Keywords: trailer, underride, guard, crash, LS-Dyna, simulation.

This paper investigates the benefits of implementing an offset trailer underride guard. Trailer underride guards are usually installed under the rear buck plate of semi-trailers. They are designed to prevent or minimize the amount of underride a passenger vehicle striking the rear of the trailer will sustain. The need for an underride prevention device is obvious since the typical trailer deck is 1 m (40 in.) above the ground and the typical car bumper is 0.5 m (20 in.) above the ground.

The most common trailer underride guard consists of two vertical members supporting a horizontal member. The verticals are designed to bear load in the rear-to-front direction and may be braced by diagonal reinforcement. When the current design of the underride guard is struck with sufficient force it folds completely inward. As the guard folds inward, it also pivots upward and it eventually disengages from the striking vehicle allowing the vehicle to travel under the trailer, largely unencumbered, until the A-pillars of the passenger compartment are engaged against the box structure of the trailer. At that point, the vehicle occupants are at much greater risk of sustaining severe injuries.

A proposed improved design longitudinally offsets the horizontal guard surface rearward with multiple load bearing supports providing an essentially L-shaped profile from the side. The horizontal offset supports are designed to bear loads in both the axial direction and bending direction. This design allows the underride guard to remain lower and continuously engaged with the striking vehicle bumper as the verticals fold inward during the collision. After the verticals have completely folded to essentially a horizontal position, the horizontal offset supports are oriented such that they function as vertical support members for a secondary underride guard for an additional vehicle ride-down.

PC-DYNA 970 [1] was used to simulate finite element (FE) models of a four door sedan impacting both underride guard configurations with an initial velocity of 20.1 m/s. The conventional underride guard absorbed 56,591 N*m while the offset underride guard absorbed 101,650 N*m. The conventional underride absorbed very little energy after 60 ms while the offset underride guard continued to absorb energy through 130 ms.

Both underride guards have the similarly designed vertical members so it is not surprising that the energy-time curves are closely aligned during the initial stage of the underride guard crush (to 60 ms). Since the conventional underride guard has essentially stopped absorbing energy after approximately 60 ms, the deceleration from 60 ms to 260 ms (the time at which the vehicle stops) is generated primarily from crushing the striking vehicle. The vehicle striking the offset underride guard decelerated primarily from crushing the striking vehicle from 120 ms to 160 ms (the time at which the vehicle stops). The offset underride guard has distributed more of the crush energy to the front structure of the vehicle than the conventional underride guard.

A very important consideration in any trailer design is weight. The offset underride guard is 22% heavier than the conventional underride guard. However, the offset underride guard absorbed 80% more energy than the conventional underride guard which provides a favourable energy absorption to weight ratio.

This paper shows that the offset underride guard provides more energy absorbing potential than a similarly designed conventional underride guard. These promising results warrant further investigation into the benefits of an offset underride guard design.

The underride guards used in this analysis were configured at the lower end of FMVSS 223 strength requirements [2]. It is recommended that further analyses with stronger underride guards be performed in addition to simulating impacts with different vehicles [3] and different impact velocities. This analysis only takes into account the energy absorption of rigidly mounted underride guards and not the performance of the underride guards when mounted to a trailer. The bumpers would have to be attached to a model more representative of an actual trailer to evaluate overall system performance. The weight of any additional trailer structures, if any, needed to secure the guard to the trailer and provide sufficient strength to withstand impact loading can not be overlooked in overall evaluation of the energy absorption versus weight performance.

1

PC-DYNA_970, Livermore Software Technology Corporation, Livermore, CA

2

CFR571.223 S5.2.1 (c) - Strength & Energy Absorption of rear impact guards.

3

National Highway Transportation Safety Administration, Research and Development http://www-nrd.nhtsa.dot.gov/departments/nrd-11/FEA_models.html

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