2. Anatomy of a knee airbag module

A knee airbag (KAB) module is a compact, self-contained restraint assembly that deploys a cushion into the lower occupant compartment to control pelvis and femur kinematics during a frontal impact. Although it is often treated as a single part number, the module is a system of five functional sub-assemblies — housing, inflator, cushion, tethering, and mounting brackets — whose interfaces with the instrument panel (IP) and, on some architectures, the steering column determine whether the restraint performs as designed. This section dissects each element, explains the design constraints that drive its geometry, and shows how the interfaces propagate load and timing requirements into the surrounding structure.

Housing: the structural backbone

The housing (also called the canister or retainer) is a stamped or die-cast metal shell, typically 1.0–1.5 mm stamped steel or an aluminium die-casting, that performs four jobs simultaneously: it retains the folded cushion, it reacts the inflator's thrust load, it provides the reaction surface for the cushion as it unfolds, and it defines the deployment door tear path. The housing's internal volume must accommodate the folded cushion at a defined compression ratio — commonly 1.6:1 to 2.2:1 for a fabric cushion — because over-compression increases fabric abrasion and can delay the first gas breakthrough.

Two housing features deserve attention during design review. First, the inflator mounting boss must be concentric with the cushion throat within roughly ±1.5 mm; eccentricity causes asymmetric gas distribution and a skewed deployment trajectory. Second, the door hinge line must be positioned so that the door rotates outward and downward, away from the occupant's shin, rather than inward. A hinge placed too close to the cushion throat produces a door that pinches the cushion during the first 10–15 ms of deployment.

Inflator: energy source and timing anchor

Knee airbags almost universally use a hybrid or pyrotechnic inflator with a smaller gas output than a driver frontal inflator — typically 0.8–1.6 mol of gas for a 15–25 L cushion. The inflator's output curve, not its peak pressure, is the design driver. A representative hybrid inflator for a KAB reaches 90 % of its total gas output within 18–25 ms at 23 °C, and the cushion must reach its full deployed depth within 30–40 ms to be in position before the occupant's knee reaches the IP lower surface.

Inflator selection is constrained by three interfaces: the electrical connector (usually a 2-pin squib with a shorting clip), the mounting flange (bolt pattern and clocking), and the gas exit ports (radial or axial). Clocking the inflator incorrectly by 90° can direct the gas jet into the housing wall instead of the cushion throat, producing a pressure spike that ruptures the cushion seam. Always verify the clocking feature — a keyway, a flat, or an asymmetric bolt pattern — against the housing drawing before assembly.

Cushion: geometry, fabric, and fold pattern

The cushion is a coated or uncoated woven fabric (typically 470–630 dtex nylon 6.6) sewn into a shape that, when inflated, presents a broad, flat reaction face to the knee and thigh. The inflated geometry is defined by the panel pattern — the 2D cutting layout — and the fold pattern, which controls the order in which the cushion unfolds. A common KAB fold is a combination of a longitudinal accordion fold along the cushion axis and a lateral roll at the outboard edges, which produces a controlled, low-trajectory deployment.

Three cushion parameters are worth tracking in a design table:

  • Throat diameter — the opening that receives the inflator gas; too small and the cushion inflates slowly, too large and the cushion loses internal pressure before it reaches full depth.
  • Vent area — the total area of the vent holes; controls the pressure decay after peak inflation and therefore the cushion's ability to absorb rebound energy.
  • Seam construction — the stitch type and thread denier; a seam that fails at 80 % of the cushion's burst pressure is a design defect, not a manufacturing escape.

Tethering: shaping the deployed envelope

Tethers are fabric straps sewn between the cushion panels or between the cushion and the housing. They do not carry the inflation load; they limit the cushion's expansion in directions where the restraint must not grow. In a KAB, tethers typically perform three functions: they prevent the cushion from expanding upward into the IP structure, they pull the cushion's outboard edge inward to avoid contact with the door trim, and they create a concave reaction surface that cradles the knee rather than presenting a flat plate.

Tether length is a first-order design variable. Shortening a tether by 10 mm can reduce the deployed cushion depth by 15–20 mm and increase the internal pressure by roughly 8–12 %, which changes both the occupant's knee trajectory and the cushion's seam loading. Tether anchor points must be reinforced with bar-tack stitching; a single-row stitch at a tether anchor is a common field-failure location.

Mounting brackets and structural interfaces

The mounting brackets transfer the deployment reaction — a short, high-magnitude impulse — into the IP substrate and, in some architectures, into the steering column support beam. A typical KAB bracket is a 2.0–2.5 mm stamped steel or glass-filled PA66 component with two or three M6 or M8 fasteners. The bracket must be stiff enough to prevent the housing from rotating during deployment; a bracket that deflects more than 3–5 mm at peak load allows the cushion to deploy at an angle, which is one of the most common causes of out-of-position (OOP) test failures.

The IP interface is defined by three features: the door outline (the tear seam geometry), the reaction surface (the IP substrate that the cushion bears against), and the clearance envelope (the free volume the door and cushion need to deploy). The steering column interface, where present, is usually a bracket that ties the KAB housing to the column's lower support structure, sharing the deployment load between the IP and the column. This shared path is beneficial for stiffness but introduces a coupling: a column adjustment or a column-mounted switch module can change the KAB's effective reaction stiffness.

Design review checkpoint: Before releasing a KAB module drawing, confirm that (1) the inflator clocking feature is unambiguous, (2) the cushion throat is concentric with the inflator boss within ±1.5 mm, (3) the tether anchor bar-tacks are specified, (4) the bracket deflection at peak deployment load is below the OOP threshold, and (5) the IP door tear seam is continuous and free of knit lines.
Knee Airbag Module — Sub-Assembly and Interface Map Housing / Canister Stamped steel 1.0–1.5 mm Inflator boss ±1.5 mm Door hinge line Cushion compression 1.6–2.2:1 Inflator Hybrid / pyrotechnic 0.8–1.6 mol gas 90 % output @ 18–25 ms 2-pin squib, clocking key Cushion Nylon 6.6, 470–630 dtex 15–25 L inflated volume Throat, vents, seams Accordion + lateral roll fold Tethering Limits expansion direction Length = 1st-order variable Bar-tack anchor points Mounting Brackets 2.0–2.5 mm steel / PA66-GF M6 / M8 fasteners Deflection < 3–5 mm Interfaces IP door tear seam IP reaction surface Steering column support retains inflates shaped by reacted by load path Deployment timing (representative, 23 °C): 0–5 ms trigger 5–20 ms inflator rise 20–35 ms cushion deploy 35–60 ms occupant contact / pressure decay

How the interfaces propagate design requirements

The module's interfaces are not passive attachments; they are load paths and timing constraints. Consider the IP door tear seam. If the tear seam is too strong, the cushion pressure must rise higher before the door opens, which delays deployment by 3–6 ms and increases the peak load on the housing. If the tear seam is too weak, the door may open during a low-severity event or even during shipping vibration, which is a warranty and safety concern. The tear seam's strength is therefore specified as a window, not a minimum.

The steering column interface introduces a different constraint. When the KAB housing is tied to the column support, the column's own stiffness becomes part of the reaction path. A column that is designed to collapse during a frontal impact will move rearward as the occupant loads it, and that motion changes the KAB's effective reaction surface. In practice, this means the KAB bracket and the column bracket must be analysed together, not separately. A useful check is to apply the peak deployment impulse to the KAB housing and measure the relative displacement between the housing and the IP substrate; if the relative displacement exceeds the door clearance, the door will bind.

Practical assembly and inspection notes

During module assembly, three checks catch the majority of build-related defects. First, verify inflator clocking by confirming that the keyway or flat aligns with the housing mark; a 90° error is visually obvious only if the mark is inspected. Second, measure the cushion throat concentricity with a go/no-go gauge; a 2 mm eccentricity is enough to produce a visible deployment skew in a static deployment test. Third, inspect the tether anchor bar-tacks under 10× magnification; a missing bar-tack will not fail a functional test at 23 °C but can fail at −35 °C when the fabric is stiff.

When a KAB deployment shows a skewed or short cushion, the fault is more often in the tether length or the inflator clocking than in the cushion itself. Measure the deployed cushion depth at three points — centre, inboard, outboard — and compare the spread. A spread greater than 15 mm points to a tether or clocking issue; a uniformly short cushion points to an inflator output or vent area issue.

Understanding the module as five sub-assemblies plus two structural interfaces gives the design engineer a clear mental model for both CAD packaging and validation planning. The next step is to translate this anatomy into the simulation model, where each sub-assembly becomes a set of elements, materials, and contacts with its own fidelity requirements.

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