direction, and the driver is going to be compensating and that is going to cause additional tire wear on the vehicle.”
Furthermore, proactive alignment versus reactive alignment will better serve a fleet’s return on investment with ever-expensive tire costs.
“People generally check alignment when they notice a tire is wearing,” Bernens says. “You have already incurred that cost and tires are one of the most expensive things that fleets spend money on. A really good fleet will check alignment during their one-year preventive maintenance check on a vehicle. Not all of them do, and they will do it after they noticed the tire is wearing, but it is already too late at that point. You want to try and prevent the tire wear. Getting a regular interval for checking alignment is definitely a best practice for keeping your costs down, as far as tire wear is concerned.”
When performing an alignment, it is always recommended to keep the vehicle aligned to the OE’s and manufacturers’ specifications. Hendrickson’s Elliott advises utilizing alignment instructions, either from suspension manufacturers or from industry sources such as the American Trucking Associations’ Technology & Maintenance Council (TMC), is a key to success. Following recommended guidelines will ensure the systems impacted through alignment will perform as engineered and designed. The frequency and extensiveness of these alignment procedures are contingent on duty cycle, application, vocation, and area of operation.
The benefits of maintaining vehicle alignment are numerous, including cost savings across multiple channels and extending the lifecycles of critical components. But, it is critical fleets conduct these services properly because the negative impacts of a misaligned vehicle are costly.
“Properly aligned trailer axles optimize fuel economy and drivability and help prevent excessive tire wear,” Hendrickson’s Elliott says. “An improperly aligned vehicle may lead to premature wear; the tires may wear unevenly, which can reduce the usable tire life and prompt early replacement. Trailer off tracking or ‘dog tracking’ also requires extra effort from the driver to keep the trailer within its lane.”
Fleets may argue proactively conducting comprehensive alignment procedures may not be reasonable given time restraints. TravelCenters of America’s Hogg advises alignment equipment manufacturers have begun to support an abridged alignment procedure, which he refers to as an alignment “quick check” or “quick alignment." Equipment manufacturers have introduced products to assist with these services, he adds, noting conducting a “quick alignment” of only the steer axle during a preventive maintenance (PM) inspection can still be beneficial.
“I recommend that an alignment check becomes part of your preventive maintenance,” Hogg says. “Whether you are going to do that at every PM cycle or you are going to [perform an alignment] once or twice a year is somewhat contingent on where and how that vehicle is operated.”
Trailer alignment
Unique from tractor alignment, trailer alignment procedures serve to maintain trailer integrity and performance, as well as ensure collaborative operation with an equally aligned tractor.
“In the case of a trailer, a suspension axle alignment is ensuring the tires are tracking true to the trailer,” says Tony Ryan, technical services and training manager, SAF-Holland. “This is usually done by aligning the front axle to the trailer kingpin and then subsequent axles to the properly realigned front axle.”
Trailer alignment takes multiple angles into consideration when establishing optimal geometry.
“The goal of a trailer alignment is to position all the trailer axles parallel to one another and perpendicular to the centerline of the trailer,” Hendrickson’s Hester says. “There are two important trailer axle angles that are considered during an alignment procedure: thrust and scrub. When thrust and scrub angles are not maintained within an acceptable range, increased rolling resistance (fuel consumption), excessive tire wear, and visible ‘dog tracking’ can result.”
Hester says that a proper trailer alignment begins with proper preparation.
“The procedure should be performed on a flat, level, and debris-free surface. The tires and wheels should be inspected to ensure they match, being of the same size and set to the same inflation pressure as prescribed by the tire manufacturer.
“Additionally, ensuring the suspension is set at its designed ride height is also critical for proper alignment,” he continues. “The trailer should be positioned in a ‘relaxed’ state without any preload applied to the pivot bushing and with kingpin set at designed height. It is also important to ensure the trailer parking brakes are disengaged during the alignment process to allow wheel rotation that occurs during alignment.”
Hendrickson’s technical document, L579, details the entire preparation procedure. After preparation, the alignment begins with the front, non-steer axle, Hester says.
“Measurements are taken between the kingpin and both ends of the front axle,” he continues. “These measurements are compared with the goal that each measurement is equal to each other, or at least within an acceptable range as indicated by the suspension manufacturer. If needed, adjustments can be made by positioning either side forward or rearward by the means provided by the suspension. The Hendrickson QUIK-ALIGN pivot connection allows for quick and easy suspension alignment without welding or use of section tools.
“Once the front axle is at an acceptable position, the subsequent axles can be adjusted to be parallel to the front axle to minimize scrub,” Hester advises. “Once all the axles are at an acceptable position, the pivot connection should be tightened per the suspension manufacturer’s recommendations.”
Within the SAF-Holland trailer air suspension, the company’s SwingAlign feature provides quick and easy axle alignment, Ryan says. With the SwingAlign, loosening of the pivot bolt connection isn’t required, as the technician can adjust the axle by turning a bolt head on the front side of the suspension pivot connection which in turn changes the alignment to the king pin by pulling the roadside wheel end closer or pushing it further away from the kingpin.
“Most modern alignment machines are also able to check and record tire camber, toe, and scrub angles,” Ryan says. “These are all issues that should be checked, and if not corrected, can cause irregular tire wear.”
Suspension inspection
When it comes to suspension maintenance, regular inspection of critical parts and components goes a long way in assuring that performance is upheld and that the fleet stays ahead of impending wear, damage, or failure. First, inspect the bushings.
“The benefit of the suspension and the design of the tractor and trailer is a lot of them are really easy to inspect by just walking around the vehicle,” says Meritor’s Bernens, on bushing inspections. “You can inspect the majority of the bushings on your suspension without having to actually crawl underneath the vehicle… And rubber bushings are pretty easy to tell when they’re starting to go. There is either too much play or they are cracking, becoming deformed. Visually, there is something wrong. It is not like you are setting up a dial indicator to measure thousandths of an inch to tell if something is right or wrong.”
A visual inspection provides a status report on the bushings’ stage of life and probable capability.
“Bushings wear more over the years,” says Link’s Molitor. “If designed properly, five to seven years on a bushing is capable.”
The bushings serve a crucial role in dampening and transferring energy, as well as providing movement compliance within the suspension system.
“You want to have compliance within the suspension system,” Molitor continues. “It can't be all metal-on-metal, otherwise things will bend and break. But, if that compliance grows as the bushings start to wear out, it is going to allow a lot more movement than the design was intended to. If you have a poor fleet practice of maintenance and these bushings are essentially disintegrated and that rubber is now allowing a half-inch of motion or three quarters of an inch of motion, that is going to start allowing the tires and the axle alignment to start being too far off. It will create wear and misalignments.”
If a worn bushing is not replaced in time, more than just the bushing itself is in danger of needing replacement.
“What ends up happening is either [a worn bushing] causes this suspension to go out of alignment, which causes tire wear, or you actually damage that [suspension] component,” Bernens advises. “Then, you are paying to replace a much more expensive component. I would say bushings are probably one of the number one things that are the easiest to inspect and can have some of the larger downstream costs if they are not replaced. They are wear items, they are designed to be replaced.”
Another important, but simple, inspection procedure on the suspension system focuses on the shocks.
“You’re certainly going to look at the shocks fairly quickly because the shocks can have a significant impact on that suspension, on that wheel end, [and] the tire wear on that particular position,” TravelCenters of America’s Hogg says. “And they are fairly easy to check.”
How simple can a shock inspection be? Perform a temperature reading, either by using a thermal imager or even just by touch.
“Are they hot?” Bernans poses. “If they are hot, they are working. If they are not hot, they are not working.”
The shocks serve to dampen the motion of the vehicle.
“You want the suspension to be moving around a lot so that the chassis isn’t moving around a lot,” Molitor says. “The ideal world is that the frame rails of the trailer and tractor are not moving at all… That does not happen when the axles are jumping around like crazy. Well, if your shocks are not working properly, they are not dampening that motion.” Molitor explains that poor shock performance increases the loading and unloading frequencies demanded of the tires, “Which is not ideal for tire wear, or for the [vehicle] geometry for that matter.”
To ensure optimal performance of suspension system components, technicians should refer to the manufacturer’s recommended inspection points and intervals, such as those detailed in Hendrickson’s L578 preventive maintenance guide, suggests Hester.
Hester explains that inspection frequency depends on the component’s exposure to a variety of forces.
“Ride components that play a role in the ‘feel’ of the trailer and interact between the tires and the ground are primarily the air spring, shock absorber, and pivot bushing,” he says. “Hendrickson has continued its efforts to maximize performance of these critical components through the development of products like ZMD Zero Maintenance Damping technology. The ZMD system eliminates the need for an external damping component such as a shock absorber. Instead, the suspension damping function has been incorporated into the suspension air springs. This eliminates the need to inspect and maintain the shock absorber while also maintaining… ride quality and performance.”
Furthermore, fasteners throughout the suspension system must be inspected and maintained in strict adherence to recommended intervals and guidelines from manufacturers to maximize the lifecycle of the system as a whole.
Link’s Molitor advises that fasteners, either installed or serviced without the proper torque, will come loose. A loose fastener can wreak havoc to the structural integrity of the suspension and pose reliability issues.
“Once you start having a fastener moving around, for pivot connections and such, it is going to go through the path of least resistance,” he says. “Say the pivot connection on a frame bracket is loose. As you accelerate, that axle is going to go back as far as that fastener clearance will allow it. Once it goes back there, it might actually start moving the metal because there is so much force there. The break cycle is cyclical, [and it is then] slammed forward. It is going to increase that motion that is allowed every shift cycle. If it was a hole to start, it will turn into a slot.”
Molitor continues, saying that holes will be deformed into oblong shapes, so gross that drivers will notice the movement. If such an instance is not corrected early enough, instead of replacing a fastener, frame brackets or control arms will need to be replaced as the metal has been worn out.
Technicians should look for worn paint or rust underneath the fasteners during inspection, Molitor advises, which is indicative of a loose fastener allowing movement. He further recommends steam cleaning the chassis, removing dirt and grime to allow for a clear visual inspection of all suspension components. After a steam clean, rust or paint removal should be easy to identify, alerting technicians that the connection is not tight.
Specific to air ride suspensions, another area pertinent to the inspection procedure is the air system and the quality of air reaching components in that suspension.
“If you maintain your air system, it can help extend the life of [air ride] suspension components like leveling valves, air bags, and any other component that has to route that air and control that air,” TravelCenters of America’s Hogg says. “If your air compressor is passing oil and your air dryer is not properly removing that all, and it is getting into your air system… that is what we find when we find failures. Most of the time when we pull an air valve off prematurely, we look at the line going into the leveling valve and we find contamination. That contamination has to be detected before it gets there.”
Hogg recommends drivers be more adequately trained so that when draining an air tank, they can perform a diagnostic. If excessive oil is vacated along with moisture when draining the tank, the vehicle should be serviced to determine the cause, advises Hogg. “It could be something as simple as your air filter is getting too dirty and it is affecting systems down the line. I wonder how many people connect those dots… When you get a contaminated air leveling valve and that valve is not maintaining the right frame height, you could be beating up a transmission. How bad would it be to have to replace a transmission when you really had a bad air filter?”
Tire maintenance and monitoring
It is no secret that tires are one of the most commanding expenses of any fleet operation. Having an understanding that suspension maintenance will play into tire performance, fleets should diligently monitor and maintain the suspension. As for maintaining the tires, first and foremost, fleets should monitor tire pressure.
“Along with alignment, make sure you are maintaining your tire pressure,” Bernens advises. “I have been in fleets where the driver just kicks the tire and goes, ‘Yep, it's good!’ I don’t know about you, but I can’t tell the difference between 90 psi and 100 psi with my foot – but your tires can tell the difference and they will wear differently.”
Tire inflation is a key aspect that affects both tread wear and suspension performance. If a tire is overinflated, it will cause accelerated wear in the center of the tire face, Link’s Molitor says. “Your center tread is going to be shallower than the outside. And if [a tire] is underinflated, then you will get cupping.”
“Proper alignment and tire inflation pressure are typically at the top of the list of factors that influence tire wear,” Hester says. “Solutions like Hendrickson’s TIREMAAX tire pressure control system help with maintaining these critical performance factors. TIREMAAX PRO is an advanced automatic tire pressure control system that is capable of actively inflating, relieving, and equalizing tire pressures across all wheel positions.”
Link offers a convenient tool called the Cat’s Eye which facilitates a visual diagnostic applicable for dual tires placed in tandem. “The Cat’s Eye connects the two and then it gives a very good visual indication to the driver if the pressure is too low,” Molitor explains. “It helps to make sure that the inner and outer tire are always at the same pressure… If you have an inside and outside tire that are different pressures, they are different diameters. And if they are different diameters, you will create tons of wear on the one that is smaller in diameter because it is going to be scrubbing.”
Tire maintenance not only impacts tire performance, but also affects suspension performance.
“Let’s say that you have drastically different tire pressures on one side of the vehicle than the other,” Bernens suggests. “You are going to have a pivot bushing then doing more work on one side than the other, and so it is going to wear faster. If your tire pressure is off enough, you can cause additional work on your suspension, which is going to lead to you replacing more components sooner than you want.”
Diligent monitoring and recording of tire wear can reveal hidden suspension issues.
“Whether it is diagonal wear across the face, scalloping, feathering, or any other tire wear symptoms, everything is cause and effect,” SAF-Holland’s Ryan advises. “Root causes of wear can be an assortment of things like worn bushings, broken shocks, loose bolt connections, overloaded axles, or misaligned axles. Identifying the root cause is the game changer that allows technicians to concisely find and fix the underlying issue. I would highly encourage fleets to give technicians access to TMC’s Recommended Practice on tire wear – RP 219. No matter the wear patterns identified, it is covered with probable causes and solutions to your issue.”
Tire shoulder wear is one key indicator of an alignment issue.
“If you see the inside or the outside groove is not as deep anymore, that is usually an alignment issue,” Molitor says. “The alignment usually shows up on one face or the other, either the outboard of the tire is wearing or the inboard of the tire is wearing too much. And then, unfortunately, you get some scalloping in the tires as well. And that is usually an indication that your tires are hopping too much.”
Uneven cross-axle wear should not be dismissed, but rather investigated.
“You could have tires on one side of the vehicle that are wearing rapidly compared to other tires,” Hogg notes. “Everybody does not always attribute that to the suspension, but if you have a weak suspension component on one side of one axle, then more weight gets shifted to that wheel position. And if more weight gets shifted to that wheel position, then the tire has to bear the brunt of that and you will see more scrub on that tire.”
In the end, establishing a practice of recording and comparing tread depth and wear patterns can create an atmosphere that can serve to diagnose cause, rather than chasing effects.
“If you are really monitoring your tread depth and you are paying attention to that and relating or correlating those tread depths to one another, you might just get a good sign, or a good indicator, that you may have a suspension problem just by looking at tire wear, [especially] how rapidly it wears on one position versus others,” Hogg continues.
It is through this performative correlation between the suspension and the tire that diligent maintenance wins out. By adhering to best practices, inspections, regular PMs, and strict monitoring and recording, fleets can not only maintain both their suspensions and tires but also use one to maximize the other.
“Suspension systems are critical to the stability of a vehicle and to maximizing many of the components on that vehicle,” Hogg concludes. “I think that the industry would be wise to spend a little extra time evaluating the condition of their suspension system during their preventive maintenance program.”
