3. Conflicting pulse scenarios

Occupant protection targets are rarely derived from a single, clean crash pulse. In real-world collisions the restraint system must satisfy a family of loading conditions whose kinematics pull the design in opposite directions. A restraint that is optimal for a 40% offset frontal impact — generous ride-down, early belt force limiting, moderate airbag pressure — can be too soft for a small overlap event and too aggressive for a low-speed rear impact. This chapter examines the five pulse families that most often impose competing requirements, and shows how to reason about the trade-offs quantitatively rather than by intuition.

Why the pulses conflict

The core conflict is between energy absorption and occupant excursion. Frontal pulses deliver large longitudinal velocity change (Δv) over a relatively long crush distance, so the restraint can spread deceleration over 80–120 ms. Small overlap and oblique pulses concentrate loading on a narrow structural path, producing a short, steep pulse with high peak acceleration and significant lateral and yaw components. Side impact compresses the available ride-down to 20–40 ms because the struck-side structure has almost no crush space. Rear impact is dominated by seat and head restraint behaviour at low Δv, where the belt and frontal airbag contribute little.

Each pulse therefore defines a different "target envelope": the combination of chest deflection, head excursion, neck load, and pelvis motion that the restraint must keep inside injury thresholds. The same hardware cannot sit at the centre of every envelope, so the calibration task is to find a set of parameters that satisfies all of them simultaneously — or to introduce adaptive elements that change behaviour per pulse.

Frontal offset: the baseline ride-down case

A 40% offset deformable-barrier test (the classic Euro NCAP / IIHS-style configuration) engages one longitudinal rail and the engine cradle, giving a crush stroke of roughly 500–700 mm. The resulting pulse is comparatively gentle: peak sled deceleration around 30–40 g, duration 90–110 ms, Δv near 56 km/h. The restraint target is dominated by chest deflection (keep below ~50 mm for a Hybrid III 50th percentile) and femur load. Here a force-limited retractor plus a dual-stage airbag with a low-power first stage works well: the belt pays out at 3.5–4.5 kN to let the chest ride down, and the airbag vent is sized so that head excursion stays within the interior.

The trap is that this calibration is too soft for the next two pulses. If the belt force limiter is set at 4 kN to protect the chest in offset, the same limiter will allow excessive forward excursion when the pulse is shorter and steeper.

Small overlap and oblique: short, steep, and asymmetric

In a 25% small overlap impact the barrier misses the main rail and loads the wheel, suspension, and hinge pillar. The occupant compartment decelerates over a much shorter distance, producing a pulse with peak acceleration often above 50 g and duration of only 50–70 ms. Because the pulse is short, the restraint must act earlier and harder: pretensioner firing thresholds drop, the airbag must fill faster, and belt force limiting must be raised or made load-path-dependent. The competing requirement is now the opposite of the offset case — the system needs a stiffer, earlier restraint to prevent the head from striking the A-pillar or the airbag from bottoming out.

Oblique impacts (15–30° barrier angle) add a lateral velocity component of 8–15 km/h and a yaw rate to the occupant. The occupant slides outboard relative to the belt, so the shoulder belt may slip off the clavicle and the head moves toward the side window. This is where the side curtain airbag and a belt with a lap-preload or "belt-in-seat" geometry become decisive. A restraint tuned only for pure frontal loading will show high head lateral excursion and elevated neck lateral bending in the oblique test.

Side impact: the compressed-time problem

Side impact is fundamentally different because there is almost no crush space between the striking vehicle and the occupant. The struck door intrudes within 20–40 ms, so the restraint has no time for a long ride-down. The dominant countermeasures are structural (B-pillar and door beam stiffness) and inflatable (side thorax and curtain airbags), not the belt. The belt's role is limited to keeping the occupant coupled to the seat so the side airbag can act on the correct body region. A restraint calibration that relies on belt force limiting for chest protection is largely irrelevant here; instead the target is to fire the side airbag within 5–10 ms of impact detection and to keep the curtain inflated long enough to cover the window during rebound.

The conflict with frontal tuning appears in the sensing and firing logic: a side impact produces a large lateral acceleration at the B-pillar sensor but little longitudinal signal, so the frontal airbag must not fire, while the side airbag must fire almost immediately. Calibration of the crash sensor thresholds is therefore part of the restraint target, not a separate activity.

Rear impact: low Δv, different injury mechanism

Rear impacts at 16–24 km/h Δv produce a long, low-amplitude pulse (100–150 ms) and are dominated by the relative motion between the head and torso — the whiplash mechanism. The belt and frontal airbag contribute little; the seatback, head restraint geometry, and seat recliner stiffness govern the outcome. The restraint requirement here is essentially a non-requirement for the frontal system, but a strong requirement for the seat: head restraint backset under 55 mm and top height above the head's centre of gravity. The conflict is that a seat stiffened for rear impact can worsen the frontal case by changing occupant kinematics, so seat and restraint must be calibrated together.

Reconciling the requirements

The practical resolution is rarely a single compromise calibration. Modern systems use adaptive restraint control: occupant weight and position sensing, multi-stage inflators, switchable force limiters, and pulse-aware firing maps. A representative strategy is summarised below; the values are illustrative engineering targets, not measured results.

When calibrating, treat the five pulses as a constraint set rather than a sequence. Start from the small overlap case because it sets the earliest firing and highest force limit; then verify that the offset case still keeps chest deflection inside its corridor by switching to the lower force-limit stage. Check the oblique case for lateral head excursion and adjust curtain coverage or belt routing. Confirm that side-impact sensing does not cross-fire the frontal airbag, and finally verify the rear-impact seat behaviour independently. The output of this loop is not a single number but a firing map and a set of stage thresholds that remain valid across the pulse family — which is exactly the occupant protection target the rest of this course builds on.