📚 Course ContentsChapter 3 Set Up Vehicle-to-MPDB Alignment and Contact Definitions

3. Set Up Vehicle-to-MPDB Alignment and Contact Definitions

Alignment Strategy: 50% Overlap and 0° Impact Angle

In the Euro NCAP MPDB frontal test, the vehicle is driven into a stationary, deformable barrier mounted on a moving trolley. The prescribed configuration is a 50% overlap with a 0° impact angle, meaning the vehicle's longitudinal centerline is offset laterally from the barrier centerline by exactly half the vehicle width, and the vehicle's heading is parallel to the barrier face normal. Getting this alignment wrong is one of the most common causes of non-representative results, because a small lateral offset or yaw error changes which structural members engage the barrier and how the load is distributed across the barrier face.

The cleanest way to enforce alignment in a solver model is to define it through a small number of explicit reference entities rather than by eyeballing coordinates. A robust setup uses:

A vehicle reference frame anchored at the vehicle's front bumper center, with +X forward, +Y to the vehicle's left, +Z up.
A barrier reference frame anchored at the barrier face center, with +X pointing toward the approaching vehicle (i.e., opposite the vehicle's travel direction), +Y lateral, +Z up.
A single alignment transform that maps the vehicle frame into the barrier frame, parameterized by lateral offset dy, longitudinal gap dx, and yaw theta.

For a 50% overlap, the lateral offset is dy = W_vehicle / 2, applied so that the vehicle's centerline sits half a vehicle width to one side of the barrier centerline. The impact angle is theta = 0°, so no rotation is applied. The longitudinal gap dx is a modeling choice: it must be small enough that the vehicle reaches the barrier within the simulated event window at 50 km/h, but large enough that no initial contact exists. A typical value is 20–50 mm of clearance between the front bumper and the barrier face.

Define the overlap sign convention once and document it. If +Y is the vehicle's left, then a positive dy shifts the vehicle to the left of the barrier centerline. Whichever side you choose, keep the same convention in the geometry, the contact definitions, and any post-processing scripts that measure overlap.

Contact Definitions Between Vehicle Front Structure and Barrier Face

Once the geometry is positioned, the contact interface must be defined so that the vehicle's front structure (bumper beam, crash boxes, longitudinal rails, hood, fenders) interacts with the barrier face and nothing else. In most explicit solvers this is done with a surface-to-surface contact definition using a node set or segment set on the vehicle side and a segment set on the barrier face side.

A practical contact setup for this case includes:

Contact pair	Vehicle side	Barrier side	Purpose
Primary frontal	Front bumper, crash boxes, rail tips, hood leading edge	Barrier face segments	Carries the main impact load
Secondary / wrap-around	Fenders, wheel arches, lower rocker front	Barrier face segments	Captures oblique engagement at 50% overlap
Self-contact (optional)	Vehicle front structure	Vehicle front structure	Prevents interpenetration during crush

Use a penalty or kinematic contact formulation consistent with the rest of the model, set a friction coefficient appropriate to the barrier face material (often 0.3–0.5 for the deformable face), and enable thickness offsets so that shell-based vehicle parts and solid barrier cells interact at their physical surfaces rather than at their reference surfaces. If the barrier face is modeled with solid cells, make sure the contact segment set covers the full face, including the edges, because at 50% overlap the vehicle's outer corner can slide past the face boundary.

Validating the Assembled Model

Before running the full impact, perform a short initialization check. The goal is to confirm three things: the alignment is correct, no initial penetrations exist, and every intended contact pair is active.

Geometry check. Query the minimum distance between the vehicle front structure and the barrier face. It should equal the intended dx clearance (e.g., 30 mm) within modeling tolerance. A negative value indicates initial penetration.
Overlap check. Project the vehicle's front silhouette onto the barrier face plane and measure the lateral overlap. It should be 50% of vehicle width within a few millimeters.
Contact activation check. Request a contact initialization report or a list of active contact pairs at t = 0. Every pair in the table above should appear with zero or near-zero penetration; missing pairs usually mean a node set or segment set was not assigned.
Angle check. Confirm the vehicle's heading vector is parallel to the barrier face normal. A nonzero dot product indicates an unintended yaw.

A common failure mode is a contact that is defined but silently inactive because the vehicle-side segment set references parts that were renamed or re-meshed after the contact was created. Always re-run the initialization check after any geometry update, and treat a missing contact pair as a hard error rather than a warning.

Practical Checklist for This Setup Step

The following items should all pass before the model is handed off to the impact run. Treat each as a binary gate rather than a soft check.

Vehicle front bumper center is offset laterally from the barrier centerline by W_vehicle / 2 within ±2 mm.
Vehicle heading vector is parallel to the barrier face normal within ±0.1°.
Minimum clearance between vehicle front structure and barrier face is positive and matches the intended dx.
All three contact pairs (primary frontal, secondary wrap-around, optional self-contact) are present in the initialization report.
No contact pair reports initial penetration greater than the solver's warning threshold.
Barrier face segment set covers the full face, including edges, so the vehicle's outer corner cannot slide past the boundary at 50% overlap.

If any item fails, correct the geometry or contact definition and re-run the initialization check. Do not proceed to the full impact simulation with a known alignment or contact defect, because the resulting kinematics will be non-representative and difficult to diagnose after the fact.