3. Crash pulse characterization for far-side events

Far-side occupant protection is fundamentally a problem of managing relative motion between the occupant and the vehicle interior. Unlike the near-side case, where the struck-side structure and side airbag dominate the occupant's immediate environment, the far-side occupant is separated from the impact by the entire vehicle cabin. The crash pulse that reaches that occupant is therefore filtered, delayed, and reshaped by the body structure, the seats, and the restraint system. Characterizing that pulse correctly is the prerequisite for choosing any countermeasure — a far-side airbag, a seat-integrated restraint, or a belt pretensioning strategy — because the pulse determines both the magnitude and the timing of the loading the occupant will experience.

Pulse Shape: What the Far-Side Occupant Actually Feels

In a barrier or pole impact, the vehicle's longitudinal or lateral deceleration is measured at a structural location such as the rocker or the tunnel. That measurement is the crash pulse: acceleration versus time, usually expressed in g and integrated to give velocity change. For far-side events, the pulse of interest is the lateral component at the occupant's seating position, and it differs from the structural pulse in three important ways.

First, the pulse is delayed. The impact energy must travel through the struck-side structure, across the floor and cross-car members, and into the seat mounting points before the far-side occupant begins to accelerate. A typical delay of 10–25 ms is common in passenger cars for a pole impact at 32 km/h, depending on body stiffness and the distance from the impact point. Second, the pulse is attenuated in peak amplitude — the far-side rocker or seat track may see only 40–70% of the peak acceleration measured at the struck-side sill. Third, the pulse is reshaped: high-frequency structural ringing is filtered out, and the remaining signal is dominated by the lower-frequency rigid-body motion of the occupant compartment.

These three effects mean that a far-side occupant is not simply a "softer" version of a near-side occupant. The delayed onset changes the phase relationship between the occupant's inertial motion and the vehicle's motion, which in turn changes when and where the occupant loads the far-side interior — the center console, the opposite door trim, or the opposite B-pillar.

When comparing pulses across vehicles, always confirm the measurement location and the filter class (e.g., SAE J211 CFC 60 or CFC 180). A pulse measured at the struck-side sill and one measured at the far-side seat track are not interchangeable, and mixing them will corrupt any delta-v or loading estimate.

Delta-v: The Integrated Quantity That Drives Occupant Kinematics

Delta-v is the time integral of the acceleration pulse over the crash duration. For a far-side event, the relevant delta-v is the lateral velocity change of the occupant compartment at the seating position. It is the single most useful scalar for comparing events, because occupant excursion and the resulting interior contact velocity scale approximately with delta-v, not with peak acceleration.

Consider a worked example. A vehicle is subjected to a 32 km/h (8.89 m/s) lateral pole impact. The struck-side sill pulse peaks at 45 g with a duration of about 90 ms. At the far-side seat track, the measured pulse peaks at 26 g, is delayed by 18 ms, and integrates to a lateral delta-v of approximately 6.2 m/s — about 70% of the struck-side delta-v, the remainder having been absorbed by structure and by the rotation of the vehicle about its vertical axis. These numbers are illustrative of a mid-size sedan and should be treated as a teaching example, not as a validated dataset.

The practical consequence is direct: a far-side occupant in this event experiences a lateral velocity change of roughly 6 m/s relative to the ground, while the vehicle interior on the far side is moving with the occupant compartment. The occupant's relative velocity toward the far-side interior is therefore governed by how quickly the occupant is coupled to the vehicle — through the belt, the seat, and any far-side countermeasure — versus how quickly the compartment itself decelerates or rotates.

Implications for Occupant Loading

The far-side loading mechanism is a two-stage sequence. In the first stage, the occupant's torso lags the compartment because the belt and seat friction cannot instantly transfer the compartment's deceleration. The occupant translates laterally across the seat toward the far-side interior. In the second stage, the occupant either contacts the far-side interior (console, door, B-pillar) or is caught by a far-side restraint. The severity of that second stage depends on the relative velocity at contact, which is a function of delta-v, pulse duration, and the restraint's engagement timing.

A short, high-amplitude pulse produces a large relative velocity early and a violent contact; a longer, lower-amplitude pulse of the same delta-v produces a smaller relative velocity but a longer excursion, which may allow the occupant to reach the far-side interior before any restraint can act. This is why pulse shape, not just delta-v, matters for countermeasure selection: a far-side airbag sized for a short pulse may deploy too late for a long, slow pulse, and a seat-integrated restraint tuned for one shape may be mistimed for another.

Far-Side Pulse: Delay, Attenuation, and Delta-v time (ms) a (g) 0 20 40 60 80 100 15 30 45 Struck-side sill pulse (peak ≈ 45 g) Far-side seat-track pulse (peak ≈ 26 g) onset delay ≈ 18 ms ∫a dt ≈ 6.2 m/s (far-side Δv) structural pulse far-side pulse integrated Δv

Reading the Pulse for Countermeasure Selection

With the pulse characterized, the countermeasure decision becomes tractable. Three pulse-derived quantities drive the choice:

  • Time-to-peak and onset delay set the deployment window. A far-side airbag must be triggered early enough that it is inflated before the occupant's torso crosses the seat centerline. If the far-side pulse peaks at 40–50 ms after impact and the occupant reaches the console at 70–90 ms, the airbag has roughly 30–50 ms of usable window — tight but feasible with a side-impact sensor and a fast inflator.
  • Peak amplitude and delta-v set the required restraint energy. Higher delta-v means higher relative velocity at contact and therefore a need for more restraint force or more stroke. A seat-integrated far-side restraint with a load limiter can be tuned to the delta-v band the vehicle actually produces.
  • Pulse duration sets the excursion. A long-duration pulse gives the occupant more time to travel laterally, so the restraint must engage over a longer stroke or the seat must provide earlier lateral support.

A practical workflow is to extract the far-side seat-track pulse from a validated full-vehicle model or a sled test with a far-side buck, integrate it to delta-v, and then run a simplified occupant model (a rigid-body or MADYMO-type model) to estimate torso excursion and contact velocity. The output of that step is a loading envelope: the range of relative velocities and contact locations the countermeasure must handle. Countermeasure selection then becomes a matching problem — does the candidate restraint engage within the envelope, and does it absorb the energy without introducing its own loading penalty?

Do not select a far-side countermeasure from delta-v alone. Two events with identical delta-v but different pulse shapes can produce occupant contact velocities that differ by 30–50%, because the relative velocity depends on the phase between occupant motion and compartment motion. Always pair the delta-v with the pulse shape and the onset timing.

Practical Characterization Checklist

The following steps form a reproducible characterization sequence for a far-side event, using a full-vehicle model or test data as input:

  1. Identify the measurement location: far-side seat track or far-side rocker, with the sensor axis aligned to the lateral direction. Record the filter class (CFC 60 is typical for occupant-relevant pulses).
  2. Extract the acceleration time history over the crash duration (typically 0–150 ms for a pole impact).
  3. Integrate to obtain velocity; subtract the initial velocity to get delta-v. Confirm the integration converges (the velocity curve should plateau).
  4. Record onset delay relative to the struck-side sill pulse, peak acceleration, time-to-peak, and pulse duration at 10% of peak.
  5. Feed the pulse into an occupant model and extract torso lateral excursion and contact velocity at the far-side interior.
  6. Compare the resulting loading envelope against candidate countermeasure engagement windows and energy absorption capacity.

If the integrated velocity does not plateau, the pulse record is truncated or the filter has removed low-frequency content; re-extract with a longer window or a lower filter class before proceeding. If the far-side peak exceeds the struck-side peak, suspect a measurement location error or a structural resonance — this is physically unusual and should be investigated before the pulse is used for countermeasure sizing.

The characterization step is where far-side protection succeeds or fails. A countermeasure chosen against a mischaracterized pulse — wrong location, wrong filter, or delta-v without shape — will either deploy too late, over-restrain the occupant, or miss the excursion entirely. Getting the pulse right, and reading it as a shape-and-integral pair rather than a single number, is what makes the subsequent countermeasure selection defensible.

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