You hear a knock under the rear of the truck on rough roads, but the shock absorbers look fine, the U-bolts are tight and the leaf spring is not cracked. The culprit is often one level higher: the spring hanger or the equalizer beam that the entire tandem suspension hangs from.
At Ressorts Laval, spring hanger and equalizer beam inspection is part of every full rear suspension check. These components rarely get attention until something breaks, and when they do fail, they take other parts with them.
How tandem rear suspension is structured
On a heavy truck with tandem rear axles, the two axles do not each have independent suspension mounts. Instead, they share a walking beam or equalizer beam system that allows both axles to move together while keeping the load balanced between them.
The key components:
- Spring hangers: the steel brackets welded or bolted to the truck frame that anchor the ends of the leaf springs. The front hanger is fixed; the rear hanger often incorporates the equalizer pivot.
- Equalizer beam (walking beam): a rocker arm mounted in the center of the tandem suspension that connects the rear ends of both axle spring packs. When one axle goes over a bump, the beam rocks and transmits the load partially to the other axle.
- Equalizer pivot bushings: the bushings in the center pivot of the equalizer beam. These wear out and cause the most common clunking symptoms.
- Torque rods (radius rods): on some suspension types, torque rods control axle position and transmit braking and acceleration forces to the frame. Their bushings wear independently of the equalizer system.
Signs of spring hanger and equalizer beam problems
Knocking from the rear tandem on rough roads
The most common symptom of worn equalizer pivot bushings. The equalizer beam rocks as normal but the worn bushing allows metal-to-metal contact at the pivot. You hear a dull knock each time the beam rocks, both on compression and rebound.
One rear axle sitting higher or lower than the other
If the equalizer beam is seized at its pivot (instead of being able to rock freely), one axle will carry more of the load than the other. You may notice this as a height difference between the two tandem axles when the truck is loaded and parked on a flat surface.
Cracked or bent spring hanger
Spring hangers are welded to the frame and take the full vertical load of the rear suspension. A cracked hanger can be visible as a fracture in the metal, sometimes accompanied by rust streaking. A bent hanger (from an overload event or a severe impact) changes the spring geometry and causes alignment issues that no alignment adjustment can correct.
Excessive rear-end movement in corners
A loose equalizer beam or failed pivot bushings allow more axle movement than the suspension was designed for. In corners, particularly in sweeping on-ramp turns at speed, the rear end feels looser and less planted than normal.
⚠️ Warning
A cracked spring hanger is a structural failure, not just a wear item. The hanger is part of what holds the rear axles under the truck. If a hanger cracks through completely at speed, the axle can shift unpredictably. Inspect immediately if you suspect one.
What we check during a tandem suspension inspection
- Visual inspection of all four spring hangers for cracks, bends and weld integrity
- Play check at the equalizer beam pivot: we move the beam by hand to detect loose bushings
- Visual check of the equalizer beam for cracks, especially at the pivot bore and at the spring seat contact zones
- Torque rod bushing play check (if applicable to the suspension type)
- U-bolt torque check on both tandem axles
- Axle height comparison between the two tandem axles under load
💡 Equalizer pivot bushings are often the first component to wear on a high-mileage tandem. Replacing them when they start to clunk, rather than waiting, avoids wearing out the equalizer beam center bore, which is a much more expensive repair.
What does tandem suspension repair cost?
| Repair | Price range |
|---|---|
| Equalizer pivot bushing replacement | $300 to $700 |
| Equalizer beam replacement | $600 to $1,400 |
| Spring hanger repair / replacement | $400 to $1,200 depending on severity |
| Torque rod bushing replacement (pair) | $300 to $600 |
| Full tandem suspension inspection | included with diagnosis |
Frequently asked questions
Can I weld a cracked spring hanger myself?
We strongly advise against it. Spring hangers are structural components under constant dynamic load. A field weld that is not properly prepared, preheated and stress-relieved is likely to crack again, often at the weld toe. Bring it to a shop with the right equipment.
How often should equalizer pivot bushings be replaced?
On a high-mileage tandem truck, inspect at every major service from 300,000 km onward. Replacement is typically needed between 400,000 and 700,000 km, depending on load and road conditions.
One tandem axle is sitting lower than the other when loaded. What does that mean?
It most often means the equalizer beam is seized at its pivot and is no longer distributing load evenly between the two axles. That axle is being overloaded every trip. Get it inspected before it affects the springs, bearings or tires on that axle.
Does suspension type affect which equalizer system is used?
Yes. Hendrickson, Neway, Watson and Chalin each have their own equalizer designs and pivot bushing specifications. We have parts for all the common systems used on trucks in Quebec.
Knock under the rear tandem? Let us find it.
Spring hangers, equalizer beams, pivot bushings: we inspect the complete tandem suspension. Ressorts Laval, Laval, since 1971.
Call us at 450-661-5157.