Automotive BIW Closures – Hood Design
- Bk Engineering
- Jun 20
- 3 min read
Table of Contents
What is a Car Hood?
A car hood (also called a bonnet in some regions) is the hinged cover that rests over the engine bay of a vehicle. It serves both protective and functional purposes — shielding the engine and other components from dirt, debris, and weather conditions while allowing easy access for maintenance.
Requirements and Functions of a Car Hood
The hood must satisfy a number of technical and safety requirements, including:
Structural Strength: To withstand aerodynamic forces and minor impacts.
Crash Safety: Should deform in a controlled way to protect pedestrians and engine components.
Heat Resistance: Must tolerate the heat generated by the engine.
Noise Reduction: Should help in damping engine noise.
Ease of Access: Should allow easy and safe opening for servicing.
How to Open a Car Hood
Opening a car hood usually involves two steps:
Inside Lever: A lever inside the cabin (usually below the dashboard) releases the primary latch.
Safety Latch: A secondary latch under the hood must be manually released to fully open the hood.
Once open, the hood must stay in the open position — this is where hood stays come into play.
Car Hood Basic Components
Outer Panel: The visible top surface.
Inner Panel: Reinforcement structure beneath the outer panel.
Hinges: Connect the hood to the car body and allow it to pivot.
Latch & Lock Mechanism: Keeps the hood securely closed.
Bump Stops: Rubber or plastic supports that help in closing softly.
Hood Stay (Support Rod): Mechanism that holds the hood in its open position.
Hood Stay (Support Rod)
A hood stay is a crucial component that keeps the hood lifted when open. It's generally mounted on the engine bay and hooked into place manually or automatically.
Types of Hood Stays
Manual Support Rod (Stay Rod):
Simple metal rod.
Requires manual placement in the hood slot.
Cost-effective and reliable.
Gas Strut (Hydraulic Stay):
Uses pressurized gas for smooth operation.
Automatically holds the hood in place once opened.
More common in premium vehicles.
Layout Condition for Support Rod (Stay Rod)
When designing a manual hood support rod, a few layout guidelines must be followed:
1. Opposite to Battery Placement
The support rod should be placed on the side opposite to the battery.
Why? This gives enough clearance for battery maintenance or replacement.
Example: If the battery is on the right, the support rod should be on the left.
2. Rod Conditions: Open & Closed
In the open condition, the rod is engaged and supporting the hood.
In the closed condition, it should nest neatly within a clip or groove in the engine bay.
3. Curvature and Length
The rod may require a bent shape (curvature) to match the geometry of the engine bay.
Length should fall within a standard range (e.g., ~35 mm arc range) depending on the opening angle and hood size.
4. Key Design Inputs
To design a hood stay effectively, you need:
Hood Opening Angle: The maximum angle the hood opens from the closed position.
Point A (Central Axis Point):
Imagine a line connecting both hood hinges.
Point A lies at the midpoint of this imaginary hinge axis.
It helps define how the rod pivots and where the load distributes.
Conclusion
Designing a car hood is not just about the outer shape. It involves careful planning of every component — especially the hood stay, which ensures safety and ease of use during servicing. Understanding the layout rules and basic geometry can go a long way in creating a functional and user-friendly design.
In the next part, we’ll discuss safety guidelines and key design criteria for robust and safe hood mechanisms.
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