2. Anatomy of the knee mapping test

The knee mapping test is a sub-system validation procedure within the Euro NCAP pedestrian protection protocol. Its purpose is not to score the vehicle directly, but to establish the geometric relationship between the vehicle's front-end surface and the upper legform impactor at a defined set of impact points. Before any mapping run can be trusted, the physical impactor, its instrumentation, and the measurement chain must satisfy a tightly specified set of requirements. This section dissects those requirements: what the impactor is made of, how it is calibrated, and what certification evidence must exist before the test is accepted.

The Upper Legform Impactor as a Measurement Instrument

The impactor used in knee mapping is the upper legform impactor defined in the Euro NCAP pedestrian testing protocol (and harmonised with UN Regulation No. 127). It represents the femur and knee region of a 50th percentile adult pedestrian. Structurally it consists of two rigid cylindrical segments — an upper femur tube and a lower femur tube — joined by a deformable knee element. Each tube carries a mass block, and the assembly is fitted with a foam-covered front face that contacts the vehicle.

The critical specification values that must be verified before use are summarised below. These are the nominal design values; the actual acceptance limits are defined in the current protocol revision and must be taken from the version applicable to the test programme.

Two points deserve emphasis. First, the mass figure is a total figure: any change in instrumentation, cabling, or mounting hardware alters it, so the impactor must be weighed as it will be used, not as a bare assembly. Second, the knee element is a consumable in the sense that its stiffness degrades with repeated use; it must be replaced or re-verified at the interval defined by the protocol, not merely when it looks damaged.

Calibration of the Measurement Chain

Calibration in this context has two distinct layers, and conflating them is a common source of audit findings.

Layer 1 — Transducer calibration. Every load cell and accelerometer in the impactor carries an individual calibration certificate traceable to a national metrology institute. The certificate states sensitivity, linearity, hysteresis, and the calibration date. The test house must confirm that each certificate is within its validity period on the day of the test, and that the sensitivity value programmed into the data acquisition system matches the certificate exactly. A mismatch of a few percent here propagates directly into the reported force and acceleration traces.

Layer 2 — System-level verification. Beyond individual transducers, the complete measurement chain — impactor, cabling, amplifier, digitiser — must be verified as a system. This is typically done with a shunt calibration or a known reference signal injected at the transducer connector, confirming that the end-to-end gain is correct. The result is recorded as a system check, separate from the transducer certificates.

Certification Requirements and Documentation

Certification here means the documented evidence that the impactor is fit for purpose on the test date. The evidence package typically comprises:

• Transducer calibration certificates, all in date, with sensitivities matching the acquisition configuration.
• Impactor mass verification record, taken with the full instrumentation fitted.
• Knee element stiffness verification record, within the protocol corridor, with the date of last replacement noted.
• Front-face foam condition record — hardness measurement and visual inspection for compression set or tearing.
• System-level measurement chain check record.
• Impactor dimensional check against the protocol drawing, particularly the tube diameters and the knee joint geometry that define the contact profile.

The certification logic is straightforward: the mapping result is only meaningful if the instrument that produced it is demonstrably the specified instrument. A mapping run performed with an out-of-date load cell certificate, or with a knee element outside its stiffness corridor, produces numbers that cannot be defended in a Euro NCAP assessment review, regardless of how carefully the vehicle was prepared.

How the Requirements Interlock

The figure below shows the dependency structure. The impactor's physical state feeds the measurement chain, which feeds the mapping data; certification is the gate that must be passed before any of it is accepted.

Worked Example: Pre-Test Verification Sequence

Consider a test house preparing for a mapping run on a compact SUV. The following sequence is a reasonable, protocol-aligned order of operations. The values shown are illustrative examples, not measured results.

1. Weigh impactor with full instrumentation fitted.
   Example reading: 13.42 kg  → within ±0.1 kg of nominal 13.4 kg  → PASS

2. Check knee element stiffness record.
   Example: last bend test 14 days ago, force at 15 mm deflection
   = 1.18 kN, corridor 1.05–1.30 kN  → PASS

3. Measure front-face foam hardness.
   Example: 52 Shore A at three points, spread 2 units  → PASS

4. Verify transducer certificates.
   Example: load cell cert valid to 2026-03-01, sensitivity
   2.014 mV/V programmed as 2.014  → PASS

5. Run system-level shunt check.
   Example: injected 1.000 mV/V equivalent, acquisition reads
   0.998 mV/V  → gain error 0.2 %, within 1 % tolerance  → PASS

6. Record all results in the certification package and sign off.

If step 5 had returned 0.985 mV/V — a 1.5 % gain error — the correct response is not to proceed with a note in the margin. The chain should be investigated: check connector torque, re-seat the transducer cable, and repeat the shunt. If the error persists, the amplifier channel is suspect and must be swapped and re-verified. Proceeding with a known gain error silently biases every force value in the mapping data.

What the Certification Package Must Demonstrate

The certification package is not a formality; it is the argument that the mapping data is admissible. It must demonstrate three things in sequence: that the impactor is physically the specified instrument (mass, geometry, knee stiffness, foam), that the measurement chain is correctly calibrated and configured (certificates, sensitivities, system gain), and that all of this was true on the test date. When those three are documented, the mapping result — the set of impact point coordinates and the associated contact geometry — can be used with confidence in the subsequent assessment. When any one is missing, the run should be treated as provisional until the gap is closed.

The practical takeaway for test planners is to build the verification sequence into the test schedule as a discrete, time-boxed activity with its own sign-off, rather than folding it into the general pre-test preparation. The impactor is the measuring instrument, and in a mapping test the instrument is the test.