4. Vehicle front-end geometry modeling

In pedestrian impact simulation, the vehicle front-end is not a cosmetic shell — it is the primary contact surface that determines where the pedestrian's lower limb, pelvis, and head are loaded, how those loads are distributed, and how the body is rotated before it reaches the ground. A MADYMO model that represents the bumper as a single rigid box and the hood as a flat plate will produce leg bending moments and head impact times that are physically meaningless. This chapter explains how to build a front-end geometry that captures the load paths that actually matter, using the three dominant sub-systems: bumper, hood, and grille.

4.1 What the Front-End Must Represent

Before any geometry is drawn, decide which physical behaviours the model must reproduce. For a pedestrian impact, the front-end must answer four questions:

• Where does first contact occur? The bumper leading edge and its vertical position relative to the knee joint determine whether the leg is struck above or below the knee.
• How much energy is absorbed before the leg is pushed rearward? Bumper foam, fascia, and the underlying beam act as a spring in series.
• How does the upper body rotate? The hood leading edge and grille contact the pelvis and abdomen, converting linear momentum into rotation.
• Where does the head land? Hood outer panel stiffness and the gap to the engine bay decide head deceleration and the risk of bottoming-out.

Each of these is a stiffness-and-geometry problem, not a styling problem. The model therefore needs three things for every front-end component: an outer surface (the contact geometry), a structural representation (beam, foam, or panel), and a force–deflection characteristic that governs how the surface deforms under load.

4.2 Bumper Representation

The bumper is the first structure to contact the pedestrian. In MADYMO it is normally built as a multi-body or finite-element assembly with three layers:

1. Fascia / cover — a thin shell that provides the outer contour. It contributes little stiffness but defines the contact surface.
2. Energy absorber — foam or honeycomb, modelled as a nonlinear spring or a crushable FE block. This is where most of the early energy is absorbed.
3. Bumper beam — the stiff cross-member behind the foam. It is usually modelled as a rigid body or a stiff beam because it does not deform significantly at pedestrian impact speeds.

The critical modelling decision is the force–deflection curve of the energy absorber. A typical compact-car bumper foam might show an initial elastic region up to about 5 mm at 1.5 kN, a plateau around 3–4 kN extending to roughly 40 mm, and then a sharp rise as the foam densifies. If this curve is replaced by a linear spring, the peak leg force will be over-predicted and the timing of knee bending will be wrong.

Vertical placement matters as much as stiffness. The bumper reference line (the horizontal centre of the bumper face) should sit at the height specified by the vehicle's design position — typically 400–500 mm for a passenger car. The knee joint of the pedestrian model is around 500 mm, so a bumper centred at 450 mm strikes slightly below the knee, producing the characteristic hyperextension moment. Shifting the bumper 50 mm upward changes the peak knee bending moment by a significant margin, so this dimension must come from the actual vehicle, not from a default value.

4.3 Hood Representation

The hood is the largest contact surface and the one most often oversimplified. It should be modelled as a deformable outer panel supported by an inner reinforcement structure, with an explicit gap to the engine-bay components beneath.

The outer panel is a thin steel or aluminium shell, typically 0.6–0.9 mm thick. Its local stiffness depends strongly on curvature and on the distance to the nearest inner panel or engine component. A flat plate of the same thickness will be far too stiff in the centre and far too soft near the edges. The practical approach is to divide the hood into zones — leading edge, mid-panel, and rear/cowl region — and assign each zone a force–deflection curve derived from either a component test or a detailed FE sub-model.

The gap to the engine bay is the single most important parameter for head injury prediction. If the hood can deflect 60 mm before striking the engine block, the head deceleration pulse is long and moderate. If the gap is only 20 mm, the head bottoms out and the deceleration spike can double. In MADYMO this is represented by placing a rigid or stiff contact plane at the bottom-out depth, so the hood spring is active only over the available travel.

4.4 Grille and Front-End Integration

The grille and the region between bumper and hood form the transition zone that contacts the pelvis and abdomen. Geometrically it is a recessed surface, often with cooling openings. For pedestrian simulation, the grille is usually represented as a semi-rigid contact surface with a stiffness between that of the bumper foam and the hood panel. Its main role is to apply a rearward and upward force to the pelvis, initiating torso rotation.

Two integration details are frequently missed. First, the vertical gap between the top of the bumper and the leading edge of the hood must be modelled explicitly; if the two surfaces are merged, the pelvis will be loaded by a continuous surface instead of two distinct contacts. Second, the hood leading edge should be allowed to rotate or translate slightly under load, because in a real vehicle it is supported by hinges and latches that have finite stiffness.

4.5 A Worked Geometry Setup

The following example shows a representative parameter set for a mid-size passenger car. These values are illustrative and must be replaced with measured data for any real study.

After assembly, verify the model by running a simple legform impact at 40 km/h against the bumper centreline. Check that the peak knee bending moment falls in the expected range for the foam stiffness used, and that the legform's upper end rotates rearward rather than being pushed straight back. If the legform stops abruptly, the foam plateau is too high or the beam is too far forward. If the legform passes through the fascia, the contact thickness is insufficient.

4.6 Verification Checklist

Before accepting a front-end model, confirm the following:

• The bumper reference line height matches the measured vehicle dimension, not a default.
• The foam force–deflection curve includes a plateau region and a densification rise.
• The hood is deformable, with a defined bottom-out depth and a rigid plane beneath it.
• The grille is a separate contact surface, not merged with the bumper or hood.
• The gap between bumper top and hood leading edge is explicitly modelled.
• A legform impact at the bumper centreline produces a knee bending moment consistent with the foam stiffness used.

When these conditions are met, the front-end geometry becomes a reliable predictor of pedestrian loading rather than a source of numerical noise. The next step — coupling this geometry to a validated pedestrian multi-body model — depends entirely on the quality of the surfaces and stiffness curves established here.