3. Side impact biomechanics

Side impact is fundamentally a problem of limited space and short time. Unlike frontal crashes, where the restraint system and crush zone can manage deceleration over hundreds of milliseconds, a struck-side occupant may be loaded by the intruding door within 20–40 ms of impact. The biomechanical consequence is that the torso is accelerated laterally through a narrow gap before any meaningful restraint can act. This chapter connects the injury mechanisms and criteria used to quantify side-impact risk to the kinematic chain that determines how much clearance an occupant actually needs.

Injury mechanisms and the criteria that quantify them

Side-impact injury is dominated by three load paths: the pelvis and lower torso loaded through the door armrest and trim, the chest loaded through the door inner panel and side airbag, and the head loaded either by direct contact with the intruding structure or by the far-side interior. Each path has a distinct injury mechanism and a corresponding criterion.

For the chest, the dominant mechanism is lateral compression and the resulting rib strain. The most widely used metric is chest deflection, expressed in millimetres, together with viscous criterion (VC), which captures the rate-dependent soft-tissue injury that pure deflection misses. VC is computed as the instantaneous product of chest compression velocity and the normalized deformation:

VC = (dD(t)/dt) × (D(t) / D_max)

where D(t) is the instantaneous half-chest deflection and D_max is the half-chest breadth. A commonly cited thoracic limit in regulatory and consumer test protocols is a half-chest deflection of about 42 mm for a 50th-percentile male, with VC thresholds around 1.0 m/s. These are protocol-dependent values, not universal constants, and must be read against the specific dummy and test configuration.

For the pelvis, the relevant metric is pubic symphysis force or pelvic acceleration, reflecting the risk of pelvic ring fracture when the door armrest loads the greater trochanter. For the head, HIC remains the standard, but in side impact the more informative quantity is often the head contact velocity and the timing of contact relative to the airbag deployment. A head that reaches the intruding B-pillar before the side airbag is fully inflated experiences a very different loading than one that is intercepted by a deployed cushion.

Occupant kinematics in the struck-side seat

The struck-side occupant's motion is best understood as a sequence of coupled phases. In the first phase, the door trim contacts the pelvis and lower torso, imparting a lateral velocity to the occupant's lower body. Because the occupant is initially restrained only by the seat and the far-side geometry, the upper torso lags, producing a lateral bend of the spine. In the second phase, the chest contacts the door or the deploying side airbag, and the head begins to rotate and translate toward the struck side. In the third phase, the head either contacts the intruding structure or is arrested by the airbag, and the neck experiences combined lateral bending and axial rotation.

Two parameters govern how severe this chain becomes: the intrusion velocity of the door and the initial lateral clearance between the occupant and the door trim. Higher intrusion velocity shortens the time available for the airbag to deploy and for the occupant to be coupled to the restraint. Smaller initial clearance means the occupant is loaded earlier and with less opportunity for the torso to rotate away from the load.

From kinematics to clearance requirements

The clearance budget is the practical bridge between biomechanics and package design. It can be written as a simple accounting identity:

C₀ = C_intrusion + C_occupant + C_margin

Here C₀ is the initial lateral gap between the occupant's struck-side surface and the door trim, C_intrusion is the maximum door intrusion into the occupant compartment, C_occupant is the lateral excursion of the occupant's torso or head during the event, and C_margin is the residual gap that must remain to avoid hard contact or to allow the airbag to function as intended. Each term is a design variable: C_intrusion is set by the door structure and the barrier, C_occupant is set by the restraint and seat, and C₀ is set by the package.

Consider a worked example. A vehicle is evaluated against a moving deformable barrier at 50 km/h. CAE predicts a maximum door intrusion of 180 mm at the armrest height. The occupant's chest is predicted to displace 90 mm laterally relative to the seat before the side airbag couples it. If the design target is a 40 mm residual margin to keep the chest from hard contact with the intruding trim, the required initial clearance is:

C₀ = 180 + 90 + 40 = 310 mm

If the actual package provides only 260 mm, the deficit of 50 mm must be recovered by reducing intrusion (stiffer door structure or a different barrier load path), reducing occupant excursion (earlier airbag coupling, a seat-mounted side airbag, or a more aggressive bolster), or accepting a smaller margin and validating that the resulting chest deflection and VC remain within the chosen criteria. This is the core trade-off of side-impact clearance development: the package, the structure, and the restraint must be balanced against the same 50 mm.

Practical assessment workflow

A defensible clearance assessment follows a repeatable sequence. First, define the reference configuration: dummy or human body model, seat position, H-point, and the barrier or sled pulse. Second, extract the time histories of door inner-panel intrusion at the armrest, chest, and head heights, together with the occupant's lateral displacement at the same heights. Third, compute the instantaneous clearance as the difference between the door surface position and the occupant surface position, and identify the minimum value and its timing. Fourth, compare the minimum clearance and the corresponding injury metrics against the criteria. Fifth, if the margin is negative or the criteria are exceeded, attribute the deficit to intrusion, occupant excursion, or restraint timing, and iterate on the responsible subsystem.

A useful diagnostic is to plot clearance against time alongside chest deflection and VC. If clearance goes negative before the airbag pressure rises, the problem is deployment timing, not package. If clearance remains positive but deflection is high, the problem is load distribution or restraint stiffness. If clearance is negative at peak intrusion but the airbag is already deployed, the problem is structural intrusion and the door or barrier load path must change. This attribution step is what turns a failed metric into an actionable design change.

The biomechanical criteria, the kinematic chain, and the clearance budget are therefore not separate topics. They form a single chain of reasoning: the criteria define what must be avoided, the kinematics explain how the occupant reaches the injurious condition, and the clearance budget converts that understanding into a package and restraint requirement that can be designed, simulated, and validated.