What is an under-run or underrun?

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Accepted Answer

An underrun, also known as a rear underride guard or bumper, is a structural reinforcement mounted at the rear of a trailer or rigid body vehicle. It is designed to prevent smaller vehicles from sliding underneath the larger vehicle in the event of a rear-end collision.

Purpose of an Underrun

Underruns serve a critical safety function by mitigating the severity of rear-end collisions, particularly those involving passenger cars and larger vehicles like trucks or trailers. By preventing the smaller vehicle from undercutting the larger vehicle, underruns can help to:

Reduce the risk of passenger compartment intrusion: Underruns prevent the smaller vehicle from sliding up under the larger vehicle's chassis, which could intrude into the passenger compartment and cause serious injuries or fatalities.
Deflect the smaller vehicle: Underruns redirect the force of the impact, potentially reducing the impact's severity and diverting the smaller vehicle away from the larger vehicle's occupants.

Types of Underruns

Underruns come in various designs and materials to suit different vehicle types and applications. Some common types include:

Rigid steel underruns: These are the most common type, providing robust protection against underride.
Energy-absorbing underruns: These are designed to absorb impact energy and deform during a collision, further reducing the risk of passenger compartment intrusion.
Removable underruns: These are used on some specialized vehicles, such as fire trucks, to allow for access to the vehicle's rear components.

Regulations and Standards for Underruns

Underruns are subject to various regulations and standards to ensure their effectiveness in preventing underride collisions. These regulations often vary by region and vehicle type.

Importance of Underruns

Underruns play a crucial role in road safety, particularly for vulnerable road users like passenger car occupants. By preventing underride collisions, underruns can help to save lives and reduce the severity of injuries in rear-end accidents.

What is an "under-run" or "underrun"?