Interpretation ID: 18345pulse
Mr. John C. Stultz
Research Scientist
Transportation Research Center Inc.
East Liberty, OH 43319-0367
Dear Mr. Stultz:
This responds to your letter asking about the acceleration curve shown in Figure 2 of Federal Motor Vehicle Safety Standard No. 213, "Child Restraint Systems" (49 CFR 571.213). I apologize for the delay in responding. You explain that Transportation Research Center (TRC) is interested in conducting certification testing for manufacturers and possibly bidding on the National Highway Traffic Safety Administration (NHTSA) compliance programs for Standard 213. However, due to features of the TRC sled and others of its type generally, you see a problem with achieving the acceleration curve specified in the standard and suggest that the pulse can be slightly revised, by manipulating time zero, to accommodate your type of sled without having an appreciable affect on test results.
Paragraph S6.1.1(b)(1) of the standard specifies that when testing child restraints to the 48 kilometers per hour (30 miles per hour) dynamic test, the acceleration of the test platform shall be entirely within the curve shown in Figure 2 of the standard.(1) The curve shown in Figure 2 begins at zero g's and zero time.
You state that your type of acceleration sled is generally unable to produce the required acceleration curve. You state that your sled "fires" by cracking a seal between a high pressure chamber and a low pressure chamber, with the flow of gas (around a metering pin, which controls acceleration curve shapes) from high pressure to low pressure providing the acceleration force. You explain that initially, the area available for gas flow is small, and a short amount of time is required for pressure to build enough to cause significant acceleration. To illustrate, you enclose an acceleration curve obtained from your sled showing the time lag between initiation of the test and appreciable acceleration of the sled.
When the curve begins at zero g's and zero time, a significant portion of the curve is not within the tolerance band required by our test procedure. However, you state that when time zero has been manipulated so that the initial acceleration pulse falls within the zero to 10 millisecond envelope, and the acceleration at time zero is 1.25 g's, the required tolerance band is achieved.
You state that the velocity change during negative time is only 0.08 mph, which you believe is inconsequential to test results. You suggest that "TRC would like NHTSA to reconsider its pulse envelope requirements to allow a small deviance at time zero so that . . . sleds [similar to TRC's] may defendably participate in certification and compliance testing."
We have considered your request and agree that it is an issue which merits consideration. However, it is not possible to interpret Standard 213 to provide for a different means of defining time zero. Instead, your suggestion would have to be addressed through a rulemaking process to amend the standard.
In December 1998, NHTSA conducted a rulemaking to amend the sled test requirement in Standard 208, "Occupant Crash Protection," by, among other things, revising how time zero is defined (63 FR 71390, December 28, 1998). The sled test in that standard tests air bags. In that rulemaking, NHTSA determined that it is impractical for that test to have time zero at 0.0 g acceleration, because of the time lag between initial movement of the sled and significant acceleration. The agency decided that the start of the sled test will be determined by a specific acceleration level for the sled which corresponds to a time at which the most rapid acceleration begins, at about 0.5 g's (63 FR at 71393).
I have enclosed a copy of the December 1998 final rule for your review. We would be interested in your views on whether the issues discussed in that rule are similar to those you raise. To discuss possible rulemaking to follow up on your letter to us, please contact Michael Huntley of NHTSA's Office of Vehicle Crashworthiness Standards at (202) 366-0029.
If you have other questions, please contact Deirdre Fujita of my staff at (202) 366-2992.
Sincerely,
Frank Seales, Jr.
Chief Counsel
Enclosure
ref:213#208
d.5/4/99
1. Our laboratory test procedure (TP) for Standard 213 (TP-213-04, September 1, 1997), specifies a "tolerance band," or "acceleration function envelope," that incorporates the upper limit of Figure 2 and that also sets a lower limit (see section D.3.3, "Impact Severity" (page 53)).