What second moment of area is required depending on the weight of the truck?

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Second moment of area is an important factor to take into account in vehicle construction as part of ensuring compliance with the requirements for underrun protection. For example, if your vehicle weighs between 12 and 21 tonnes, in the ‘mid-range segment’, you can adapt the strength of the vehicle frame against the weight of the vehicle according to our table to be sure that the vehicle can handle UNECE R58 rev3.

What is second moment of area?

and is denoted with I, and units expressed as cm4, mm4 or m4. The higher the value of I, the more rigid the structure.

Since the requirements for underrun protection are based on a specific maximum deflection, truck frames must correspond to the rigidity levels that components are designed and tested for at VBG.

Simply put, small frames are weaker than large frames, but because the boundary can be difficult to determine, you need to be able to understand and calculate the value for your own structures. Geometry data can usually be downloaded from automakers’ bodybuilder instructions.

How is it calculated?

The calculation bases for beam bending are shown below, where I is an important element in the formula for deflection.

What is the requirement?

I = 5,700 cm4 is a minimum requirement and means that the frame on the truck must equal or exceed this value in order for a potential deflection to remain within certified levels. Owing to the new UNECE R58 Revision 3 regulations, we need to develop products with greater resistance to bending/deformation than before.

If you are interested in knowing more about what the new regulations mean, you can read our summary of regulations UNECE R58 revision 3.

When is the vehicle frame strong enough to attach the underrun protection?

All VBG underrun protection is certified according to the new directive ECE R 58.03. The products are distinguished by the suffix 100 or 180 in the product name, where 180 corresponds to vehicle weights of 21 tons, or more, and 100 corresponds to vehicle weights of up to 12 tons.

In order for the certificate to be valid during an assembly, requirements are imposed on the strength of the vehicle frame. As mentioned, this is referred to as second moment of area and is measured in cm4.

• For heavy vehicles with vehicle weights over 21 tonnes, an underrun protection with the suffix 180 and a vehicle frame with min I = 5700 cm4are required

• For vehicles lighter than 21 tonnes but weighing more than 12 tonnes, an underrun protection with a suffix of 180 is required. However, there is a ‘staircase’ for how strong the vehicle frame needs to be for each vehicle weight. This works since the requirement for test power decreases with the weight of the vehicle. The respective requirements for the vehicle frame can be read off from the table below.

Extract from certificate: SBSA

• For vehicles up to 12 tonnes, an underrun protection with the suffix 100 and a vehicle frame with at least 3,100 cm4 are required. It is the sum of the right and left sides that is counted.