What a Hendrickson suspension does
A Hendrickson suspension is an air suspension system used on heavy trucks and trailers to support the vehicle's weight and absorb shock from the road. Instead of traditional steel springs, Hendrickson suspensions use pressurized air bags to cushion the ride and keep the load level. The system automatically adjusts air pressure as the truck's weight changes, which is why you'll see it on dump trucks, flatbeds, tankers, and other commercial vehicles that carry varying loads.
The suspension connects to the truck's frame through a series of linkages and control arms. Air is supplied by the vehicle's air brake system — the same compressor that powers your brakes also feeds the suspension. When everything works correctly, the truck rides smoothly and the load stays stable even on rough roads or when weight shifts during loading and unloading.
Hendrickson is one of the largest suspension manufacturers for heavy trucks in North America, so if you own or operate a commercial vehicle, there's a good chance you have their system. Understanding how it works helps you spot problems early and know what to expect when repairs are needed.
Key Takeaways
- Hendrickson suspensions use air bags instead of steel springs to support weight and absorb road shock on heavy trucks and trailers.
- The system draws pressurized air from your truck's main air brake compressor, so brake problems and suspension problems can be connected.
- Common failures include leaking air bags, broken control arms, and worn bushings — each one changes how the truck handles and can damage tires or the frame.
- A sagging suspension or uneven ride usually means an air bag is leaking and needs replacement within days, not weeks.
- Maintenance involves checking air pressure, inspecting linkages for cracks or corrosion, and replacing air bags and bushings on a schedule.
How the air bag system pressurizes and holds weight
The air bags in a Hendrickson suspension are rubber and fabric cylinders that inflate and deflate to support the truck's weight. When the truck is parked and the engine is off, the bags stay pressurized so the truck doesn't sag to the ground. When you start the engine and the air compressor runs, it tops up the pressure in the bags to the correct level for the current load.
A height control valve (also called a load-leveling valve) is the brain of the system. It senses how high or low the suspension is sitting and automatically adds or releases air to keep the truck level. If you load heavy cargo, the suspension drops slightly, the valve senses this, and it adds air to bring the truck back to the correct height. If you unload, the suspension rises, and the valve releases air to prevent the truck from sitting too high.
This automatic adjustment is why Hendrickson suspensions are popular on vehicles with unpredictable loads. A dump truck that's empty one hour and full the next doesn't require manual adjustment — the system handles it. However, this also means the system has more parts that can fail: the air bags themselves, the height control valve, air lines and fittings, and the compressor that supplies the air.
Signs that your Hendrickson suspension needs attention
The most obvious sign of suspension trouble is a sagging or uneven ride. If one side of the truck sits noticeably lower than the other, or if the entire suspension sags when the truck is parked, an air bag is likely leaking. You may also hear a hissing sound near the wheels, especially when the truck is parked and the compressor is running — this is air escaping from a crack or failed seal.
A rough or bouncy ride is another red flag. If the truck bounces excessively after hitting a bump, or if it feels unstable when turning, the suspension linkages or bushings may be worn. Worn bushings allow the suspension arms to move too freely, which throws off the truck's handling and can cause uneven tire wear.
Check your air pressure gauges regularly. Most Hendrickson suspensions have a gauge on the dash or near the driver's seat. If the pressure drops steadily over days or weeks, you have a slow leak. If it drops suddenly, you have a fast leak or a failed component. Either way, the truck should not be driven long distances until the problem is found and fixed.
Visible damage is also a reason to inspect the suspension. Cracked welds on the frame, bent control arms, or corroded air lines should be addressed before they cause a failure on the road. Rust on air lines is especially important — corrosion can weaken the line until it ruptures under pressure.
Air bag replacement and when it's necessary
Air bags are the most commonly replaced part of a Hendrickson suspension. They wear out because they're under constant pressure and flex thousands of times as the truck moves. The rubber and fabric eventually crack or separate, and air escapes. A leaking air bag cannot be repaired — it must be replaced.
Replacement involves lifting the truck, removing the old air bag from its mounting points, and installing a new one. The job usually takes one to two hours per bag, depending on how corroded the bolts are and how much space the mechanic has to work. You'll need to have the suspension re-leveled and the height control valve adjusted after installation to make sure the truck sits at the correct height.
Air bags typically last five to ten years depending on how much the truck is used and how well the suspension is maintained. Trucks that sit idle for long periods may have bags fail sooner because the rubber dries out. Trucks that are driven regularly but maintained properly often get closer to ten years. If you own the truck, ask your mechanic how old the bags are and plan for replacement before they fail completely.
Control arms, bushings, and linkage maintenance
The control arms and linkages that connect the air bags to the frame also wear out over time. Bushings — rubber or polyurethane sleeves that allow the arms to pivot — crack and harden, especially in cold climates. When bushings fail, the suspension becomes loose and the truck's ride deteriorates quickly.
Worn bushings and control arms are usually caught during a pre-trip inspection or when a mechanic is working on the suspension for another reason. The fix involves removing the old bushing or arm and installing a new one. This is more labor-intensive than an air bag replacement because the parts are often bolted in tight and may require cutting or pressing to remove.
Regular inspection of the linkages — looking for cracks, bending, or excessive movement — can catch problems before they cause damage to the frame or tires. A mechanic can grab the control arm by hand and check for play; if there's more than a quarter-inch of movement, the bushings or arm itself needs replacement.
Height control valve problems and diagnostics
The height control valve is a sensitive component that can fail in several ways. If the valve sticks in the open position, air continuously leaks from the bags and the truck sags. If it sticks closed, the bags over-pressurize and the truck sits too high. If the valve's sensing arm is bent or disconnected, the system can't read the truck's height and won't adjust properly.
Diagnosing a valve problem requires a mechanic to check the air pressure at different points in the system and observe whether the pressure changes when the truck is loaded or unloaded. A functioning valve will add air when the truck sags and release air when it rises. If the pressure stays the same regardless of load, the valve is likely faulty.
Height control valves can sometimes be rebuilt or cleaned if the problem is dirt or corrosion inside the valve. However, if the internal seals are damaged or the valve body is cracked, replacement is the only option. A new valve typically costs more than an air bag but less than a complete suspension overhaul.
Maintenance schedule and preventive care
Preventive maintenance on a Hendrickson suspension starts with regular visual inspections. Every month, walk around the truck and look for leaks, cracks in welds, corrosion on air lines, and signs of uneven wear on the tires. Check the air pressure gauges and note whether the pressure is stable or dropping over time.
Every six months or at your truck's scheduled service interval, have a mechanic inspect the suspension more thoroughly. This includes checking the height control valve function, testing the air bags for leaks, and examining the control arms and bushings for wear. Corrosion on air lines should be cleaned or the lines replaced if the corrosion is deep.
Air bags should be replaced before they fail completely — waiting until a bag ruptures can leave you stranded and may cause damage to the frame or other suspension parts. If your bags are five years old or older, discuss replacement timing with your mechanic. Bushings and control arms should be replaced when wear is detected, not when they fail.
Keep records of all suspension work. This helps you track how long parts last on your truck and plan for future maintenance. It also provides documentation if you sell the truck, as buyers want to know the suspension history.
Frequently Asked Questions
Can I drive the truck if the suspension is sagging?
You can drive it short distances to a repair shop, but not long distances. A sagging suspension means an air bag is leaking, and the truck will continue to sag as more air escapes. This changes the truck's handling, can cause the frame to contact the wheels, and puts stress on other suspension parts. Get it repaired within a day or two.
What's the difference between a Hendrickson suspension and a spring suspension?
Spring suspensions use steel leaf springs or coil springs to support weight, while Hendrickson uses air bags. Air suspensions ride smoother, adjust automatically to different loads, and are easier on cargo. Spring suspensions are simpler and cheaper but don't adjust to load changes and provide a rougher ride on rough roads.
How much does it cost to replace an air bag?
The cost varies by truck model and your location, but typically ranges from $400 to $800 per bag including labor. If you need multiple bags replaced at once, the total can be $1,500 to $3,000 or more. Ask your mechanic for a quote before authorizing work.
Why is my truck sitting higher on one side than the other?
This usually means the air bag on the lower side is leaking or has failed. It could also mean the height control valve on that side is stuck or the control arm is bent. A mechanic can diagnose the problem by checking air pressure and inspecting the components. Don't ignore this — it affects handling and tire wear.
Do I need to bleed air from the suspension before working on it?
Yes. Before any work on the suspension, the air must be released from the bags. This is done by opening a bleed valve or disconnecting an air line while the compressor is off. Never work on a pressurized air bag — the pressure can cause injury or damage. Your mechanic will handle this, but always make sure they know the suspension is air-operated.