Interpretation ID: nht71-5.44
DATE: 09/22/71
FROM: AUTHOR UNAVAILABLE; Lawrence R. Schneider; NHTSA
TO: Toyota Motor Co., Ltd.
TITLE: FMVSS INTERPRETATION
TEXT: This is in reply to your letters of June 16 and September 1971, regarding the computation of the severity index under Standard No. 208. I apologize for our delay in replying.
Your question is whether the severity index for an acceleration time history with two(Illegible Word), one caused by initial impact and the other caused by rebound, is computed on the basis of both peaks. Our reply is that both peaks must be used, even though the accelerations may be in opposite directions and separated by a measurable interval. The severity index computation is based on the entire event from onset of acceleration until the end of motion.
SINCERELY
TOYOTA MOTOR CO., LTD.
September 14, 1971
Mr. Douglas W. Toms, Admin. National Highway Traffic Safety Administration
Re: Our letter to you dated June 16, 1971.
On June 16, 1971, I wrote a letter to you requesting clarification of obtaining the Severity Index which is required in the Motor Vehicle Safety Standard No. 208.
To date, I have not received your reply.
As we have developed a passive restraint system to meet Standard 208, your clarification is quite necessary for the evaluation of our own system.
I would very much appreciate your reply as soon as possible.
K. Nakajima Director/General Manager
attachment
June 16, 1971
Mr. Douglas W. Toms, Admin. National Highway Traffic Safety Administration
This is to request clarification of obtaining the Severity Index which is required by the Motor Vehicle Safety Standard No. 208 -- Occupant Crash Protection.
Figure 1 (attached) is an example of head acceleration time histories of an anthropomorphic test device in a front passenger seat which were recorded during a 30 mph barrier crash test for the evaluation purpose of passive restraint systems. These time histories indicate two acceleration peaks. The first peak occurs when the device in a passenger seat is restrained at the time of impact to the barrier by a passive restraint system which is installed in the front of the device. About 0.1 seconds after the first impact, the second peak follows when the device rebounds to strike the front seat back.
The Severity Index is calculated at 840 for the first impact to the front restraint system and calculated at 343 for the second impact to the front seat back. Therefore, the Severity Index of these two combined impacts is 1183.
Several questions on obtaining the Severity Index have arisen from the two impacts. We interpret these two
P2 impacts to be independent phenomena from each other because of the following reasons:
a. The impact areas on the vehicle's interior are different between the first and second impact.
b. The directions of the acceleration of the device are opposite, and the impact areas of the device are different. The first impact is to the front of the head, and the second impact is to the rear of the head.
c. A zero acceleration period is observed between the first impact and the second impact.
d. The time interval of these two peaks is about 0.1 seconds which seems to be enough time for the human brain to recover from the effect of the first impact.
Therefore, we believe that the evaluation of the Severity Index on the acceleration time histories should be done separately and should not be added together. In other words, the restraint system, the performance of which is shown in Figure 1, meets the requirement of Section 6.2. Is our interpretation correct?
Also, please advise us of the minimum time interval of these two impacts or the other bases of judgement which can be evaluated separately.
Your consideration is greatly appreciated.
K. Nakajima General Manager
Attachment
The first head impact to a passive restraint system. S.I. = 840
The second head impact to the front seat back.
S.I. = 343
Longitudinal accel.
Transverse accel.
Vertical accel.
Figure 1. Head acceleration time histories of an anthropomorphic test device in the front passenger seat at 30 mph barrier impact.
(Graphics omitted)