Mr. Frank Smidler
Director of Engineering
Wabash National Corporation
P.O. Box 6129
Lafayette, IN 47903

Dear Mr. Smidler:

This letter responds to your December 11, 1996, letter asking several questions about the National Highway Traffic Safety Administration's January 24, 1996, (61 FR 2004) rear impact protection (underride guard) standards. You asked about using an elastomeric bumper facade to meet the energy absorption requirements of Federal Motor Vehicle Safety Standard (FMVSS) No. 223, Rear impact protection. The short answer to your questions is that the elastomeric material, by itself, cannot be used to meet the energy absorption requirements because the requirements call for plastic, not elastic, deformation of the guard. Your specific questions are answered below, in the order that you posed them.

Question 1: S5.2.2 Guard energy absorption states ". . . shall absorb by plastic deformation within the first 125 mm of deflection at least 5,650 J of energy . . .". Is energy absorption through elastic deformation acceptable? (Emphasis in original).

You urge us to interpret the standard to allow for elastic deformation for two reasons. First, you believe that a guard designed to absorb energy by "simple plastic deformation of steel" is more likely to be damaged by repetitive normal impacts of a vehicle with a Gross Vehicle Weight Rating of 80,000 lb with loading docks, thus rendering it useless for the intended purpose of protecting colliding vehicles. Second, you state that elastomeric material would be easier to apply to trailers with low floor heights and other special applications where it is not possible to mount a bumper that hangs down and can pivot back on a long arm during impact.

The standard's required energy absorption cannot be met by elastic deformation of the guard. We interpret the language "by plastic deformation" in S5.2.2 to mean that the required energy absorption will be provided exclusively by plastic deformation. In other words, any energy that the guard returns to the force application device when the load is removed (i.e., the elastic component of the deformation) is subtracted from the total energy absorption for purposes of meeting the requirement. The typical energy absorption curve in Figure 2 (61 FR 2034) shows that the area under the force-deflection curve where the guard rebounds elastically from 125 mm of deflection to about 90 mm of deflection is not included in the shaded area (the shaded area represents the amount of energy absorption counted against the requirement in the standard). There is a discussion in the preamble to the final rule (61 FR 2011) of NHTSA's reasons for requiring plastic deformation.

Please note that the standard does not prohibit the use of elastomeric material as supplemental shock absorbers. The material might be useful in preventing shock-induced metal fatigue for certain guard designs. However, NHTSA believes that the strength requirements of the standard are sufficiently high that most guard designs would stand up to the stresses of normal use without significant degradation in performance.

Please also note that the standard is not prescriptive about guard design, as your letter implied. There is no requirement that "[s]imple plastic deformation of steel" provide the energy absorption. Other materials may provide the deformation, and the deformation may be as complex as desired. There is also no requirement that trailers with low floor heights or any other trailers have a "bumper that hangs down and can pivot back on a long arm during impact to absorb energy." The final rule specifically mentioned that vertical supports were not required (61 FR 2013). Even if the conventional vertical strut guard design would not work well for certain vehicles, other designs could be engineered for these vehicles without resorting to elastic materials.

Question 2: S6.6(c) states that when testing for energy absorption ". . . apply the force to the guard until displacement of the force application device has reached 125 mm." S5.2.2 states ". . . shall absorb by plastic deformation within the first 125 mm of deflection . . .". If our test bumper absorbs the required amount of energy at less than 125 mm of deflection (i.e., ". . . within the first 125 mm . . .") do we have to continue to test load to the full 125 mm of deflection? (Emphasis in original)

No. The test procedures in the standard describe how NHTSA will test guards for compliance with the standard's requirements, and are not binding upon guard manufacturers. They may certify their guards based on other kinds of testing or even engineering analysis, if these provide a reasonable basis for certification. If a guard can pass NHTSA's test after less than 125 mm of displacement, it would be reasonable to assume that it will pass the test if displacement was continued to the full 125 mm, because more displacement will only result in more energy absorption, up to a certain point.

Even if the guard appears to have absorbed the required amount of energy before the displacement has reached 125 mm, NHTSA will continue the test because S6.6(c) states "[i]f conducting a test . . . for . . . energy absorption . . . apply the force . . . until displacement . . . has reached 125 mm." NHTSA does this because it does not know how much elastic rebound the guard will exhibit once the load is removed, and the energy returned during the rebound will have to be subtracted when calculating the total energy absorbed.

Question 3: S6.6(b) states that "If conducting a strength test . . . the force is applied until the forces specified . . . has been exceeded, or until the displacement of the force application device has reached at least 125 mm, whichever occurs first." Is it correct to take this to mean that the strength requirements must be met at or before 125 mm of deflection? The ". . . at least. . . " is slightly confusing.

Your assumption is correct. The words "at least" do not imply that NHTSA will continue to displace the guards beyond 125 mm. If the required level of strength has not been achieved by 125 mm, the guards will have failed the test. The test procedures are based on demonstrating compliance with the requirements, and the relevant requirement, S5.2.1, states "[t]he guard must resist the force levels specified . . . without deflecting by more than 125 mm."

Question 4: Hydraulic guards that are velocity sensitive have been excluded from the energy absorption test with the statement in the January 24, 1996 Federal Register that the NHTSA is unaware of any nonhydraulic guards that are velocity sensitive. We are looking at the contribution of energy absorption of air escaping from an elastomeric bumper facade as it is compressed under load and at high velocity. What must be done to allow the energy absorption of a pneumatic chamber deflating?

Hydraulic guards are defined in S4 as ". . . a guard designed to use fluid properties to provide resistance force to deformation." (emphasis added). Pneumatic guards use the fluid properties of air to provide resistance to deformation. However, the word "hydraulic" is defined as "operated by, moved by, or employing water or other liquids in motion."(1) Therefore, the word "hydraulic" limits the meaning of the word "fluid" to liquids. This is also the common sense meaning of the word "hydraulic."

Like hydraulic guards, guards based on pneumatic resistance of escaping air might not provide sufficient resistance to the slow application of force in Standard No. 223's quasi-static test. Therefore, the quasi-static test is inappropriate for testing guard designs based on the principle of pneumatic resistance of escaping air. Only a change in the standard will allow relying on such a chamber to provide energy absorption.

Question 5: Will we be allowed to use a full width elastomeric bumper facade that does not meet the quasi-static test using an 8" x 8" input plate but that in total provides more energy absorbing potential that a structure utilizing plastic deformation of steel supports that does meet the quasi-static test using an 8" x 8" plate?

No. The requirements are not based on the total energy absorbing potential of the entire structure, but on the energy absorbing potential at the P3 test points using an 8" by 8" plate. Writing a standard based on calculating the "total energy absorbing potential" would be impractical due to the many possible guard designs. In addition, that is not an appropriate measure for guard performance, because the force of an underriding vehicle will normally be concentrated in a certain area, rather than distributed along the entire surface of a bumper facade.

We note that NHTSA has received petitions for reconsideration on certain aspects of the energy absorption requirements, although their resolution probably will not affect our answers to your questions. If you have any further questions, please feel free to contact Paul Atelsek of my staff at (202) 366-2992.


John Womack

Acting Chief Counsel



1. Random House Dictionary of the English Language, unabridged edition, 1966.