Interpretation ID: 77-1.18

TYPE: INTERPRETATION-NHTSA

DATE: 02/02/77

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Blue Bird Body Company

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Blue Bird Body Company's November 2, 1976, request for confirmation that head and knee contact areas specified under S5.3.1.3 and S5.3.2.2 of Standard No. 222, School Bus Passenger Seating and Crash Protection, may be measured by the use of the direct transfer of a medium (such as spray paint) from the head or knee form to the seating surface with the addition of the area that falls within nonintersecting lines that are tangent to the outer bounds of the areas of direct contact. You also request confirmation that the impact requirements of S5.3.1.2, S5.3.1.3, and S5.3.2.2 may be conducted on seating that is attached to a test fixture instead of being mounted in a bus body. In both cases you suggest rulemaking action to make your recommended procedures a part of the standard.

When a standard does not specify a particular aspect of a requirement, such as the means to measure contact area, a manufacturer is entitled to use any reasonable method it chooses to demonstrate, in the exercise of due care, that the regulated vehicle or item of equipment in fact conforms to the requirement. For your information, the agency intends to use a contact medium test similar to the test you describe and will include in its computation of "contact area" the maximum area that falls within nonintersecting line segments that are tangent to the outer bounds of the areas of direct contact. The outer bounds of direct contact do not include areas that represents splattering of the transfer medium without contact of the head or knee form.

In response to your second request, Standard No. 222 is a vehicle standard. Therefore, the impact requirements have meaning only as they apply to seating when installed in a vehicle. For this reason the agency intends to conduct its compliance testing with the seating installed in a bus. The agency's contemplated procedure may involve the removal of seating around the seating being tested, and the test device may be mounted to the floor in place of the seating that is removed.

However, the requirement that the seating conform as it is installed does not prohibit a manufacturer from using a different test procedure from that specified, in view of the NHTSA's expressed position on the legal effect of its regulations. To certify compliance, a manufacturer is free to choose any means, in the exercise of due care, to show that a vehicle (or item of equipment) would comply if tested by the NHTSA as specified in the standard. Thus, the NHTSA test procedures need not be duplicated by each manufacturer or compliance test facility. Blue Bird, for example, is free to conduct its test on a test fixture outside the bus as long as it can certify that its vehicle would comply if tested by the NHTSA according to the standard.

In view of this disposition of your requests, the agency does not intend to undertake modification of Standard No. 222 at this time. The NHTSA will continue to monitor the results of tests conducted to determine compliance with the head and kneeform contact area requirements of the standard and will modify the standard if warranted.

SINCERELY,

November 2, 1976

Frank Berndt Acting Chief Counsel National Highway Traffic Safety Administration

The purpose of this letter is to discuss two problems associated with FMVSS 222, School Bus Seating and Crash Protection, which are caused by the technical state of the art of seat impact testing and to ask for approval to proceed based on the two proposals described below.

First problem - Contact Area S5.3.1.3 and S5.3.2.2

Background 1. FMVSS 222 requires a headform contact area of 3 square inches when impacted at 5 feet per second and a knee form contact area of 3 square inches impacted at 16 feet per second.

2. The requirement of 3 square inches is based on bio-mechanical data and is, therefore, not disputed.

3. It is our understanding that the feasibility of the 3 square inch requirement was verified based on impact tests on foam without upholstery.

4. It is believed that the problems of measuring contact area with upholstery were not anticipated.

Problems of Contact Area with Upholstery:

1. The tension of the upholstery is virtually uncontrollable as it is affected by many factors including tolerances of the seat back, foam pad, and upholstery; installation techniques; temperature; etc.

2. There are several types and weights of upholstery offered as standard equipment and as options. Various state specifications require different upholstery materials.

3. The grain of the upholstery varies and can affect contact area results.

4. The method of determining contact area has not been defined and such factors as transfer mediums, area measurement techniques, and documentation methods can cause large variations in results.

Example of Effects of Upholstery on Contact Area:

The following table shows the significant difference that the use of upholstery can make at several impact locations of Blue Bird drawing #0833079 (copy attached). These contact areas were determined using black spray paint sprayed on the head and knee form as the transfer medium, with the imprint transferred to tracing paper, and the area measured with a Planimeter. Figure Impact Location Contact Area (square inches) Number Dwg. No. 0833079 with 42 oz. upholstery Direct transfer Bounded by tangent lines 1 H5 2.76 3.20 2 H7 2.53 4.85 3 H11 2.53 5.24 4 H12 2.14 4.29 5 K3 wall side 2.92 X 6 K3 aisle side 3.26 X 7 K4 center 2.45 X

Figure no upholstery Number Direct transfer 1 8.75 2 8.30 3 9.90 4 10.90 5 5.80 6 5.89 7 5.05

Discussion: 1. It is felt that the contact area problems associated with upholstery are measurement problems.

2. It is felt that our seat is in compliance with the contact area requirement with or without upholstery, however, due to inadequate measurement techniques we cannot verify compliance when upholstery is used.

3. Obviously, we meet the spirit of the requirement since it was based on 3 square inches of contact area without upholstery and we accomplish this in all areas with a significant margin.

4. The problems of contact area with upholstery have been discussed with the Office of Crash Worthiness and Office of Standards Enforcement on several occasions. The latest of these were meetings on October 28th and 29th, 1976.

Recommendations:

1. Because the contact area problem is one of measurement methodology, we recommend that the NHTSA develop a proper methodology and incorporate it into the standard through the Rulemaking Procedure.

2. Since in the meantime we must release designs, commit tooling, and order material we are asking for approval to proceed based on a conservative approach of defining contact area as that area circumscribed by tangent lines drawn between points of direct paint transfer. See figures 1 to 7.

Second Problem - Impact Test Inside Bus S5.3.1.2, S5.3.1.3, S5.3.2.2

Background: 1. The NHTSA has indicated it plans to conduct all seat performance tests including impact test inside a bus body.

2. Blue Bird Body Company agrees that it is most desirable to conduct impact tests inside a bus body, however, impact testing inside a bus requires a compact, portable, and adjustable test fixture incorporating a fired projectile.

3. All of the impact testing conducted at Blue Bird Body Company has been done outside a bus body using cable suspended pendulums to impact seats. The seats are mounted on bus body floor sections secured to rigid fixturing. The fixturing allows the seat to be positioned to different impact locations and different impact angles in order to meet the requirements of S5.3.1.2 ". . . when any contactable surface . . . is impacted from any direction . . . ."

Problems: 1. Blue Bird Body Company has done development work on a pneumatic fired impact testing device but have not been able to perfect one because of the rigid requirements such a device must meet.

2. The primary factors that have prevented us from perfecting a successful testing device are:

a. The standard requires that the headform exhibit no resonant frequencies below 3000 Hz and the knee form, no resonant frequencies below 1800 Hz and specifies the data channel class for each. Section S6.6.2 and S6.7.2.

b. The standard specifies the total equivalent weights and the shapes of both the head form and knee form. Sections S6.6 and S6.7.

c. The measurement of Force-Distribution S5.3.1.3 requires a very sensitive data acquisition system capable of accurately sampling low level data (0 to 13.04 "G's" compared to up to 200 "G's" for the H.I.C. requirement) at a high sampling rate (approximately 10,000 data samples per second).

3. Within the above size, shape, and weight requirements we have been unable to build a testing device that had sufficient rigidity of the head and knee forms and the supporting and guiding fixtures to eliminate undesirable and intolerable signal inputs produced by the test fixture, with the exception of a cable suspended pendulum which cannot be used inside a bus body.

Examples of Fixturing Induced Signals:

1. The top half of figure 8 shows an oscillograph trace of a Head Impact with our cable suspended impact pendulum. Note the first portion of the trace, 0 to 13.04 G's, which is the area of the curve used in determining Force Distribution, is free of fixture imparted - low amplitude - high frequency signal and can be accurately sampled at a 10 KG sampling rate by a computer in order to calculate a Force Distribution value.

2. The lower half of figure 8 shows an oscillograph trace of a Head Impact with our most successful pneumatic fired impact fixture. The signal, prior to and during the critical initial part of the impact, has a low amplitude, high frequency signal superimposed on it. This is believed to be a fixture imparted signal. This signal cannot accurately be used manually or with a computer to determine a Force Distribution value.

Discussion:

1. It is felt that resonant frequency and fixture imparted signal are fixturing problems and because of the complexity and interplay of these problems we know of no impact test system, fired projectile or otherwise, that could be used inside a bus at all angles and at all impact locations required to comprehensively evaluate seat impact performance.

2. It is felt that our seat is in compliance with the impact requirements because impacting a seat mounted on a rigid fixture is considered to be a more severe test than impacting a seat mounted in a bus.

3. Obviously we meet the spirit of the requirements since we are testing under conditions we believe to be more severe than intended and have spent considerable effort and resources in trying to develop new test fixturing.

Recommendations:

1. Since we know of no other alternative we recommend that Impact Testing for Compliance be performed outside a bus on a "rigid" fixture which can be used to orient the seat in front of the impact form. We request immediate rulemaking action be taken by the NHTSA to require impact testing be performed outside the bus.

2. For the reasons mentioned earlier we are asking for approval to proceed based on impact test data generated on a rigid fixture outside a bus.

In Summary: May we have immediate approval to:

1. Proceed based on tangentially circumscribed contact areas,

2. Proceed based upon impact test data generated on a rigid fixture outside a bus.

Thank you for your careful consideration.

W. G. Milby Manager, Engineering Services

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