4. Effect of Pole Diameter on Far-Side Head Injury in Side Impact

Why Pole Diameter Matters for the Far-Side Occupant

In a side pole impact, the pole is a narrow, rigid barrier. Its diameter controls two competing effects: how much of the vehicle's side structure the pole engages, and how concentrated the contact load becomes. A larger pole spreads the load over a wider patch of the door and B-pillar, which tends to reduce local intrusion velocity but increases the total mass of structure pushed into the occupant compartment. A smaller pole concentrates the load, producing deeper local intrusion and a sharper acceleration pulse at the struck side. For a far-side occupant — the person seated opposite the struck side — the injury mechanism is dominated by the occupant being thrown across the cabin toward the struck side, loading the far-side shoulder belt and eventually the head against the intruding structure or the near-side interior. Pole diameter therefore changes both the timing and the severity of the head contact, not just the intrusion depth.

This case isolates that variable. Three pole diameters — 200 mm, 250 mm, and 300 mm — are run against an otherwise identical side pole impact model, and the far-side head injury metrics are compared. The goal is not to declare one diameter "worst" in the abstract, but to show how geometry shifts the head contact timing and the resulting HIC and peak head acceleration, so the result can be interpreted as a design sensitivity rather than a single number.

Model Setup and the Parameters That Must Stay Fixed

The comparison is only meaningful if everything except the pole diameter is held constant. The table below lists the fixed inputs used for this example. These are teaching values chosen to be representative of a mid-size sedan side pole model; they are not measured results from a specific test.

Two modeling details deserve attention because they are common sources of non-comparable results. First, the pole must be positioned so that its centerline strikes the same longitudinal station on the vehicle for all three diameters; otherwise the diameter change is confounded with an impact-location change. Second, the contact definition between pole and door must use the same friction coefficient and the same penalty stiffness scaling across runs, since a stiffer contact on the smaller pole can artificially sharpen the pulse.

Running the Diameter Sweep

The sweep is a simple parametric loop over the pole radius in the model's include file. In a typical LS-DYNA deck the pole is defined as a rigid cylinder, so the diameter is set by the radius of the cylinder primitive and, if a mesh is used, by regenerating the pole mesh at the new radius. The key is to change the geometry, not to scale the whole pole part, because scaling would also change the pole length and mass distribution.

# pole_diameter_sweep.k — driver include, one run per diameter
*PARAMETER
Rdia    200.0        # pole radius in mm; sweep 100, 125, 150
*INCLUDE
../model/vehicle_side_pole.k
../model/occupant_far_side.k
../model/pole_rigid.k
*CONTROL_TERMINATION
0.200
*DATABASE_HISTORY_NODE
# far-side head CG node and near-side head-contact node IDs
100201  100305
*DATABASE_BINARY_D3PLOT
0.002

For each run, set Rdia to 100, 125, and 150 mm (giving 200, 250, and 300 mm diameters), keep the pole centerline at the same vehicle station, and confirm that the pole mesh is regenerated rather than scaled. After each run, extract from the head CG node the resultant acceleration time history and compute HIC36 using the standard 36 ms window. Also record the time of peak head acceleration and the time of first head contact with the near-side interior, since these are the timing metrics that reveal how geometry shifts the event.

Reading the Results: Severity and Timing Together

The figure below shows the expected qualitative relationship between pole diameter and the far-side head response. It is a schematic of the mechanism, not a plot of measured data: as diameter decreases, the intrusion pulse becomes sharper and the head contact occurs earlier and with higher peak acceleration, while a larger diameter delays contact and lowers the peak but lengthens the loading duration.

Two metrics should be tabulated for each run: HIC36 and the time of peak head acceleration. The expected pattern is that the 200 mm pole produces the highest HIC and the earliest peak, the 300 mm pole the lowest HIC and the latest peak, with the 250 mm case falling between them. If a run violates that monotonic ordering, the cause is usually a contact or initialization artifact rather than a genuine geometric effect, and the run should be inspected before the trend is reported.

Interpreting the Trade-off

The diameter effect is not a simple "smaller is worse" story. The smaller pole concentrates load and produces a sharper intrusion pulse, which drives the far-side occupant across the cabin faster and loads the head earlier and harder. But the larger pole engages more of the side structure, which can increase total intrusion depth and change where the head finally contacts. In practice, the far-side head injury metric is governed by the combination of intrusion velocity at the occupant's head height and the timing of the head's lateral excursion relative to the intruding structure. A diameter sweep that reports only HIC without the contact timing will miss the mechanism and may lead to the wrong countermeasure — for example, tuning belt force limiting when the real driver is intrusion timing.

For this reason, the deliverable from the sweep is a small comparison table rather than a single "worst diameter" claim. Record HIC36, peak head acceleration, time of peak, and time of first head contact for each diameter, and note whether the head contacts the near-side interior, the far-side belt, or both. That table is what makes the result usable in the next design step, where restraint parameters are optimized against the critical diameter identified here.

Quick Reference for the Sweep

• Variable: pole radius 100 / 125 / 150 mm (200 / 250 / 300 mm diameter).
• Fixed: impact velocity 32 km/h, 90° lateral, rigid pole, same longitudinal station, same contact friction and stiffness scaling.
• Extract per run: HIC36, peak head resultant acceleration, time of peak, time of first head contact.
• Sanity check: repeat one diameter identically; if results differ, fix model non-determinism before comparing diameters.
• Expected trend: smaller diameter → earlier and higher peak; larger diameter → later and broader. Non-monotonic results indicate a modeling artifact, not a geometric finding.