Shaken and Stirred

You think you've got a tough workload? Try doing what a heavy-duty tractor suspension does from day to day. A tractor suspension carries, supports, and protects the load being carried as well as the load of the vehicle itself, which includes the cab and the driver. It controls vehicle stability and protects from rollover. It is important in braking and acceleration as the braking force goes from the brakes to the axles.


Whether you are working on brand-new equipment or maintaining older suspensions, both new and old require a multi-point inspection to be sure everything operates as it should.

Bill Nash, manager for technical services with Hendrickson Truck Suspension Systems in Woodridge, IL, says, "If you look at just one part without looking at the whole system, you can create a suspension that excels in one area but is missing in others." Nash spent a number of years in drivetrains and notes that he likes to see a suspension that maintains good driveline alignment. "The driveshaft has to be properly aligned, otherwise you can damage the drive line, U-joints, transmissions, and more," he says.

A checklist of items on a visual inspection appears in the back of the Hendrickson Technical Procedure maintenance manual. Nash offers that they supply inspection items and interval information to the OEM. Hendrickson recommends that suspensions be inspected at 25,000 miles or at six months, adding that some suspensions require inspections more often and some less. It is always good to first check the owners or maintenance manual.

Floyd Frick, manager of service operations with Freightliner LLC in Portland, OR explains that suspension inspections include general condition, damage, indications of wear, and bushing play. Inspection intervals should follow specified vehicle manufacturer's maintenance schedules.

Maintaining proper U-bolt torque is a primary concern as loose U-bolts (low torque values) can lead to spring failure through the center bolt. Frick says there is a suspension U-bolt torque check for initial maintenance. For Severe Service, this is the first 1,000 miles; for Short Haul, it's the first 10,000 miles; and for Long Haul, the first 25,000 miles. Then, as an M2 operation, it's every 5,000, 50,000 and 100,000 miles respectively.

Frick recommends paying close attention to other suspension bolts. Technicians should check the mounting bolts, control rod bolts, and crossmember bolts.

If unchecked, loose mounting bolts can lead to crossmember and frame rail cracks. Loose U-bolts can lead to axle misalignment or shifting and air bag leaning, and loose control rod bolts could lead to broken control rod bolts, resulting in suspension, driveline, tire, and wheel damage.


According to Frick, front air suspensions are growing in popularity and while the airing system is more complex, the ride benefits outweigh potential service requirements. With the AirTek front suspension, the ride height setting is also important to ensure ride quality. The rear suspension ride height setting—particularly on the torque reactive air-ride suspensions—should be set carefully. An incorrect suspension height often leads to driveline vibration and a rougher ride

"If you've got air springs, you've got air lines going to it to be inspected," says Nash. "Know how the air system functions.

Check ride height and adjust ride height according to the owners or maintenance manual. When riding on air—too low you could damage the bag and have a harsh ride; too high, the same thing—the drive line needs to be aligned correctly to limit drive line vibrations.

"Shocks are part of the suspension and are a maintenance issue," says Nash. "People don't really know how to diagnose shock problems properly. If the vehicle is loaded and comes back from a trip, the shocks should be warm to the touch. If they are cold, they are not working. However long it takes the vehicle to warm up, that's how long you wait to check the shocks' temperature."

He adds that there is a multitude of shocks in today's vehicles. The air-ride seat may have a shock on it, the cab is an air-ride cab, and there is air-ride suspension.


OEMs can affect the suspension by stretching the frame or putting a dump body or tank on it.

"They'll put different tires on the steer axle and you have to compensate for moving the load from the steer axle to the tandem," says Nash. "Many times, they don't realize what they are doing when they add things to the original design."

Torque rods are wear items that are very important to the way the suspension works and are often overlooked.

"Technicians need to look at the condition because many times, techs will replace other parts and it may have been because of a torque rod problem and not anything else," says Nash.

"It's so important for the technician to get familiar with the suspensions before laying hands on them," says Roger Elkins, product manager for truck, bus and motor home suspensions with The Holland Group in Holland, MI. "Go to the manufacturer's website to tap into the latest version of the maintenance manual. Technicians also need to spend some quality time with that information, and this is especially important for new technicians. When working on suspensions, they need to look at the system as a whole."

Open communication between technicians and drivers goes a long way for preventative maintenance as well as in troubleshooting current problems. If the driver doesn't start the dialogue, it's up to the technician.


Elkins offers that an alleged bushings problem may not be the root cause.

"Track bars keep drive axles centered underneath chassis and aid in steering," he explains. "If it's a rubber-bushed track bar, the rubber bushings wear out and allow the axle to ‘walk out' from its position and this puts more wear on the suspension."

He says that technicians would probably just replace the bushings. "What should happen is that they should replace the bushings and the track bar. Suspensions are affected by other systems so technicians need to take a systems approach as the root problem may not obvious. My suggestion is that if technicians bump into a situation where they can't determine a root cause, call the OEM tech lines to help with troubleshooting."

Using genuine parts, not only for fasteners and bushings, but for any replacement parts needed is important to Elkins. "They don't call them genuine for nothing," he says. Elkins advises that keeping to original specifications will keep performance of the suspension consistent. Warranties also can be affected if you do not use genuine parts.


"Techs aren't getting enough training," says Nash. "They don't have enough on suspensions as well as enough training on the myriad of components on the entire vehicle. It's not that they don't want training -- they're overwhelmed with work and it is difficult to be out of the shop. Plus, if you don't work on a product soon after the training, you don't retain the information."

He continues, "We need to rethink and change how we deliver training and information. We need to move to a Just-In-Time training opportunity. Techs don't need hours or days of in-depth training because they probably won't ever use all of the information."

Nash feels the biggest training deficit is on the part of the component supplier. "We're not supplying the information the way the tech needs it," he says. "Some component suppliers are working to change that. The big thing that will help is getting the computer into the shop. Online access right to the service bay will go a long way to improve troubleshooting and maintenance."

"Nobody gets enough training because there is so much equipment to know about," says Elkins. He says The Holland Group has teamed up with Kenworth to develop a training module and workbook for dealer technicians that includes technical drawings, reference charts, and details specific to Holland Group products.


Freightliner LLC began offering air front suspensions a few years ago says Frick, who explains that technically, these are a combination of steel spring and air and now represent a large portion of Freightliner LLC's line haul build rate. "Although current suspensions are a combination of spring and air, we expect that these will transition to full air suspensions," he says.

Freightliner LLC's most recent introduction is in axle suspensions—a monoleaf style spring front suspension, which is expanded across the Freightliner Trucks and Sterling product lines. The monoleaf suspension is designed to reduce friction in the spring to improve the ride and reduce weight by having fewer leaves than those found in multileaf suspensions. This technology is now standard in the 8,000-, 10,000-, and 12,000-lb. front suspensions in the highway truck category, where the ride and weight improvements are best realized. According to Frick, Freightliner LLC is the first to offer this system in a front suspension in North America.

Over at Hendrickson, Nash says his drivetrain background gets him excited over the new HTB, which is a heavy-duty, on-highway air suspension that is non-torque reactive like it's off-highway cousin, the PRIMAAX. The big benefit with the HTB is it is very light in weight due to aluminum used in construction. The HTB also has a torque box, which takes the place of torque rods, but it works the same and requires less maintenance according to Nash.

Frick explains that while technology exists for front and rear active suspensions, his company does not see the payback for the customer with these products in the near future. Citing initial cost along with potential complexity, they feel that it will be a number of years before these suspensions are prevalent. Similarly, independent front suspensions, while gaining popularity in motor home and other rear-engine applications, Freightliner expects that it will be at least another generation of highway trucks before independent front becomes a factor in a forward engine chassis.

"Space requirements and structural requirements on a forward engine chassis make application of an independent front extremely challenging," says Frick. "The day will come, but not in the near future."