The physics of tipping trailers

Tags:
Tippers Product Tips & tricks
The physics of tipping trailers

At STAS, safety isn't just a feature—it's a fundamental part of our engineering philosophy. We know that tipping a fully loaded trailer involves complex mechanics, and the stakes are high for drivers, operators, and fleet managers alike.

Our commitment to operational safety focuses on two key areas:

1.    Maximizing Safety Margins: Through rigorous engineering, high-quality materials, and optimized manufacturing, we ensure our trailers are safe under all conditions.
2.    Educating Operators: By sharing insights and best practices, we help ensure trailers are used safely and effectively.

Safety is not just a checkbox; it's a continuous challenge and a shared mission between STAS and our end users.

Literature on the topic of safe tipping often provides a theoretical approach, but real-world situations can demand split-second decisions that go beyond what’s written in the manual. Unfortunately, this gap between theory and practice can lead to accidents on-site.

 

In this blogpost, we'll start at the beginning and take a look at how stability is defined and what happens to the weight distribution during unloading. 

This blogpost is the first part of a blogpost series on tipper safety. Navigate to the other blogposts here:

Understanding stability

Stability is a matter of weight distribution.

A scientist would say that an object is stable whenever the center of gravity of the object is positioned above the base of support of that object. If the center of gravity is no longer above the base of support, the object rolls over.

Center of gravity?

The center of gravity is the theoretical balance point of an object. It’s where the weight is spread out evenly in all directions. If you hold something at its center of gravity, it won’t tip over.

  • The first object on the left is the most stable because the forces that are acting on the base of support are in perfect balance between left and right.
  • The second object in the middle is less stable but not falling over because the center of gravity is still above the base of support, there’s more weight on the left corner than on the right though.
  • In the case of the third object, all the weight is on the left corner and the center of gravity is no longer above the base of support. The object irreversibly falls over.

How stable an object is, is a matter of how much margin the object has until the center of gravity is no longer above the base of support. In other words: moving the center of gravity closer to the edge of the base of support decreases the stability of that object. 

Weight distribution when tipping

Tipping a tipping trailer moves the center of gravity in two directions:

  • Upwards
  • Rearwards

In this example the load does not exit the body while tipping. This is not a normal situation, but we use this situation as a base for all strength simulations and tests because this is a worst-case-scenario from an engineering perspective.

Consequences of the center of gravity moving rearwards

Looking at the trailer from the side, the first thing we need to know is where we can find our base of support.

In the front of the trailer, the base of support boundary is the kingpin. This is where the weight is resting on the fifth wheel of the truck.

The rear base of support boundary is the center of the axle tridem. The air bellows that support the weight are all connected to each other through air lines so all six air bellows act as one air cushion and carry exactly the same amount of weight, so the base of support boundary is the center of the air cushion.

(In this example, the air bellows are inflated during tipping. Click here for more information on the effects of inflating or deflating the air bellows during unloading.)

Now that we have our base of support, it’s a lot easier to see the physical consequences:

If the load does not exit the body, the weight on the axle tridem increases to 34 tonnes, while the kingpin load goes from 14 tonnes to only 4 tonnes.

Knowing this, we can draw some conclusions with regards to tipping safety:

  1. A ground surface that is solid enough to drive on is not necessarily safe to tip on because the load on the ground surface can potentially increase with more than 40% locally, meaning the ground could start to give way as soon as tipping starts.
  2. Tires and axles get used to their technical limits when tipping, so tires and suspension are among the most safety-critical components on a tipper.

Consequences of center of gravity moving upwards

In theory, the upwards movement of the center of gravity has no effect on the weight distribution on the base of support. However, stability is about how much margin there is between perfect balance and the point where the center of gravity is no longer above the base of support.

This is where theory clashes with reality because no ground surface is 100% level.

On the left: no difference in weight distribution on the base of support.

On the right: on a base of support that is not fully level, the weight distribution between left and right is compromised even further by pushing up the center of gravity. 

Moving the center of gravity upwards reduces the safety margin because inclinations on the base of support are magnified.

Conclusions

Moving the center of gravity closer to the base of support boundary reduces stability, so how safe a tipping procedure is, depends on how much margin the center of gravity has before it’s no longer above the base of support.

From a physics point of view, the best way to ensure stability and therefore safety while tipping is to keep the base of support perfectly level, and the center of gravity as low as possible.

Knowing this, there is a lot we can do to increase safety margins, both as an operator and as an equipment manufacturer.

 

Learn how drivers and operators can maximize safety margins during tipping:

Best practices for safe tipping.

How we engineer the highest safety margins into our trailers.