1. Post-NPRM Test Program
As discussed in the NPRM, NHTSA had initiated a test program in response to the TREAD Act to assess seat parameters of production seats, working with the U.S. Naval Air Warfare Center Aircraft Division at Patuxent River, Maryland (PAX). PAX analyzed seat geometry data, including seat cushion angle, seat back angle, seat cushion length, seat back length, tether anchor locations, child restraint anchorage system anchor locations, and seat belt locations.
After publication of the NPRM, PAX conducted a series of dynamic tests using a revised test seat assembly that had been constructed incorporating the changes to the test seat assembly proposed in the NPRM. These tests were conducted with the dummies currently specified in FMVSS No. 213 (the newborn and TNO 9-month, and Hybrid II 3- and 6-year-old dummies), and with various types of child restraints (rear-facing infant only, rear- and forward-facing convertible, forward-facing "hybrid boosters" (a child restraint that can be used as a forward-facing restraint with harness for toddlers up to 40 lb and as a belt-positioning booster with children over 40 lb), and both backless and high-back boosters).
The results from this series of dynamic sled tests were compared to actual compliance tests that the agency had conducted to determine what effect, if any, the revisions to the test seat assembly might have on the dynamic performance of child restraints. NHTSA compared measurements taken for seat back rotation in rear-facing tests, and HIC, chest acceleration, and head and knee excursion in forward-facing tests. All of the proposed changes were simultaneously incorporated into the test seat assembly, and were not individually assessed for its effect on child restraint performance.
i. Seat Back Rotation
The effect of the revised test seat assembly on measured seat back rotation in rear-facing tests did not show a clear pattern.
Rear-facing tests were conducted using the revised test seat assembly with rear-facing infant only seats using the newborn dummy, and rear-facing convertible restraints using the newborn and Hybrid II 9-month-old dummies. In tests of rear-facing restraints, HIC and chest acceleration are not currently measured, since the newborn and 9-month-old dummies are not instrumented. Further, head and knee excursion are not measured. The only measured parameter in testing rear-facing child restraints is provided in S5.1.4 of FMVSS No. 213, which specifies that when a rear-facing child restraint is tested, the angle between the system's back support surface for the child and the vertical shall not exceed 70º.
The seat back rotation measured in these tests is compared to the seat back rotation measured in NHTSA compliance tests of the identical child restraints in Table 1 below.
TABLE 1: SEAT BACK ROTATION IN REAR-FACING TESTS
|Child Restraint||Type||Dummy||Seat Back Rotation (degrees) Relative to Vertical||Change(%)|
|Test Seat Assembly|
|Evenflo On-My-Way||Infant only||Newborn||43||51.5||+19.8|
|Century 560||Infant only||Newborn||46||42.5||-7.6|
|Evenflo On-My-Way||Infant only||9-month||57||53.9||-5.4|
|Century 560||Infant only||9-month||52||52.9||+1.7|
|Century Accel||Convertible||Newborn||Not tested||50.7||N/A|
|Century STE 2000||Convertible||Newborn||Not tested||40||N/A|
|Cosco Triad (LATCH)||Convertible||Newborn||Not tested||43.1||N/A|
|Century STE 2000||Convertible||9-month||42||50.6||+20.5|
The data indicated no clear effect of the revised test seat assembly on measured seat back rotation in rear-facing tests. In tests using the newborn dummy and two different rear-facing infant-only child restraints, the seat back rotation angle increased by 19.8 percent over that measured in the comparable compliance test in one, and decreased by 7.6 percent in the other. When the same infant-only seats were tested rear facing with the 9-month-old dummy, the restraint that had previously shown increased seat back rotation with the newborn dummy decreased by 5.4 percent over that measured in the comparable compliance test, while the restraint that had shown decreased seat back rotation with the newborn dummy increased by 1.7 percent over that measured in the comparable compliance test. In all cases, the measured seat back rotation was well under the FMVSS No. 213 limit of 70°.
Tests were conducted using the revised test seat assembly on three different rear-facing convertible child restraints with the newborn dummy. In each case, the measured seat back rotation angle was well below the FMVSS No. 213 limit.
PAX also conducted tests of two different rear-facing convertible child restraints with the 9-month-old dummy using the revised test seat assembly. In each of these tests, the seat back rotation increased by at least 20 percent over that measured in the comparable FMVSS No. 213 compliance tests conducted on the existing test seat assembly. Again, however, the rotation was within the allowable limits.
ii. HIC Measurements
Generally speaking, HIC increased in tests with the Hybrid II 3-year-old dummy, and decreased in tests with the 6-year-old.
Sled tests were conducted using the revised test seat assembly with the Hybrid II 3-year-old dummy in forward-facing convertible restraints, and in forward-facing hybrid boosters using the restraint's internal harness (in the toddler seat mode), and with the Hybrid II 6-year-old dummy in both backless and high back belt-positioning booster restraints. The HIC measured in these tests is compared to the HIC measured in NHTSA compliance tests of the same model child restraints in Table 2 below.
TABLE 2: HIC IN TESTS OF
FORWARD-FACING CHILD RESTRAINTS
|Test Seat Assembly|
|Century Breverra||Hybrid Booster||3-year||659||670||+1.6|
|Cosco High Back Booster||Hybrid Booster||3-year||535||446||-16.6|
|Cosco Grand Explorer||Backless BPB||6-year||438||267||-39.0|
|Cosco Grand Explorer||Backless BPB||6-year||438||328||-25.1|
|Century Breverra||High-back BPB||6-year||308||209||-32.0|
|Cosco High Back Booster||High-back BPB||6-year||399||381||-4.6|
The effect of the revised seat assembly on HIC measurements appear to be varied, and largely dependent on the dummy used in the testing. In three of four tests conducted with the 3-year-old dummy, the measured HIC was higher using the revised test seat assembly as compared to compliance tests performed on the existing test seat assembly. This includes both tests conducted using forward-facing convertible restraints, and one of two tests using a forward-facing hybrid booster with its internal harness system. However, in each of four tests conducted with the 6-year-old dummy, two each with backless boosters and high back boosters, the measured HIC was lower than in the identical compliance tests conducted on the existing test seat assembly. Overall, some measurements differed by as much as ± 40 percent between tests conducted on the two different test seat assemblies. All HIC measurements were well within the existing limit of 1000.
iii. Chest Acceleration
Chest acceleration measurements were recorded using the Hybrid II 3- and 6-year-old dummies in the same series of tests outlined in Table 2 above. Table 3 details the recorded chest acceleration in these tests as well as the comparable compliance tests of the identical child restraints. The measured chest accelerations decreased in each of the tests using the 3-year-old dummy in the revised test seat assembly. The measured chest accelerations generally increased in tests using the 6-year-old dummy in the revised test seat assembly.
TABLE 3: CHEST ACCELERATION IN TESTS
OF FORWARD-FACING CHILD RESTRAINTS
|Test Seat Assembly|
|Century Breverra||Hybrid Booster||3-year-old||40||29.2||-27.0|
|Cosco High Back Booster||Hybrid Booster||3-year-old||44||41.6||-5.5|
|Cosco Grand Explorer||Backless BPB||6-year-old||44||49.2||+11.8|
|Cosco Grand Explorer||Backless BPB||6-year-old||44||38.6||-12.3|
|Century Breverra||High-back BPB||6-year-old||33||35.1||+6.4|
|Cosco High Back Booster||High-back BPB||6-year-old||40||42.4||+5.5|
All chest acceleration measurements recorded were well within the current limit of 60 g's maximum. It is noted, however, that while most chest acceleration measurements were comparable in magnitude between the two test seat assemblies, there was one test in which the measured values differed by 42 percent for the same child restraint.
iv. Head Excursion
It is not evident whether use of the revised test seat assembly will have a positive or negative effect on measured head excursion.
In the tests outlined in Tables 2 and 3, supra, head excursion was measured. In addition, head excursion was measured in sled tests performed with the TNO 9-month-old dummy on two different forward-facing convertible restraints. Head excursion was compared to the head excursion measured in compliance tests of the identical child restraints using the same dummies. Table 4 provides this comparison.
TABLE 4: HEAD EXCURSION IN TESTS
OF FORWARD-FACING CHILD RESTRAINTS
|Test Seat Assembly|
|Century Breverra||Hybrid Booster||3-year-old||483||572||+18.4|
|Cosco High Back Booster||Hybrid Booster||3-year-old||432||572||+32.4|
|Cosco Grand Explorer||Backless Booster||6-year-old||381||363||-4.7|
|Cosco Grand Explorer||Backless Booster||6-year-old||381||457||+20.0|
|Century Breverra||High-back Booster||6-year-old||457||500||+9.4|
|Cosco High Back Booster||High-back Booster||6-year-old||432||447||+3.5|
In three of four tests conducted using forward-facing convertible child restraints, a decrease in head excursion was observed in tests using the revised test seat assembly. However, in tests conducted on the revised seat assembly using forward-facing hybrid boosters, backless and high back belt-positioning booster seats, a marginal increase in head excursion was observed. All measured head excursions, on the existing and revised test seat assemblies, were well within the established 813 mm limit prescribed in FMVSS No. 213. 
v. Knee Excursion
For the tests of forward-facing child restraints outlined in Table 4 above, NHTSA also measured the dummy's knee excursion. These results were compared to the knee excursion measured in compliance tests of the identical child restraints using the same dummies. The knee excursion measurements did not demonstrate a direct correlation between tests conducted with the revised test seat assembly versus the existing test seat assembly, or with the type of child restraint used or the test dummy used. Table 5 presents the results. As with the other injury criteria discussed above, all knee excursion measurements were well within the established 915 mm limit prescribed in FMVSS No. 213.
TABLE 5: KNEE EXCURSION IN TESTS OF
FORWARD-FACING CHILD RESTRAINTS
|Test Seat Assembly|
|Century Breverra||Hybrid Booster||3-year-old||584||696||+19.1|
|Cosco High Back Booster||Hybrid Booster||3-year-old||635||660||+4.0|
|Cosco Grand Explorer||Backless booster||6-year-old||686||610||-11.1|
|Cosco Grand Explorer||Backless Booster||6-year-old||686||653||-4.8|
|Century Breverra||High-back Booster||6-year-old||610||500||-17.9|
|Cosco High Back Booster||High-back Booster||6-year-old||686||701||+2.2|
vi. Summary of PAX Testing
Overall, while differences were seen in tests using identical child restraints on the existing versus the revised test seat assembly, NHTSA did not identify any specific trends along specified parameters, i.e., child restraint type, dummy, etc. All of the measured injury criteria in the tests were well within the established limits of FMVSS No. 213. This leads the agency to conclude that the changes to the standard test seat assembly will not have a significant effect on compliance test results of child restraint systems that meet the current requirements of the standard. Manufacturers will not need to redesign their restraints due to the changes in the seat assembly.
2. Response to Comments
There was unanimous support for amending Standard No. 213's specifications for the test seat assembly used to test child restraints in the agency's compliance tests. Almost all of the commenters believed that the test seat assembly should be more representative of the seats of newer passenger vehicles.
i. Seat Back and Cushion Angles
Amending the seat cushion angle by increasing it from 8 degrees off horizontal to 15 degrees was generally supported. Several commenters viewed these changes as aligning the bench seat more with the ECE Regulation 44 seat assembly bench. Ford believed that the proposed change to the seat cushion angle would help make rigid attachment LATCH infant seats commercially viable in the U.S., and would help facilitate the use of infant restraints by reducing the need for consumers to add towels or pool noodles as spacers under the restraints. Ms. Bidez and Public Citizen opposed the proposed change to the seat cushion angle, stating that seat cushion angle should represent the average angle of a 9-year-old vehicle, not a new vehicle. Ms. Bidez stated that older seat cushions are more horizontal and do not contain any anti-submarining structural components.
The agency has decided to revise the seat cushion angle as proposed. Increasing seat cushion angle from 8 degrees off horizontal to 15 degrees will make the seat assembly more representative of currently manufactured vehicle seats and will reduce or eliminate the need for supplementary devices, such as rolled towels or swimming noodles, now being used with infant seats to compensate for the difference in seat cushion angle of the current seat assembly and new vehicle seats. The agency does not agree with Ms. Bidez and Public Citizen that the seat assembly should be representative of seats in 9-year-old vehicles. Such a rearward-looking approach ensures the obsolescence of the standard, since seats in the vehicle fleet are already in the process of being replaced by the seats of more modern design.
UMTRI expressed concern that tests of child restraints on a seat assembly with a seat cushion at the proposed 15 degree angle to horizontal generally resulted in decreased head excursion values of about two inches and increased chest accelerations by an average of 4 g. UMTRI suggested reducing the allowable head excursion limit in Standard No. 213 by two inches to compensate for the change. JPMA disagreed with UMTRI's comment that the head excursion limit should be changed, stating its belief that there is no difference in safety since the reference point from which head excursion is measured is unchanged. JPMA further stated that--
the fact that the increased angle allows the child's head to travel a longer distance in the real world will permit the manufacturers to utilize that additional movement to manage some of the crash energy without making other, perhaps less desirable, changes to other restraint parameters. For example, the harness system could include measures and/or devices to add energy absorption similar to vehicle retractor torsional load limiters, which were implemented with air bags as a means to reduce chest compression. Such devices require that a small amount of additional head excursion be permitted in the real world to achieve a longer ride-down and take advantage of the vehicle's ‘crumple zone.'…
The agency does not agree that testing on the new seat assembly will result in across-the-board reductions in dummy head excursions as compared to head excursions of dummies tested on the current assembly. It is not evident from the agency's test data that use of the revised test seat assembly will have a positive or negative effect on measured head excursion. Table 4, supra, provides test results comparing head excursion measurements in a total of 10 tests using the revised test seat assembly and using the existing test seat assembly (compliance test results). These tests were conducted using (1) the 9-month-old dummy in two different forward-facing convertible restraints, (2) the 3-year-old dummy in two forward-facing convertible restraints and two forward-facing hybrid booster restraints, and (3) the 6-year-old dummy in two backless boosters and in two high back belt-positioning boosters. In three of four tests conducted using forward-facing convertible child restraints, a decrease in head excursion was observed in tests using the revised test seat assembly. However, in tests conducted on the revised seat assembly using forward-facing hybrid boosters, backless and high back belt-positioning booster seats, a marginal increase in head excursion was observed.
While differences of up to +32.4 percent and –24.6 percent were measured in tests using the revised and existing test seat assemblies, there was no distinctive trend across dummy or child restraint types. Thus, the agency cannot conclude that the new seat assembly necessarily results in a less rigorous test of a child restraint's ability to limit head excursion as compared to the existing seat assembly. Further, all measured head excursions on the existing and revised test seat assemblies in NHTSA's program were well within the established 813 mm limit prescribed in FMVSS No. 213. Thus, the agency does not believe that there has been a showing of a safety need to reduce the head excursion limit to take account of the effect of testing on the new test assembly.
In response to JPMA's comment about increased head excursion benefiting overall child restraint performance due to increased "ride down" of crash forces, the agency agrees that generally speaking, increased ride down can help reduce head, neck and chest accelerations. However, increased ride down obviously must not come at the cost of increased risk of head impacts due to excessive head excursions in a crash. Thus, the agency does not concur with any implication that head excursions beyond what is permitted by Standard No. 213 is acceptable. The agency is concerned that child restraints that might meet the head excursion requirements of the standard when tested on the new test seat assembly might allow excessive head excursion when used in actual vehicles whose seat cushions are more like the current seat assembly. The agency asks the public for help in monitoring this situation and providing information of a real world problem should one occur. If there are unreasonable excessive head excursions due to child restraints being used on vehicle seats that are flatter than the revised seat assembly, reducing the head excursion limit of the standard will be considered by the agency.
Amending the seat back angle by increasing it from 15° off vertical to 22° was widely supported. Several commenters viewed these changes as aligning the bench seat more with the ECE Regulation 44 seat assembly bench, which has a seat back angle of 20 ± 1°. In response to commenters and in further consideration of the agency's efforts to harmonize its standards where possible, the agency amends the seat back angle by increasing it to 20 ± 1° to make it consistent with the test seat assembly of ECE Regulation 44. The agency believes that the difference between 22° and 20 ± 1° is negligible and should have no significant effect on child restraint performance.
ii. Belt Systems On The Standard Seat Assembly
The commenters generally agreed with the proposals for amending the seat belts on the test seat assembly. Almost all of the commenters supported increasing the spacing between the anchors of the lap belt from 222 millimeters (mm) to 392 mm in the center seating position and from 356 mm to 472 mm in the outboard seating positions. JPMA stated that it does not object to the proposal, but noted that the potential effect in side impact testing is unknown. Ms. Bidez suggested that the anchors should be set not at an averaged spacing but at the maximum anchorage spacing "now allowed" for vehicle manufacturers in any seat position.
This final rule adopts the proposals, except the spacing between the anchors of the lap belt in the center seating position will be 400 mm, rather than 392 mm as proposed. The agency believes that the 8 mm difference between 400 and 392 mm is negligible, yet the 400 mm specification will make the spacing identical to that of the test seat assembly of ECE Regulation 44, so it is adopted. The lap belt anchorage spacing in the outboard seating position is revised to 472 mm, as proposed. (The ECE regulation specifies a spacing of 400 mm for both lap only tests and lap/shoulder tests. The agency cannot conclude that the difference between 472 mm and 400 mm is insignificant, so the agency is not adopting the ECE specification.) In response to Ms. Bidez, the Federal motor vehicle safety standards specify a minimum spacing for the anchorages, not a maximum. As to setting the anchorages at the maximum spacing that the agency has measured in its test program, the agency declines this suggestion. The agency does not have sufficient information to form the basis for a conclusion that a safety need exists to set the anchorages at the widest spacing observed on a vehicle seat. Further, setting the anchorages at the maximum spacing was not proposed in the NPRM or evaluated in the agency's test program at PAX River.
A few commenters expressed some concerns about certain aspects of the test seat assembly's seat belts that were not addressed by the NPRM. GM, the Alliance, and ARCCA, Incorporated (ARCCA), stated that the seat belt lower anchors for both the center and outboard seating configurations do not represent typical anchorage locations found on new vehicles. As stated by the Alliance, "The lap belt anchorages are too far back and too low and the lower anchors for the outboard seat are too high to represent a typical rear seat." GM and the Alliance also believed that the current two-piece lap and shoulder belt should be replaced with a three-point continuous loop shoulder/lap belt with a simulated retractor. Ford suggested that, to improve reproducibility of test results, the standard should specify a "reasonably tight" tolerance of 8 % ± 1% elongation at 10,000 N for the belt webbing used on the standard test bench.
The agency did not pursue revising the fore-and-aft and vertical placement of the seat belt anchorages in response to the TREAD Act. This was due in part to the short deadlines of the TREAD Act. In addition, information from a 1994 test program indicated an absence of a need to change those anchorage locations. In 1994, the agency explored locating lap and shoulder belt anchorages on the standard seat assembly in a test program supporting rulemaking amending FMVSS No. 213 to facilitate the production of belt-positioning booster seats. The agency found that the fore-aft and vertical placement of the lap belt had a negligible effect on the performance of the child restraints evaluated in the program. 59 FR 37167, 37171; July 21, 1994. Nonetheless, in that rulemaking the agency placed the inboard anchor to reflect the location of the average condition identified by the research. The agency believes that those fore-aft and vertical locations are still sufficiently representative of current vehicles so as to provide a true and thorough evaluation of a child restraint's performance in a crash.
Given agency resources and rulemaking priorities, NHTSA does not anticipate exploring in the near future whether the fore-aft and vertical placement of the lap belt anchorages should be changed, or whether the current two-piece lap and shoulder belt should be replaced with a three-point continuous loop lap/shoulder belt with a simulated retractor. Our assessment of the safety need for such a rulemaking could change, if new information arises that indicates that these issues should be explored.
In response to the issue raised by Ford, the elongation of the standard belt webbing used in FMVSS No. 213's compliance test was not discussed in the NPRM. It should be noted that specifying elongation of the webbing was addressed by NHTSA in the July 21, 1994 final rule on belt-positioning boosters (59 FR at 37171). Under current FMVSS No. 213 test procedures, NHTSA tests child restraint systems using webbing that is typical of that installed in vehicles. NHTSA obtains webbing material from seat belt suppliers. These suppliers also furnish vehicle manufacturers with the webbing used in motor vehicles. This aspect of the compliance test increases the likelihood that the belts used to attach child restraints to the standard seat assembly are those that will actually be used by consumers to attach the restraints to their vehicle seats.
The belt webbing is required by FMVSS No. 209 (S4.2(c)) to meet elongation requirements. Ford believed that the elongation allowed by that standard is too varied ("from zero to twenty percent for a lap belt, …up to 30 percent for the pelvic portion of a lap/shoulder belt, and … up to 40 percent for the upper torso portion of a lap/shoulder belt. Such a large permitted variation in choice of belt webbing elongation could markedly affect FMVSS 213 dynamic test results.") Ford did not provide data substantiating that differences in test results were obtained that were attributed to the use of webbing with different elongation characteristics. The agency also cannot conclude that testing with webbing with a "tight tolerance" of 8 percent, as Ford suggested, is preferable over testing with webbing with a larger tolerance, e.g., closer to the 30 or 40 percent limit. Given agency resources and priorities, the agency can not conclude that a need exists to initiate rulemaking on this aspect of FMVSS No. 213 in the near future.
iii. Fixed Seat Back
Commenters did not see eye-to-eye on the proposal to change the seat back to represent a fixed vehicle seat. Graco, TraumaLink, the Alliance, Safekids, Evenflo, JPMA and Xportation supported the proposal. JPMA stated that a fixed seat back replicates today's motor vehicle seat back and harmonizes with the test bench setups for ECE, Canadian and Australian regulations. Xportation said that it did not believe that motion of seat backs in vans is significant to the performance of child restraints. On the other hand, General Motors agreed with the proposal that a fixed seat back would be more representative of the rear seat of today's passenger cars, but expressed concern that a fixed back would not be representative of free-standing seats in vans and other multipurpose passenger vehicles. GM believed that it was unclear how fixing the seat back would affect child restraint system performance and suggested that NHTSA should study the issue. Advocates and Ms. Bidez expressed concern that changing to a rigid seat back may result in a less rigorous test condition, even though, the commenter believed, "many children will be seated in seats with flexible seat backs." ARCCA believed that the configuration that results in the more severe test of a child restraint should be selected.
In an effort to assure that the proposed fixed seatback configuration does not pose a less stringent test condition for dynamic tests of child restraints than the existing flexible seatback, NHTSA conducted a series of rigid versus flexible seatback tests at the agency's Vehicle Research and Test Center (VRTC) on September 23-27, 2002. The proposed seatback and seat base angles were used.
Six pairs of tests using rigid and flexible seatbacks were conducted using the CRABI-12-month, and the Hybrid III 3- and 6-year-old dummies in rear- and forward- facing seat configurations, all with lap or lap and shoulder belt attachments (a top tether was not used). Charts providing plots of the normalized injury criteria measurements from these tests for HIC, chest acceleration and head and knee excursions are provided in the document titled, "Comparison of Flexible and Rigid Seat Backs—FMVSS No. 213 Test Assembly," which has been placed in the docket.
The CRABI 12-month-old dummy was tested in a rear-facing infant-only child restraint with both the rigid and the flexible seat backs. Charts A and B of the aforementioned document provide plots of the normalized injury criteria measurements from these tests for HIC and chest acceleration. There are no established head and knee excursion limits for rear-facing child restraints.
The Hybrid III 3-year-old dummy was tested in three forward-facing child restraints – a 5-point harness, an overhead shield, and a shield-type booster with the shield in place – using both the rigid and flexible seat backs as in the tests with the CRABI dummy. Charts C through K provide plots of the normalized injury criteria measurements from these tests for HIC, chest acceleration, head and knee excursion.
Similar tests were conducted using the Hybrid III 6-year-old dummy in both a backless belt-positioning booster and in a high-back belt-positioning booster seat. The plots of the normalized injury criteria measurements are provided in Charts L through Q of the document.
In each of the tested configurations (e.g., 3-year-old dummy in an overhead shield convertible restraint), only one set of rigid versus flexible comparison tests was run. As such, the data used to evaluate the effects of the seat back are limited at best. The data were inconclusive as to whether a rigid seat back represents a less vigorous test. Review of the data indicates that, in some cases, the move to a rigid seat back resulted in a reduction in measured dummy response (lower HIC and chest g's for the 3-year-old dummy in overhead shield convertible). However, other cases show increases in dummy response when the rigid seat back is used (higher HIC for 3-year-old dummy in 5-point harness convertible, shield booster; also for 6-year-old dummy in backless belt-positioning booster). Importantly, NHTSA notes that where differences in performance were noted for a particular injury criteria in a tested configuration, those differences were typically very small. Furthermore, in nearly each instance, results for both the rigid and the flexible configurations were within a 20 percent compliance margin indicating a level of performance that is well within the established limits.
Based on the above data, NHTSA concludes that any differences seen between testing conducted with a rigid versus a flexible seat back would be minimal, and therefore, a move to a rigid seat back would not represent a less stringent test for child restraints. Further, the agency notes that there are more passenger cars (with rigid seat backs) than vans and multipurpose vehicles (with more flexible seat backs) in the existing vehicle fleet. As such, the move to a rigid seat back would more closely represent the existing vehicles on the road. The rigid seat back, on balance, will not be a less stringent requirement, and that it will allow child restraint performance optimization more representative of the vehicle fleet. In addition, a rigid seat back further harmonizes the standard's test seat assembly with ECE Regulation 44, which specifies a rigid seat back in testing child restraints to that standard. For the above reasons, NHTSA is adopting the rigid seat back as proposed in the NPRM.
Figure 1A of FMVSS No. 213 is revised to reflect the above changes, as is the drawing package of the seat assembly that is incorporated by reference into the standard. (This final rule makes a technical amendment to 49 CFR 571.5 to provide information on obtaining copies of the drawing package).
iv. Future Work
The agency tentatively decided in the NPRM that certain features of the bench seat need not be changed because they either reflected the design of production seats or are different but the difference was deemed not to have an effect on child restraint performance in dynamic testing.
Seat Cushion Length: NHTSA found that the current FMVSS No. 213 seat assembly has a seat pan length that is about 50 mm longer than the average seat pan length observed in today's vehicle fleet. The agency did not believe that the difference was significant. Commenters Consumers Union, Ms. Bidez, SafetyBeltSafe and ARCCA believed that the agency should consider shortening the length of the seat cushion to reflect a more demanding test condition. Ford commented that the current seat cushion is about the same length as a typical rear seat cushion, but suggested that the support for the seat cushion be extended to more realistically support the front edge of the cushion.
NHTSA continues to believe that the length of the seat cushion of the standard seat assembly need not be changed, as it closely reflects production seats and because there is no information indicating that the difference in seat cushion length may affect child restraint performance on the seat. In addition, in view of the time constraints of the TREAD Act, NHTSA did not assess seat cushion support. However, the agency does not believe that seat support is critical. While some existing passenger cars will likely have a seat cushion that is supported more fully toward the leading edge of the cushion, vans and SUVs with bench-type seats that are removable or foldable, or individual seats such as "Captains Chairs" typically found in the second row of seating positions, will likely have much less support toward the leading edge of the seat cushion than in passenger cars. The agency does not anticipate undertaking efforts to evaluate which of these conditions would provide a more stringent test.
Test Bench Floor: Graco and Ford indicated support for the addition of a floor onto the test bench for testing or rating  child restraints. NHTSA does not believe that the standard seat assembly needs a floor because child restraints must meet the requirements of FMVSS No. 213 when attached to the seat assembly by use of the seat belts and LATCH system, without use of supplemental floor braces or other attachments. The commenters also suggest that an agency consumer information program rating the performance of child restraints should utilize all features with which the restraint is equipped, including those that are optional, i.e., that are not necessary for the restraint to meet Standard No. 213. The agency will consider the suggestion when developing its upcoming consumer information pilot programs relating to child restraint performance.
Seat Cushion Stiffness: The question of the stiffness of the seat cushion attracted most of the comments relating to features of the seat assembly that the NPRM did not propose to change. The NPRM stated that the agency was interested in increasing the stiffness of the cushion, but was uncertain what differences, if any, could be seen in dynamic testing. Comments were requested on what the stiffness should be (67 FR at 21812).
Several commenters believed that the stiffness of the seat cushion has a strong effect on child restraint performance. Consumers Union (CU) commented that it believed that cushion stiffness plays a major role in child restraint installation and suggested that further tests and analysis were needed. UMTRI expressed concern that the foam of the present test seat assembly is softer than many seats in the current fleet: "Instead of representing a worst-case scenario, the response of the soft foam and its tendency to bottom-out on to the unrealistically stiff plywood backing can lead to misleading results that can reduce the level of child passenger safety." Ms. Bidez believed that cushion stiffness has a critical influence on child restraint performance relative to head excursion. These commenters did not provide supporting data.
Some commenters were uncertain whether performance would be affected. JPMA stated that it conducted a small group of tests to evaluate the effect of foam in the tests, but the results "yielded more questions than it answered." Without elaborating on its statement, JPMA provided data from a test program it conducted on foam that was 4 inches thick with a 25 percent compression/deflection resistance of 49.5 lb.  The effect on the performance of test dummies in various types of child restraints was varied. JPMA stated that it did not believe that there is yet enough information to evaluate what the foam firmness and density should be, or how child restraint performance would be affected by changing the foam. In its own comment, Graco also expressed that it was unsure of how performance would be affected and suggested that testing and research be completed before changing the foam.
Commenters had different views as to how the seat cushion foam should be changed. JPMA expressed cautious support for changing the foam to resemble more closely the foam thickness and compression of rear seats in real-world automobiles. UMTRI suggested that the agency characterize the overall seat stiffness of several modern vehicles and select a foam stiffness that matches a mean response. Ford stated that current rear seats are typically thinner and firmer than the test bench seat cushion. Ms. Bidez believed that the test cushion must reflect the softer seats of the majority of used vehicles on the road today. ARCCA believed that the seat cushion in Standard No. 213 may be too thick to match the vehicle seats, thereby allowing more deflection before becoming stiffer. The commenter suggested that the standard "should err on the side of a softer cushion which will likely result in increased occupant excursion…."
After reviewing the comments and considering the agency's research needs and limited resources, NHTSA has decided not to endeavor at this time to change the stiffness of the standard seat assembly's seat cushion foam. As discussed in the NPRM, NHTSA is aware of data that indicate that the stiffness of the seat assembly cushion might not have a marked effect on child restraint performance. The agency conducted a study in 1988 comparing the stiffness characteristics of the seat assembly cushion with the characteristics of then current seats. 67 FR at 21812. Most vehicle seats were stiffer than the FMVSS No. 213 seat assembly. Sled tests were performed in the study to compare the dummy responses of the standard's seat cushion, a representative seat cushion that was softer, and a stiff cushion. The agency concluded that dummy response differences were not sufficiently large or consistent to warrant specifying a different cushion than that used in the current test seat assembly. Because possibly revising this parameter of the seat assembly would require further research, utilizing scarce agency resources, for disproportionate safety benefits, the agency will not pursue changing seat cushion stiffness for the time being.
Harmonize With Transport Canada: Several commenters concurred with the NPRM that the proposed changes to the test seat assembly would advance harmonization with ECE Regulation 44 in that the seat cushion and seat back angles would be similar, as would the lateral spacing of the seat belt anchors and the rigidity of the seat back. However, the Alliance, General Motors and Evenflo noted that the test bench would differ from that used by Transport Canada in testing child restraints to the Canadian child restraint standard. These commenters urged NHTSA to work with Transport Canada to ensure that the test benches are harmonized.
NHTSA regularly coordinates its vehicle safety plans and programs with Transport Canada and the agencies work closely on regulatory initiatives concerning child restraint safety. Harmonizing the countries' requirements to the extent consistent with the safety needs of each country is a goal shared by both entities. Specifically with respect to the TREAD Act, NHTSA has discussed each of the revisions with Transport Canada. Transport Canada is aware of the changes, and the agencies will continue efforts to harmonize regulations to the extent possible.
 "LATCH" stands for "Lower Anchors and Tethers for Children," a term that was developed by manufacturers and retailers to refer to the standardized child restraint anchorage system required by FMVSS No. 225. This preamble uses the term to describe either an FMVSS No. 225 anchorage system in a vehicle or a child seat that attaches to an FMVSS No. 225 child restraint anchorage system.
 Excursions are measured from Point Z identified in Figure 1B of FMVSS No. 213, which is located in the same place on both the existing and revised test seat assemblies.
 Section 14(g) of the TREAD Act directed NHTSA to establish a child restraint safety ratings program. The agency has established an ease of use ratings program and will be conducting pilot programs on possible ratings programs geared toward rating child restraint performance in sled tests and vehicle performance in frontal vehicle crash tests. 67 FR 67491 (November 5, 2002)(Docket 02-10053).
 The foam in the current seat assembly is thicker and softer than the foam tested by JPMA. The foam in the current seat assembly is 6 inches thick. Two pieces of foam (one 2 inches and one 4 inches) may be used to achieve the required dimension. To be suitable for compliance testing, foam inserts must compress 25 percent under the following load limits: (1) 45-55 pounds for the 2-inch thick foam, and (2) 21-27 pounds for the 4-inch thick foam.