4. Building a baseline CRS model

A baseline Child Restraint System (CRS) model is the reference configuration against which every subsequent variation — belt routing changes, shell thickness adjustments, foam stiffness tuning — is compared. If the baseline is geometrically inconsistent or the belt paths are wrong, every downstream result inherits that error. This chapter walks through the practical construction of a MADYMO baseline CRS model: importing CAD geometry, defining the shell as a rigid or deformable body, and building the belt and harness systems that connect the CRS to the vehicle and the occupant to the CRS.

4.1 Importing CAD Geometry into MADYMO

MADYMO does not natively read parametric CAD formats such as CATIA .CATPart or NX .prt. The standard workflow is to export the CRS shell, foam, and any rigid attachment points from the CAD system as a neutral format — typically STL for faceted surfaces or IGES/STEP for NURBS surfaces — then import into MADYMO's MADYMO Workspace or via the madymo command-line preprocessor using FEMESH or MESH keywords.

For a typical Group 0+ infant carrier, the CAD assembly contains three distinct geometric entities that must be handled differently:

• Shell (carrier body): The structural plastic shell. In a baseline model this is often represented as a rigid body with a defined mass and inertia tensor, or as a deformable FE shell mesh if shell deformation is of interest. For a first baseline, rigid representation is faster and more stable.
• Foam padding: Energy-absorbing foam (e.g., EPS or EPP). This is usually modeled as a deformable FE solid or as a MADYMO FE body with hyperelastic material properties.
• Fabric/soft goods: The cover and harness webbing. These are not imported as solid geometry; they are represented by belt elements and contact surfaces.

When exporting from CAD, keep the coordinate system consistent with the vehicle coordinate system used in the sled or full-vehicle model. A common convention is: X forward, Y left, Z up, with the origin at the vehicle centerline at the front axle or at the sled reference point. Document this convention before import — mixing coordinate systems between the CRS and the vehicle model is one of the most frequent sources of baseline errors.

4.2 Defining the Shell as a Rigid Body

For a baseline model, the shell is typically defined as a rigid body using the BODY.RIGID keyword. The required inputs are mass, center of gravity, and the inertia tensor. These can be extracted from the CAD model if material density is assigned, or measured from a physical carrier.

A representative baseline for a Group 0+ carrier might use:

BODY.RIGID
  ID = 1
  MASS = 3.2          ! kg, shell + foam + fabric
  IXX = 0.045         ! kg·m²
  IYY = 0.038
  IZZ = 0.052
  CG = 0.0, 0.0, 0.28 ! m, relative to CRS local origin
END

The shell geometry is attached to this rigid body as a SURFACE or FE mesh for contact interaction. In MADYMO, contact between the CRS shell and the vehicle seat, and between the CRS shell and the occupant, is handled through CONTACT definitions. The shell surface must be assigned a contact thickness and friction coefficient. A typical value for plastic-on-fabric contact is μ = 0.3–0.4; for foam-on-fabric, μ = 0.5–0.6.

4.3 Belt and Harness Definition

The belt system in a CRS model has two distinct subsystems: the vehicle belt (the adult seat belt that secures the CRS to the vehicle) and the harness (the internal straps that restrain the child). Both are modeled using MADYMO belt elements, but their routing and attachment differ significantly.

Vehicle belt routing

For a Group 0+ carrier installed with a lap-and-diagonal belt, the belt path runs through the carrier's belt routing guides. In MADYMO, this is modeled as a BELT system with a series of BELT.SEGMENT definitions. Each segment connects two points, and the belt is anchored at the retractor and buckle points. The key parameters are:

• Belt stiffness: Typically 200–400 N per mm of elongation for a standard seat belt. This is defined via a LOAD_CURVE or CHARACTERISTIC.
• Pre-tension: For a baseline model without pre-tensioner, set to zero. If modeling a pre-tensioning retractor, apply a force of 2–4 kN over 20–30 ms.
• Friction between belt and routing guide: μ = 0.1–0.2. This affects how the belt slides through the guide during loading.

The belt must be routed so that it contacts the CRS shell at the correct guide locations. In MADYMO, this is done by defining the belt nodes at the guide entry and exit points, and using BELT.SEGMENT with CONTACT to the shell surface. A common error is to route the belt through the shell geometry rather than around it, which produces artificial belt forces and incorrect CRS kinematics.

Harness routing

The internal harness is a five-point or three-point system. For a baseline model, the harness is typically modeled as a set of belt segments connecting the following anchor points:

1. Two shoulder slots on the shell (left and right)
2. Two lap anchor points on the shell
3. One crotch strap anchor
4. The buckle/latch point where all straps converge

Each harness segment is defined with its own stiffness and pre-tension. A typical harness webbing stiffness is 150–250 N/mm. The harness must be routed over the occupant's shoulders and pelvis, and contact between the harness and the occupant is defined via CONTACT between the belt segments and the occupant's FE or multibody surface.

4.4 Shell Components and Contact Definitions

The shell is not a single rigid body in reality — it has multiple components: the main carrier, the handle, the base (for infant carriers), and the canopy. For a baseline model, the handle and canopy are often omitted or lumped into the main shell mass. The base, if present, is modeled as a separate rigid body connected to the carrier via a joint or a rigid constraint.

Contact definitions are the most computationally sensitive part of the baseline model. The following contacts must be defined:

Each contact requires a penalty stiffness and a contact thickness. For shell-to-seat contact, a penalty stiffness of 100–500 N/mm is typical. Too low a value allows penetration; too high a value causes numerical instability. Start with 200 N/mm and adjust based on the force-penetration curve.

4.5 Baseline Model Verification

Before running any crash simulation, verify the baseline model with a static or quasi-static check:

• Gravity settling: Run the model under gravity for 50 ms. The CRS should settle onto the seat without oscillation or penetration. Check that contact forces are stable.
• Belt pre-tension: Apply a small pre-tension (e.g., 50 N) to the harness and vehicle belt. The occupant should be pulled into the CRS without excessive deformation.
• Mass check: Verify total model mass against the physical CRS mass. A 10% discrepancy indicates missing components or incorrect density.
• Coordinate check: Confirm that the CRS local origin and orientation match the CAD reference. A simple visual check in MADYMO's post-processor is sufficient.

The following SVG illustrates the baseline CRS model architecture, showing the relationship between the imported CAD geometry, the rigid shell body, the belt and harness systems, and the contact definitions that connect them to the vehicle and occupant.

The baseline model is now structurally complete: CAD geometry imported and assigned to a rigid shell body, vehicle belt and harness systems defined with realistic stiffness and routing, and contact definitions established between all interacting pairs. The verification checks confirm that the model is numerically stable and geometrically consistent before any crash load is applied. This baseline becomes the reference for all subsequent parameter studies — any change in belt stiffness, shell mass, or contact friction is measured against this configuration.