Shaken and Stirred

The importance of maintaining your suspension system

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."

This content continues onto the next page...

We Recommend