Evaluation of Tether Anchor Zones for FMVSS No. 225

Wietholter, Kedryn; Smith, Joshua · 2025 · ROSA P / United States. Department of Transportation. National Highway Traffic Safety Administration

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Summary

This report evaluates potential regulatory updates to Federal Motor Vehicle Safety Standard (FMVSS) No. 225 regarding the placement of tether anchors for child restraint systems. The study was motivated by a 2015 Notice of Proposed Rulemaking (NPRM) that proposed a vehicle clearance criterion requiring tether anchors to be located at least 165 millimeters behind or below the head restraint to facilitate proper strap tightening. In response to manufacturer concerns and questions regarding this proposal, the University of Michigan Transportation Research Institute (UMTRI) developed an alternate reference zone based on the H-point and R-point. The National Highway Traffic Safety Administration (NHTSA) conducted this evaluation to compare the current FMVSS No. 225 zone, the NPRM-proposed 165-millimeter distance, and the UMTRI alternate zone, while also validating the 165-millimeter length against actual child restraint hardware. The methodology involved surveying six passenger vehicles and one pickup truck, measuring tether anchor locations in rear driver and center positions using a SAE-J826 3D manikin and coordinate measurement machines. Researchers calculated the distance from the seat back to the tether anchor and determined compliance with the three criteria. Additionally, 20 child restraint systems were surveyed to measure tether hardware lengths. Installation tests were performed in two vehicles to assess the practical ease of tightening tethers under different anchor configurations. Results indicated that all surveyed tether anchors complied with the current FMVSS No. 225 zone. However, only five of the 11 seating positions met the NPRM-proposed 165-millimeter distance requirement, whereas nine of the 11 positions satisfied the UMTRI alternate 325-millimeter zone. Regarding hardware, 16 of the 20 child restraints had tether lengths of 165 millimeters or less. The study found that sufficient space existed to relocate anchors to meet the NPRM requirement in all vehicles except the pickup truck, though structural redesign might be necessary. Crucially, installation tests revealed that vehicles failing the 165-millimeter distance criterion still allowed for proper tether tightening, particularly when vehicle owner’s manuals specified routing the tether over or under adjustable head restraints. The significance of these findings lies in the conclusion that strict geometric measurement requirements alone may not accurately reflect installation ease. The UMTRI alternate zone required less space behind the seat back than the NPRM proposal for rear deck mounts. The authors conclude that ease of installation can be improved through clear vehicle owner’s instructions regarding tether routing rather than relying solely on rigid distance measurements. This suggests that regulatory criteria should account for practical installation variables, such as head restraint adjustability and manufacturer guidance, to ensure effective child restraint use.

Key finding

Nine of eleven surveyed tether anchor positions met the alternate 325-millimeter zone criteria, compared to only five that met the proposed 165-millimeter distance requirement, yet proper child restraint installation was achievable in vehicles that did not meet the proposed distance due to factors like head restraint adjustability and owner instructions.

Methodology

field_study

Sample size: 7

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• child passenger safety