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The Technician’s Log: Trailer Suspension Damping Best Practices

Welcome to the latest installment in our ongoing series, The Technicians Log. Each time we dive into the technician’s log, we examine a topic that diesel technicians likely have on their mind. These are trucking shop technicians’ definitive guides. So, what’s today’s topic? It’s a wonky one: Trailer suspension damping.

Let’s start with what suspension damping is, then we can dive into the ride benefits, maintenance challenges, and costs. We aim to educate you on the evolution of damping methods so that you can better manage the technique as you work on your fleet vehicles and trailers.

A Primer on Suspension Damping

Vehicle OEMs, from passenger cars to big rigs, design their suspension systems with damping in mind. What is the primary goal of a suspension system? Well, to carry the vehicle load while providing compliance between the chassis or trailer body and the suspension, tires, wheels, and brakes. OEMs use damping devices (such as shock absorbers) to create a smoother ride and add a greater level of control through the damper’s motion resistance.

Damping itself specifically refers to the process of absorbing the energy of bumps and other road inputs that make their way through the suspension system. Damping reduces the severity of bumps and other road perturbations, which provides more comfort to the truck driver and extends the life of other vehicle components.

In short suspension damping systems are designed to do the following:

  • Decrease the frequency and intensity of road vibrations.
  • Reduce the amount of energy transmitted from road inputs to the cargo and cab.
  • Ensure the tires maintain consistent contact pressure with the road.
  • Enable a comfortable and smooth ride.

Suspension Damping Degradation

Problems arise over the suspension system’s lifetime. Why? Because most suspension damping devices degrade over time, which causes a decrease in either comfort or maintenance – or both. It can be a tough balancing act for OEMs and technicians to ensure truck driver comfort while also meeting protection, safety, and vehicle compliance measures.

Excessive road vibrations also cause grater wear and tear on vehicle components, from door hinges to seals, light fixtures, trailer fasteners, and more. Ensuring consistent and reliable suspension damping extends the life of your vehicles, creates greater truck driver satisfaction, and protects your cargo, especially when you haul fragile loads.

The key is to understand the fundamentals of a suspension damping system. Provided your technicians understand the evolution and wear and tear of a damping system, they can better manage long-term wear, maintenance, and operation costs. And they can do all this without compromising truck driver safety, comfort, or satisfaction.

Multi-Leaf Suspension

There are several types of suspension damping systems, but the oldest and most widely known are multi-leaf springs. If you go back to the very first big rigs, you will find multi-leaf springs. In fact, this method can be dated all the way back to the Middle Ages when horse-drawn buggies used multi-leaf spring setups.

The problem with multi-leaf spring suspension systems is that their primary function is not damping, though by design they do provide some damping effects. The damping occurs through interleaf friction. Essentially the movement of one spring relative to the other creates a damping effect. Road bump energy is absorbed as the springs bend and then return to their original position.

Another issue with multi-leaf suspension setups is in motion control. While the springs capably absorb and distribute road bumps, due to something called hysteresis and disparities between interleaf friction, the vehicle will tend to bounce and bob around. This can also produce dissimilar damping rates on the same vehicle. Damping consistency is a problem with multi-leaf suspension. As a result, you can experience poor ride quality and require more maintenance on your trucks and trailers.

Focus on the U-Bolt

For multi-leaf suspensions, interleaf friction will be entirely dependent on the u-bolt clamp load. To maintain a proper clamp, shop technicians will need to conduct regular re-torqueing of the u-bolts themselves. Still, this can take quite some time to complete and is not easy.

When you replace a leaf spring, expect the new spring’s surface condition and damping capability to be different from other existing springs in the pack. What you will find is that the springs themselves have different friction thresholds. This can then cause different damping rates on the vehicle and impact ride quality and performance.

Multi-leaf springs have been around forever, and they do represent a tried-and-true method to controlling motion and supporting suspension control. But there is still another method that is just as old and required one less leaf.

Mono-Leaf Spring Suspension

Mono-leaf suspension systems address inconsistent damping rates of multi-leaf setups. Yet, mono-leaf suspension still requires the addition of damping components such as shock absorbers or external suspension bump stops. These components do a good job at increasing mono-leaf damping, but they also cost time and money to install.

You will also need to ensure you are regularly inspecting these additional components. To maintain a high level of damping and motion control, shock absorbers and bump stops must be replaced at regular intervals. Yet they degrade at different time intervals and levels, so you may need to check and replace them at different times.

Mono-leaf suspension systems also eliminate the problem of manufacturing variations. With multi-leaf suspension, the two leaves may suffer from inconsistent manufacturing specifications. That isn’t a problem when you use only one leaf.

Air Suspension Systems

Trailer air suspension systems are very common and work well for effective damping. Instead of using leaf springs to absorb road energy, they use air springs as the interface between the axles and the vehicle or trailer.

How do they work? Essentially, air springs act as trailer isolators. They reduce the number of road inputs that are transmitted into the trailer. They provide many of the same benefits that multi- and mono-leaf suspension systems do, but with less drawbacks. Their modern design ensure they also have a longer lifespan.

Air springs typically react to vertical loads and maintain a trailer or vehicle’s static height. Still, to ensure a comfortable ride and proper motion control, many air suspension systems still require extra components, such as shock absorbers.

A Primer on Shock Absorbers

Modern shock absorbers are essentially nothing more than specialized hydraulic cylinders. They are designed to control the motion of the piston within the absorber using a liquid medium. This viscous liquid medium allows the piston to pass through a set of orifices and check valves. This action converts the energy from the motion of the shock absorber into excess heat, which is then dissipated through the body of the absorber itself.

The components of a shock absorber include:

  • Upper mounts
  • Piston rods
  • Rod seals
  • Hydraulic oil
  • Dust shields
  • Piston valves
  • Reserve Cylinders
  • Base Valves
  • Pistons
  • Seals
  • Lower mounts

Since their invention, major improvements have been made to the overall design of a basic shock absorber. Still, though these designs are more effective, shock absorbers are not without limitations. These limitations include the amount of oil contained in the absorber, movement wear and tear, and degrading seals. If not properly maintained, these problems can degrade the effectiveness of the shock absorber over its lifetime.

Evaluating Damping Loss and Wear and Tear

Shock absorbers are very effective and provide a high level of suspension and motion control when they are brand new. But over time, due to consistent road bumps and inputs, shock absorbers will wear at different levels as they age. The problem is that it can be hard to tell when they have begun to degrade. In many cases, other than a small loss of suspension and damping, they tend to show little outward indication that there is a problem.

Two primary problems with shock absorbers include misting and leaking of their internal fluids. A leaking shock absorber indicates that it needs to be immediately replaced. The seals and tight fittings have loosened and there is no way to fix that with shop tools. Shock absorbers that are lightly misted on the outside may still be fine and working within normal parameters. It is leaking you must primarily worry about.

When a shock absorber is working properly, it essentially acts as a heat sink/generator and then it dissipates that heat back into the atmosphere. But how do you know if a shock absorber is working properly? The easiest way to do that is to touch the outside of the shock absorber immediately after it has been in use. If it is hot or warm, that means it is offering some level of damping.

What a touch check does not do is tell you the level of damping actually occurring, it only indicates that damping is occurring. A touch test is also quite subjective and is not something technicians usually add to their scheduled trailer maintenance routine.

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