4. Frontal impact assessment

Frontal impact remains the highest-volume crash mode in Global NCAP assessment, and it is also the area where protocol-year differences are most consequential. A vehicle that scores well under one protocol revision may lose stars under the next, not because the structure changed, but because the barrier, the impact speed, the occupant dummy, or the injury reference values changed. This section explains the two frontal test families — full-width rigid barrier and offset deformable barrier — and shows how each has evolved across protocol years so that engineers can map test results to the correct assessment basis.

Two Test Families, Two Different Questions

The full-width frontal test drives the vehicle into a rigid barrier that covers the entire front end. Because the barrier does not deform, the vehicle must absorb essentially all of the kinetic energy through its own structure. This configuration is a severe structural test: it loads both longitudinal rails, the firewall, and the floor simultaneously and exposes restraint system performance at high deceleration. It answers the question: can the occupant cell survive a symmetric, high-energy frontal load?

The offset deformable barrier (ODB) test, by contrast, strikes only part of the vehicle front — historically 40% of the width — against a deformable honeycomb barrier. The barrier crushes and absorbs energy, while the vehicle is loaded asymmetrically. This configuration answers a different question: does the structure manage an asymmetric load without excessive intrusion, and do the restraints control occupant motion when the vehicle yaws and rotates? The two tests are complementary, and a complete frontal assessment requires both.

Barrier Types and Their Mechanical Role

The barrier is not a passive fixture; its stiffness defines the crash pulse. A rigid full-width barrier produces a short, high-amplitude deceleration pulse because the vehicle must crush against an immovable plane. A deformable offset barrier — typically an aluminium honeycomb block with defined crush strength — produces a longer, lower-amplitude pulse and allows the vehicle to penetrate and rotate. The honeycomb specification (cell size, density, crush strength) is fixed by the protocol and must not be substituted, because a different honeycomb changes the pulse and therefore the injury outcome. In practice, barrier condition is a calibration item: honeycomb that has been previously crushed, or that has absorbed moisture, will not reproduce the specified crush strength, and the resulting pulse will be softer than the reference.

Impact Speeds Across Protocol Years

Impact speed is the single most sensitive parameter in frontal assessment, because kinetic energy scales with the square of velocity. A modest speed increase produces a disproportionate rise in energy that must be managed. The table below summarises the representative frontal configurations used across Global NCAP protocol generations; values are indicative of the protocol families and should be confirmed against the specific protocol-year document applicable to a given assessment.

The key point is not the exact number in any single row but the trend: later protocol years tend to raise the assessment bar through additional occupants, additional body regions, and tightened injury reference values, even where the nominal impact speed is unchanged. An engineer comparing a 2015 result with a 2022 result must therefore compare the assessment basis, not just the crash configuration.

Injury Metrics and How They Are Read

Frontal assessment converts dummy measurements into injury criteria, and each criterion has a reference value that defines the boundary between acceptable and unacceptable loading. The most common frontal metrics are:

• HIC (Head Injury Criterion) — computed over a defined time window from the resultant head acceleration; sensitive to both peak magnitude and duration.
• Head acceleration 3 ms exceedance — the peak acceleration sustained for 3 ms, used as a complementary head metric.
• Neck force and moment — tension, compression, shear, and bending moment, assessed against reference values that differ by occupant size.
• Chest deflection and Viscous Criterion (VC) — deflection measures compression of the rib cage; VC combines deflection rate and compression to capture soft-tissue injury risk.
• Femur force and knee displacement — load transmitted through the lower limbs into the occupant cell.

Each metric is normalised against its reference value to produce a ratio; ratios above unity indicate loading beyond the reference and drive point deductions. The critical subtlety is that reference values are protocol-year dependent. A chest deflection that was acceptable under an earlier revision may exceed the reference under a later one, and the same physical test result can therefore produce a different score. When reconstructing or benchmarking a result, always record the protocol year alongside the metric values.

Reading the Assessment Chain

The SVG below shows how a frontal test result is produced: the configuration and barrier define the crash pulse, the pulse drives occupant kinematics, the dummy channels are converted into injury criteria, and those criteria are compared against protocol-year-specific reference values to yield a score. The diagram also highlights the two places where protocol-year changes enter the chain — the barrier/occupant set and the reference values — which is exactly where cross-year comparisons most often go wrong.

Worked Example: Interpreting a Frontal Result

Consider a hypothetical offset test result for a compact sedan, assessed under a later protocol revision. The measured chest deflection for the driver is 42 mm, and the protocol-year reference value for that occupant and metric is 40 mm. The normalised ratio is 42 / 40 = 1.05, which exceeds unity and therefore attracts a deduction. Under an earlier revision with a reference of 45 mm, the same 42 mm measurement would yield a ratio of 0.93 and no deduction. The physical test did not change; the assessment basis did. This is why a result must never be quoted without its protocol year, and why a vehicle re-tested under a newer protocol can appear to regress even when its hardware is unchanged.

The same logic applies to HIC. If the measured HIC is 720 and the reference is 650, the ratio is 1.11 and the head metric is over the limit; if the reference under a different revision is 1000, the ratio is 0.72 and the metric passes comfortably. Engineers reconstructing a historical result should therefore extract the reference values from the protocol document of that year rather than applying current values retroactively.

Practical Checks Before Running or Reviewing a Frontal Test

1. Confirm the configuration and overlap against the protocol-year document — full-width and offset are not interchangeable, and the overlap percentage is fixed.
2. Verify the barrier specification: rigid face for full-width, honeycomb with the specified crush strength for offset. Inspect honeycomb for prior crushing or moisture uptake.
3. Record the actual impact speed and confirm it is within the protocol tolerance; a speed deviation changes the energy and invalidates comparison.
4. Confirm the occupant set and dummy type match the protocol year, since later revisions add occupants and change instrumentation.
5. Extract the year-specific reference values for every metric before computing ratios, and store the ratios alongside the raw measurements.

Frontal assessment is therefore less about a single crash number and more about a disciplined chain: configuration and barrier define the pulse, the pulse defines occupant loading, and the protocol year defines the reference against which that loading is judged. Keeping that chain explicit — in test records, in reconstruction, and in cross-year benchmarking — is what makes a frontal result meaningful and comparable.