The physics of tipping trailers

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.

At STAS, we understand that knowledge is power—especially in critical moments. That’s why our team of expert engineers is dedicated to bridging the gap, sharing practical, in-depth knowledge to empower operators and equip them to make safe, informed decisions when it matters most.

In this blogpost, we’ll dive deep into essential tipping trailer knowledge, giving operators the insights and background knowledge they need to stay safe and efficient on the job.

This blogpost is the first part of a blogpost series on tipper safety.

Navigate to the other blogposts here:

1. The physics of tipping
2. Best practices for safe tipping

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.

The first example 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 example 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 third example, 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.

Weight distribution when tipping

Tipping rotates the body around the tipping axle in the rear. This rotational movement inherently means that the center of gravity moves in two directions:

• Upwards
• Rearwards

In this theoretical example, we use a scenario where 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 the ultimate worst-case-scenario.

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.

In the rear however, the answer is not as obvious. The air bellows that support the weight are all connected to each other through air lines so all six air bellows carry exactly the same amount of weight and act as one air cushion. Therefore the rear base of support boundary is found in the center of the axle tridem.

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

If for some reason the load does not exit the body, the weight on the axle tridem theoretically increases to about 34 ton, while the kingpin load goes from 14 ton to only 4 ton.

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: big difference in weight distribution on the base of support, solely by moving the center of gravity upwards.

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.

Click here to learn the best practices for drivers and fleet operators that use tippers:

Best practices for safe tipping