2. Occupant protection performance metrics

FMVSS 208 does not regulate "injury" directly. It regulates measurable physical responses of anthropomorphic test devices (ATDs) and compares them against thresholds called Injury Assessment Reference Values (IARVs). Understanding the metric definitions, the sign and coordinate conventions, and the filtering used to derive each channel is a prerequisite for any compliance or countermeasure work. A restraint system that "looks fine" on video can still fail because a single filtered channel crossed its IARV by a few percent.

Head Injury: HIC and the 15 ms Clip

The Head Injury Criterion is computed from the resultant translational acceleration at the head CG, filtered per SAE J211 Channel Class 1000 (CFC 1000):

HIC = max over (t1, t2) of [ (t2 - t1) * ( 1/(t2-t1) * ∫ a(t) dt )^2.5 ]
      with t2 - t1 ≤ 36 ms
HIC15 = same computation restricted to a 36 ms window
HIC36 = same computation restricted to a 36 ms window (legacy)

In practice, the resultant acceleration a(t) is expressed in g, and the window search is performed over every candidate interval up to 36 ms. FMVSS 208 uses HIC15 ≤ 700 for the 50th-percentile Hybrid III male in the frontal rigid-barrier test, and HIC15 ≤ 700 for the small female in the same condition. The 36 ms variant (HIC36 ≤ 1000) appears in older rulemakings and in some FMVSS 213 child-restraint contexts; do not mix them. A common error is computing HIC on the unfiltered signal or on a single-axis channel — both produce non-comparable numbers.

Chest Deflection and the Viscous Criterion

Chest deflection is measured by a potentiometer or IR-TRACC string between the sternum and the spine box of the Hybrid III. The sign convention is positive = compression (sternum moving toward the spine). FMVSS 208 limits are:

  • Hybrid III 50th male: ≤ 63 mm (driver and passenger, frontal barrier)
  • Hybrid III 5th female: ≤ 52 mm

Deflection is filtered at CFC 180. The Viscous Criterion (VC) is a derived metric that combines deflection rate and instantaneous compression:

VC(t) = 1.0 * [ dD(t)/dt ] * [ D(t) / (0.229 m) ]
VC_max = max |VC(t)| over the event

where D(t) is the half-chest deflection in meters and 0.229 m is the reference chest depth of the 50th male. VC is not an FMVSS 208 pass/fail criterion but is widely used in NCAP and in countermeasure development because it captures soft-tissue injury risk that pure deflection misses.

Neck Injury: Nij

The Neck Injury Criterion combines axial force and bending moment into a single dimensionless number:

Nij = ( Fz / Fzc ) + ( My / Myc )

Fzc = 6806 N (tension) or 6160 N (compression)
Myc = 310 N·m (extension) or 135 N·m (flexion)

Each of the four combinations (tension-extension, tension-flexion, compression-extension, compression-flexion) is evaluated, and the maximum Nij is reported. FMVSS 208 requires Nij ≤ 1.0. The critical values above are for the 50th male; the 5th female uses different Fzc/Myc values. Sign conventions follow SAE J211: +Fz is tension, +My is extension. A frequent mistake is applying the male critical values to the female ATD, which inflates or deflates Nij depending on the loading mode.

Femur Load and Knee Displacement

Femur load is measured by load cells in the femur shaft. FMVSS 208 limits the axial compressive force to ≤ 10 kN per femur for the 50th male, and ≤ 6.8 kN for the 5th female. The load is filtered at CFC 600. Knee displacement is a secondary criterion in some test modes; the 50th male limit is ≤ 15 mm per knee. Femur load is highly sensitive to knee bolster geometry and to the presence of a knee airbag; a 10 mm change in bolster standoff can shift peak load by 1–2 kN.

Head Excursion

Head excursion is a kinematic metric, not a force metric. It is the maximum forward displacement of the head CG relative to a reference point on the vehicle, measured in the X (longitudinal) direction. FMVSS 208 limits:

  • Belted, 50th male, frontal barrier: ≤ 813 mm from the seating reference point (SgRP) to the head CG
  • Unbelted, 50th male: ≤ 813 mm (with the steering column and knee bolster as the primary energy absorbers)

Excursion is measured with a photogrammetric or string-pot system and is filtered at CFC 60. It is the metric most directly tied to occupant compartment integrity: if the A-pillar or roof rail intrudes, excursion can exceed the limit even when HIC is low.

IARVs: The Compliance Envelope

An IARV is the threshold value assigned to a specific ATD channel for a specific test condition. The table below summarizes the FMVSS 208 frontal barrier IARVs for the two most common ATDs.

MetricHybrid III 50th MaleHybrid III 5th FemaleFilter Class
HIC15≤ 700≤ 700CFC 1000
Chest deflection≤ 63 mm≤ 52 mmCFC 180
Nij≤ 1.0≤ 1.0CFC 600
Femur load (each)≤ 10.0 kN≤ 6.8 kNCFC 600
Knee displacement≤ 15 mm≤ 15 mmCFC 600
Head excursion≤ 813 mm≤ 813 mmCFC 60
Filtering is not optional. Every channel must be filtered at its specified CFC before any metric is computed. Computing HIC on raw data, or Nij on unfiltered Fz/My, produces values that are not comparable to the IARV and will not survive a compliance review.

Worked Example: Reading a Compliance Report

Consider a 50th-male frontal barrier test at 56 km/h. The post-processed channels show:

HIC15        = 612          (limit 700)   PASS
Chest defl.  = 58.4 mm      (limit 63)    PASS
Nij          = 0.87         (limit 1.0)   PASS
Femur L/R    = 8.1 / 7.6 kN (limit 10)    PASS
Head exc.    = 842 mm       (limit 813)   FAIL

The failure is kinematic, not force-based. The correct diagnostic sequence is: (1) verify the excursion measurement reference point and the CFC 60 filter; (2) inspect the high-speed video for belt spool-out, anchor slip, or pretensioner firing timing; (3) check seat track and recliner integrity; (4) confirm that the head CG target was tracked without occlusion. A common root cause is a pretensioner that fires 10–15 ms late, allowing the belt to pay out before lock-up. Correcting the firing timing — not adding a new airbag — is usually the fix.

Practical rule: when a single metric fails, always check the filter class and the sign convention before changing hardware. Roughly a third of "failures" in early development are post-processing errors, not physical ones.
FMVSS 208 Frontal Barrier — Metric Chain and IARV Envelope ATD Channels Head accel (3-axis) Chest pot / IR-TRACC Neck load cell, femur LC SAE J211 Filter CFC 1000 / 600 CFC 180 / 60 Per-channel class Derived Metrics HIC15, VC, Nij Femur load, excursion Peak / max window IARV Comparison HIC15 ≤ 700 Chest ≤ 63 mm, Nij ≤ 1.0 Femur ≤ 10 kN, Exc ≤ 813 mm HIC15 a_res(t) filtered CFC 1000 Window ≤ 36 ms Exponent 2.5 Limit: 700 Chest Deflection Sternum → spine + = compression Filtered CFC 180 Limit: 63 mm (M50) Nij Fz/Fzc + My/Myc 4 mode combinations Filtered CFC 600 Limit: 1.0 Femur / Excursion Axial compression Head CG X-displacement CFC 600 / CFC 60 10 kN / 813 mm Sign Convention (SAE J211, consistent across all channels) +Fz = tension | +My = extension | +chest deflection = compression | +X = forward (head excursion) Single-Metric Failure Diagnostic Order 1. Verify filter class → 2. Verify sign convention → 3. Inspect video → 4. Check hardware timing → 5. Re-run

Putting the Metrics to Work

In a compliance program, these six metrics form a coupled system. Reducing chest deflection by softening the belt often increases head excursion; reducing femur load by lowering the knee bolster can raise chest deflection. The IARVs are not independent targets — they define a feasible region, and the engineering task is to find a restraint calibration that sits inside all of them simultaneously. The next chapter examines how restraint design variables map onto this feasible region.

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