3. Barrier geometry and mass parameters
Side impact test outcomes are governed less by the vehicle than by the barrier that strikes it. A rigid pole concentrates energy into a narrow contact patch; a moving deformable barrier (MDB) spreads load across a honeycomb face and adds its own crush behaviour; a trolley is the carrier that determines mass, velocity, and impact angle. Before any acceptance check can be interpreted, the geometry and mass parameters of these three elements must be fixed, documented, and verified against the governing standard. This section defines the parameters, shows how they interact, and provides a verification workflow with worked example values.
Why geometry and mass are coupled
Impact severity in a side test is not a single number. It is the product of three independent parameter families: the barrier's mass (which sets momentum), its velocity (which sets kinetic energy), and its contact geometry (which sets the load distribution and the local deformation mode of the struck vehicle). A pole with a 254 mm diameter and a 30° oblique approach produces a fundamentally different occupant loading than a 1300 kg MDB with a 1500 mm wide honeycomb face at 50 km/h. Changing any one parameter without re-baselining the others invalidates the comparison. This is why the standard fixes all three families together, and why the test house must verify each one independently before a run is accepted.
Rigid pole: geometry dominates
The rigid pole is the simplest barrier and the most sensitive to geometric error. Its defining parameters are the pole diameter, the pole's vertical orientation (perpendicular to the ground plane), the longitudinal and lateral position of the pole axis relative to the vehicle reference point, and the impact angle. A typical side pole test uses a 254 mm ± 3 mm diameter pole, rigidly anchored, with the pole axis vertical and the vehicle towed at an angle so that the pole strikes the driver or passenger side at a defined point. Because the pole does not deform, all the crush energy is absorbed by the vehicle's side structure over a narrow contact width — roughly the pole diameter plus the local structural wrap. A 3 mm diameter error is a 1.2% change in contact width, which is small; but a 10 mm error in the lateral position of the pole axis relative to the vehicle centreline can shift the contact point onto a different structural member and change the occupant response materially. The pole's mass is effectively infinite (ground-anchored), so the parameter that matters is position, not mass.
Moving deformable barrier: mass, face geometry, and crush
The MDB is the more complex object. Its parameters fall into three groups. First, total mass: the barrier plus trolley assembly is specified to a tolerance, commonly around 1300 kg ± 20 kg for a typical side impact MDB, though the exact value is standard-dependent and must be taken from the governing document rather than assumed. Second, face geometry: the honeycomb impact face has a defined width, height, and contour, and the honeycomb's crush strength is specified so that the barrier deforms in a repeatable way. Third, the barrier's own stiffness: the honeycomb is not rigid, so part of the impact energy is absorbed by the barrier itself, which is why the MDB cannot be treated as a point mass.
The practical consequence is that MDB verification requires both a mass check and a geometry check. Mass is verified by weighing the complete barrier-and-trolley assembly on a calibrated scale, with the honeycomb face installed and all ballast in place. Geometry is verified by measuring the face width and height, the face contour against a template, and the height of the face centre above the ground. A barrier that passes the mass check but has a deformed or mis-mounted honeycomb face will produce a non-representative load distribution even though the momentum is correct.
Trolley: the carrier that sets the impact vector
The trolley carries the MDB (or, in pole tests, carries the vehicle) and determines the impact vector. Its parameters are the trolley mass (often included in the barrier mass figure, but sometimes specified separately), the track gauge and wheel condition, the tow velocity at the moment of impact, and the alignment of the trolley path relative to the vehicle. Velocity is the most tightly controlled parameter: a typical side impact specifies an impact velocity with a tolerance of about ± 1 km/h, measured at a defined point just before contact. Alignment errors — a trolley path that is not parallel to the intended impact direction — introduce a lateral velocity component that changes the effective impact angle and is a common cause of rejected runs.
Parameter summary and interaction
| Element | Key geometry parameters | Key mass/velocity parameters | Dominant error mode |
|---|---|---|---|
| Rigid pole | Diameter (e.g. 254 mm ± 3 mm), verticality, axis position relative to vehicle, impact angle | Effectively infinite (ground-anchored); vehicle tow velocity | Lateral position of pole axis |
| MDB | Face width/height, face contour, face centre height | Combined mass (e.g. 1300 kg ± 20 kg), honeycomb crush strength | Honeycomb face deformation or mis-mounting |
| Trolley | Track gauge, path alignment, wheel condition | Trolley mass, impact velocity (e.g. ± 1 km/h) | Path misalignment introducing lateral velocity |
The figure below shows how the three elements combine into the impact vector and where each verification measurement is taken.
Verification workflow with worked example values
The following sequence is a practical pre-run check. Values are illustrative and must be replaced by the governing standard's limits.
- Weigh the complete MDB-and-trolley assembly. Place the assembly on a calibrated platform scale with the honeycomb face installed and all ballast secured. Record the reading. Example: scale reads 1302 kg against a 1300 kg ± 20 kg window — pass. If the reading is 1285 kg, the assembly is 15 kg light; locate the missing ballast before proceeding, because a light barrier under-delivers momentum.
- Measure the honeycomb face. Use a contour template and a steel rule. Record face width, face height, and face-centre height above ground. Example: width 1500 mm, height 500 mm, centre height 600 mm — compare each against the standard's nominal and tolerance. A face that is bowed by more than a few millimetres will not load the vehicle symmetrically.
- Check pole geometry (pole tests). Measure pole diameter at three heights with a calliper; verify verticality with a spirit level or plumb line; measure the lateral offset of the pole axis from the vehicle centreline at the intended contact height. Example: diameter readings 253.8, 254.1, 254.0 mm — within ± 3 mm; lateral offset 2 mm from nominal — acceptable if the standard's positional tolerance is, say, ± 10 mm.
- Verify trolley velocity and alignment. Confirm the tow system's set velocity against the standard, and check that the trolley path is parallel to the intended impact direction using a laser or string line along the track. Example: set velocity 50 km/h, measured at the pre-contact point within ± 1 km/h; path deviation 3 mm over 2 m — acceptable.
- Record and sign off. Log every measured value against its nominal and tolerance. A run should not proceed if any parameter is outside tolerance, regardless of how small the deviation appears.
Geometry and mass parameters are the foundation of side impact acceptance. The pole is governed by position and diameter, the MDB by combined mass and face geometry, and the trolley by velocity and path alignment. Each must be measured independently, recorded against tolerance, and signed off before the run. When a result is questioned, these three parameter families are the first place to look, because they set the input to everything downstream.