Interpretation ID: sues
Mr. James L. Sues
Freedman Seating Company
4545 West Augusta Blvd.
Chicago, IL 60651
Dear Mr. Sues:
This responds to your letter concerning the "Seat performance forward" requirements of S5.1.3 of Standard No. 222, School Bus Passenger Seating and Crash Protection. I regret the delay in this response. You ask about testing a seat that is 444.5 millimeters (mm) (17 inches) wide.
Background
S5.1.3 requires seat backs to deflect in a controlled manner when specified loads are applied in the forward direction. The forces applied to the seat and the energy absorbed by the seat are given as multiples of "W." "W" represents the number of seating positions on a bench seat, and is calculated as the bench width in millimeters divided by 381 and rounded to the nearest whole number (S4.1 of Standard No. 222). For a seat that is 444.5 mm (17 inches) wide, W = 1.
The loads are applied through two loading bars. You ask about the load applied by the upper bar. The upper bar applies a forward horizontal force to the seat back until 452W Joules of energy have been absorbed in deflecting the seat back (S5.1.3.4). Under S5.1.3(a) and (b), "the seat back force/deflection curve shall fall within the zone specified in Figure 1 [of the standard]," and "seat back deflection shall not exceed 356 mm."
Discussion
You ask: "Is it permissible for the force/deflection curve of the W = 1 case to fall outside of the zone specified in Figure 1 provided 452 Joules of energy are absorbed by the seat?"
The answer is no. Figure 1 states: "Seat back force/deflection curve shall not enter shaded areas," i.e., outside of the zone specified in the figure. However, note that in order to meet the energy absorption requirement, a seat back in a school bus does not have to deflect the full 356 mm specified by S5.1.3. The energy absorption requirement will be met as long as the force applied through the upper loading bar and the resultant deflection of the seat back stay within the unshaded area shown in Figure 1 throughout the application of the force during the test, and the resultant energy "absorbed" by the seat is at least 452W Joules after the force applied by the upper bar returns to zero. The force on the upper bar and the deflection of the upper bar returning to zero force after the applied load is released are included in the calculation of applied load, and are not limited by the shaded areas shown in Figure 1. (1)
Your second question asks whether the applied force or the resultant deflection is "the more important parameter." I note that nowhere in Standard No. 222 is there language placing more importance on either the applied force or the resultant deflection. Therefore, both parameters must be satisfied.
I hope this information is helpful. If you have any further questions, please feel free to contact Dorothy Nakama of my staff at this address or at (202) 366-2992.
Sincerely,
Frank Seales, Jr.
Chief Counsel
Enclosure
ref:222
d.9/5/00
1. We also note that only the "force applied" and the resultant seat back deflection are restricted in Standard No. 222. In a July 30, 1976 interpretation letter to Thomas Built Buses Inc. (copy enclosed), NHTSA noted that any recoil energy returned to the upper loading bar by the seat is not considered to be part of the absorbed energy. Specifically, the letter stated that seats have to "absorb" (i.e., receive without recoil) a specific amount of energy to provide adequate protection: "This value is represented by the amount of energy that is not returned to the loading bar as it is withdrawn. Described graphically, the area that represents returned energy under the seat back force/deflection curve must be subtracted from the entire area that lies under the curve in order to calculate the energy 'absorbed' by the seat back."