4. Adult occupant protection side impact

Side impact is the most demanding part of the adult occupant protection (AOP) assessment for most vehicle programmes, because the occupant sits only a few hundred millimetres from the struck surface and there is almost no crush space to manage. Euro NCAP evaluates this with two complementary configurations: the Mobile Deformable Barrier (MDB) test, representing a car-to-car collision, and the oblique pole test, representing a single-vehicle impact with a rigid narrow object such as a tree or utility pole. Both are scored within the AOP body-region framework, but they load the occupant and the restraint system in fundamentally different ways, and a programme that optimises for one can easily lose points on the other.

What each test actually represents

In the MDB test, a deformable barrier face mounted on a moving trolley strikes the stationary test vehicle from the side. The barrier face is honeycomb-based and is intended to reproduce the front-end stiffness of a typical striking passenger car, so the struck vehicle sees a distributed but still highly localised load spread over the door and B-pillar region. The impact speed and barrier mass are defined by the protocol and are calibrated so that the resulting intrusion and occupant loading are representative of real-world urban and rural car-to-car side crashes. Because the barrier deforms, part of the kinetic energy is absorbed by the barrier itself; the struck vehicle therefore experiences a somewhat softer, longer-duration pulse than in the pole test.

The oblique pole test uses a rigid, non-deforming pole. The vehicle is propelled sideways (or at a defined oblique angle, depending on the protocol revision) into the pole at a prescribed speed, so the pole strikes a narrow band of the door structure. There is no barrier crush to absorb energy, which means the pole drives deeply into the occupant compartment over a very short time. This produces high local intrusion velocities and a sharp acceleration pulse — precisely the conditions that make head and thorax injury likely if the side airbag and curtain do not deploy correctly or the door trim does not manage the contact.

Treat the two tests as a pair of design constraints, not as two separate scores. The MDB test rewards a stiff, well-connected B-pillar and door beam that spreads load into the sill and roof rail; the pole test rewards a structure that can absorb a narrow, high-velocity strike without the pole reaching the occupant. A pillar that is too stiff for the MDB may transmit excessive acceleration in the pole test, and a pillar that is too soft for the pole may allow excessive intrusion in the MDB.

Seating positions and the anthropomorphic test devices

Euro NCAP side impact assessment is performed on the driver and, where applicable, the rear passenger seating positions on the struck side. The protocol specifies which ATDs are used — typically the WorldSID 50th percentile male for the driver position and, in the rear, either a 5th percentile female or a 50th percentile male depending on the seating position and the protocol version. The ATD is seated in the nominal seating position defined by the manufacturer's design intent, with the seat adjusted to the protocol's reference position (seat back angle, cushion height, and fore-aft position). Getting this reference position wrong is one of the most common causes of non-representative results: a seat set 10 mm too far rearward can change the measured rib deflection by a meaningful margin.

The ATD is instrumented to measure the parameters that map directly to the AOP scoring criteria: head acceleration and head injury criterion (HIC), neck forces and moments, chest deflection (rib deflection), chest acceleration, abdominal force, and pelvis force. For side impact, rib deflection is the dominant thorax metric, because the loading is lateral and the ribs are the primary load path. The protocol defines injury risk curves for each body region, and the measured values are converted into points. The exact thresholds are protocol-version dependent, so always confirm the current revision before setting targets.

Door intrusion metrics and what they tell you

Intrusion is measured at defined points on the door inner panel and the B-pillar, using displacement transducers or high-speed photogrammetry. The key metrics are the peak intrusion at each measurement point and the intrusion velocity — the rate at which the door moves inward. Peak intrusion tells you how much space remains between the door and the occupant; intrusion velocity tells you how hard the door is moving when it contacts the ATD. A door that intrudes 150 mm slowly is far less injurious than one that intrudes 80 mm at high velocity, because the occupant's body has time to be accelerated by the restraint rather than struck by the trim.

In practice, the intrusion profile is read together with the ATD response. If rib deflection is high but intrusion is moderate, the problem is likely restraint timing or airbag coverage. If intrusion is high and rib deflection follows it closely, the problem is structural — the door and B-pillar are not managing the load. This distinction drives completely different countermeasures, so make it explicitly in the test report.

MDB test — car-to-car Oblique pole test — rigid narrow object Deformable barrier face (honeycomb) moving trolley struck vehicle (stationary) door + B-pillar WorldSID side airbag intrusion acceleration pulse longer duration, lower peak rigid pole vehicle propelled into pole door + B-pillar WorldSID curtain airbag deeper, faster intrusion acceleration pulse shorter duration, higher peak MDB: distributed load, barrier absorbs energy → moderate intrusion, longer pulse Pole: narrow rigid strike, no energy absorption → deep fast intrusion, sharp pulse

Restraint system implications

The two tests impose different requirements on the restraint system, and the calibration must satisfy both. For the MDB test, the side airbag must be large enough to cover the gap between the door trim and the occupant's thorax and pelvis, and it must deploy early enough that it is inflated before the door reaches the occupant. The bag's venting and internal pressure determine how it manages the thorax load: a bag that is too stiff will produce high rib deflection even with low intrusion, while a bag that is too soft will bottom out against the door. The curtain airbag must cover the head path to the B-pillar and the upper door frame, and its deployment timing must be matched to the door's intrusion velocity.

For the pole test, the curtain airbag is the critical element for head protection, because the pole can reach the head region directly. The curtain must extend far enough down and forward to cover the head path, and it must be inflated before the pole reaches the occupant. The side airbag for the thorax must also be robust to the higher intrusion velocity: the bag must not be pushed aside by the door, and its mounting must survive the higher loading. Seat-mounted side airbags are common because they move with the seat and maintain coverage regardless of seat position, but door-mounted bags can be equally effective if the door structure is well controlled.

Restraint calibration is therefore a coupled problem. The deployment threshold (the crash pulse level at which the airbag fires) must be low enough to fire in the pole test, where the pulse is sharp, but not so low that it fires in the MDB test before the occupant has moved into position. The bag's fill rate and venting must be tuned so that the bag is at its effective pressure when the occupant loads it in both tests. In practice, this is done with a combination of sled testing and full-vehicle testing, with the ATD response used to iterate on bag volume, vent size, and inflator output.

Reading the results: a practical interpretation sequence

When the test data comes back, read it in a fixed order so that you do not confuse structural and restraint effects. First, check the intrusion profile: peak intrusion at each measurement point and the intrusion velocity. Second, check the ATD response: rib deflection, head acceleration, and pelvis force, and compare the timing of each peak against the intrusion timing. Third, check the airbag deployment: was the bag fully inflated when the occupant loaded it, and did it cover the loaded region? Fourth, check the seat and belt: did the occupant stay in the nominal position, or did the belt allow excessive lateral excursion?

A useful diagnostic pattern is the intrusion-led versus restraint-led signature. If the rib deflection peak coincides with the intrusion velocity peak and the airbag is fully inflated, the structure is the limiting factor and the countermeasure is structural (door beam, B-pillar reinforcement, sill connection). If the rib deflection peak occurs before the intrusion peak and the airbag is only partially inflated, the restraint is the limiting factor and the countermeasure is deployment timing or bag geometry. If the head acceleration peak coincides with the pole contact and the curtain is not covering the head path, the countermeasure is curtain coverage and timing.

Do not tune the restraint to compensate for a structural deficiency without confirming that the structure is acceptable in its own right. A bag that is oversized to mask intrusion will pass the test but will not protect the occupant in a real crash where the intrusion is greater than the test condition. Structural and restraint countermeasures must be validated independently before they are combined.

Programme-level checklist

  • Confirm the protocol revision and the exact ATD specification for each seating position before setting targets.
  • Define the seat reference position and document it; verify it on the test buck before the full-vehicle test.
  • Measure intrusion at the protocol-defined points and record both peak intrusion and intrusion velocity.
  • Record airbag deployment timing and coverage relative to the occupant loading, not just the bag's presence.
  • Separate structural and restraint effects in the test report so that countermeasures are correctly targeted.
  • Validate the combined countermeasure in both MDB and pole configurations before sign-off.

The side impact assessment is where the structural and restraint engineering teams must work from a single, shared dataset. The intrusion profile and the ATD response are two views of the same event, and the programme that reads them together will find the countermeasure that satisfies both the MDB and the pole test without over-engineering either.

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