4. Ride-down space metrics

Ride-down performance is ultimately judged by how much room the occupant still has, and how that room changes while the vehicle is still moving. Three metric families cover this: occupant compartment intrusion (how far structure moves into the occupant's envelope), survival space (the residual volume or clearance that must remain unobstructed), and dynamic excursion measures (how far the occupant and the restraint system travel relative to the interior during the event). These are not interchangeable. A body structure can show modest static intrusion yet produce a large dynamic excursion because the occupant loads the belt and moves forward before the restraint catches. Conversely, a stiff structure with small intrusion can still fail a survival-space criterion if the occupant's kinematics drive the head into a pillar or roof rail.

The metrics only make sense against a defined reference frame. In this course we adopt the vehicle coordinate system: x positive forward, y positive to the occupant's right, z positive upward, origin at the vehicle datum used by the CAE model. Intrusion is reported as a displacement of structure along the direction of the impact, measured at named reference points. Excursion is reported as occupant or restraint displacement in the same frame, so the two can be compared directly. Any switch of frame — for example reporting occupant motion in a sled frame — must be stated explicitly, because sign conventions flip and comparisons become meaningless.

Intrusion: measuring what the structure gives up

Intrusion is a structural response, not an occupant response. It is measured at discrete intrusion reference points placed on the occupant-side surfaces: the toe pan, the brake pedal, the instrument panel lower edge, the steering column, the door trim at hip and shoulder height, the B-pillar, and the roof rail. Each point has a nominal position in the undeformed vehicle; the metric is the peak displacement of that point along the impact direction, plus the time at which the peak occurs.

A practical measurement workflow on a CAE model or a physical test uses the same logic:

1. Define the reference points on the undeformed geometry and record their nominal coordinates.
2. Track each point through the event at the model or sensor output rate (typically 10 kHz for physical tests, matching the CAE output interval).
3. Compute displacement per point as the vector difference from nominal, then project onto the impact axis.
4. Report peak value, time of peak, and residual (permanent) displacement after the event settles.

For example, a side-impact model might report a B-pillar inner point at hip height with a peak intrusion of 145 mm at 42 ms and a residual of 118 mm. The peak tells you the worst-case envelope violation; the residual tells you how much permanent deformation the occupant must live with during extraction. Both matter, and reporting only the peak hides the residual problem.

Survival space: the envelope that must stay clear

Survival space is the region the occupant must be able to occupy without injurious contact. It is defined as a clearance envelope around the occupant's seated position, bounded by the interior surfaces. The metric is the minimum clearance between the occupant envelope and any intruding or displacing surface, evaluated over the whole event, not just at the end.

Two conventions are common and must be chosen deliberately. The first is a static survival space: the clearance measured after the event, with the occupant at the design H-point. The second is a dynamic survival space: the clearance measured continuously while the occupant moves under restraint loading. The dynamic version is the one that correlates with injury, because the head and chest are moving while the structure is still deforming. A vehicle can pass a static check and fail dynamically when the head arrives at the pillar exactly as the pillar reaches its peak intrusion.

Dynamic excursion: occupant and restraint travel

Dynamic excursion measures how far the occupant and the restraint system move relative to the interior. Three quantities are typically tracked:

• Occupant excursion — displacement of a body-region marker (head CG, chest, pelvis) from its initial position, projected onto the impact axis.
• Restraint excursion — belt payout or airbag deployment travel, i.e. how much the restraint system extends before it arrests the occupant.
• Relative excursion — the difference between the two, which is the actual gap the occupant closes against the interior.

These are read together. A large occupant excursion with a small restraint excursion means the restraint is stiff and the occupant is being stopped abruptly — good for space, potentially bad for loading rate. A large restraint excursion with a small occupant excursion means the restraint is compliant and absorbing energy over distance — good for ride-down, but it consumes space. That tradeoff is the heart of this course, and the excursion metrics are how you quantify it.

The figure makes the coupling visible. In panel B, the intrusion curve peaks near 42 ms, the minimum-clearance curve bottoms out near 45 ms, and the occupant excursion is still rising at that moment. The clearance minimum is the number that matters, and it occurs after peak intrusion because the occupant is still closing the gap. If you only report peak intrusion, you miss the worst clearance condition entirely.

Working the numbers: a reproducible example

Consider a frontal sled model with a 50th-percentile male occupant, a three-point belt with a load limiter, and a 40 g, 120 ms pulse. Suppose the model reports the following at the driver position (values are illustrative, not measured):

Read the table as a chain. The chest excursion of 180 mm minus the belt payout of 120 mm gives a 60 mm relative excursion — which matches the minimum dynamic clearance of 60 mm at 48 ms. That consistency is the check: if the relative excursion and the minimum clearance disagree, one of the reference frames or marker definitions is wrong. The 60 mm margin is thin; a design change that adds 30 mm of belt payout would consume half of it, which is exactly the ride-down-versus-space tradeoff the next chapters quantify.

Reporting discipline

Three habits keep these metrics usable. First, always report peak value, time of peak, and residual for intrusion; a single number is not a metric, it is a headline. Second, always report the minimum clearance and its timing relative to peak intrusion, because the worst clearance is a dynamic event, not a final state. Third, always report occupant excursion, restraint excursion, and their difference together, because the difference is the physical gap the occupant actually closes. With those three habits, the metrics become a consistent basis for the energy-management tradeoffs that follow.