Design of backdoor

Car Back Door Design

Aim: To design car back door from given styling surface data under given design rules and regulations.

Car Back Door and Components:

The Back door of a car is a component which hinges at the back of the car to cover the boot space or luggage area, But there are more functions that come with it which has to be considered while making a design of the same like rear visibility criteria and boot space.

This part is usually designed keeping the master section of the rear roof rail because of the sealant flange and also various other considerations, as it is also in contact with the side body of the car.

It has back lights/ Tail lamps and Braking lights which is termed as "Third brake light" mounts on it, rear windshield fixes between inner and outer panel, in some back door spare wheel can be mounted on it, sometimes car registration number plate mounts on it and it gives a complete closed design to the car.

The back door is a hinged door/tailgate at the rear of the vehicle. It may be used for loading and unloading cargo, or to get passengers in and out, in case there are seats present at the rear. The back door comprises of certain components which form the back door assembly together.

Fig.1 Typical Car Back Door

The components of back door are as follows,

• Inner Panel

The inner panel, as the name suggests, is the inner part of the door. It is where most of the reinforcements and components are mounted. It is manufactured either by casting (aluminum) or sheet metal stamping (steel). Thickness of inner panel is 0.75 mm.

Fig.2 Inner Panel

• Outer Panel

As the name suggests, it is the outer part of the rear door and is mostly responsible for aesthetics of the vehicle. Outer and inner panels are connected by mastic sealants in center and by hemming process at edges. The outer door handle and in some cases, the number plate may also be installed on the outer panel of the back door. We have considered a 2-part outer panel for our design, outer upper panel and outer lower panel prospectively. Outer panel is manufacture by stamping sheet metal. Thickness for outer panel is 0.75 mm

Fig.3 Outer Panel

• Reinforcements

Reinforcements are provided to provide support and add extra material to the sheet metal part to increase its strength near component installation regions and minimize the impact of collisions. We have considered following reinforcements during our design. The reinforcements maybe manufactured by stamping. Thickness of all reinforcements is 1.5 mm.

Fig.4 LH and RH Hinge Reinforcements

Fig.5 LH Gas Stay Mounting Reinforcement

Fig.6 RH Gast Stay Mounting Reinforcement

Fig.7 Wiper Motor Mounting Reinforcement

Fig.8 Latch Striker Reinforcement

Design Considerations:

• Rear Visibility for safety and functionality
• Support Structures like Hinge and Gasstay/ Power lifter
• Rear Crash regulations
• Boot Space
• Mounts for wiper motor and lights
• Strength criteria
• Draft Analysis

• Gas Stay

Gas stay is basically a gas spring that relies on elastic deformation of a compressed gas which is enclosed in a cylinder. It is used to reduce required effort while opening the back door. Although all the cars don’t come with gas stays, it may be required in case of a heavy back door.

Fig.9 Typical Car Back Door Gas Stays (Highlighted)

• Wiper and wiper motor mounting

As the name suggests, wiper is used to wipe off the dust and water from the rear windscreen of the vehicle.

Fig.10 Car Back Door Wiper

• Hinges

Hinges connect the door with the vehicle body and allow the door to rotate around a fixed axis for opening and closing movement. Generally, hinges are a 2-part assembly, where one part is bolted to door and other is bolted to vehicle body.

Fig.11 Car Back Door Hinges (Highlighted)

Design Methodology:

We start with the styling surface that is provided to us, and consider top-down approach for this assembly modeling. First, we create the assembly and then create the components in the assembly file itself. Inner and outer panel are the largest parts and all other parts depend on them so we start with them. Also, the given styling surface is part of the outer panel so we create it first. Using various commands and features we design the outer panel from the styling surface. We have 2 parts for outer panel, which are separated by a flange, hence we create outer panel upper and lower respectively. They will be spot welded with each other. It is necessary to make sure that all the surfaces are properly joint to their adjacent surface and there are no disjoints at their intersection edges to avoid further errors. Also, Inner and outer panels are connected by hemming process so we can come back to outer panel and do that that after completion of inner panel.

Fig.12 Outer Panel Upper Part

Fig.13 Outer Panel Lower Part

Now, as we know inner panel is at a particular distance from outer panel, we offset the outer panel surface and design inner panel on the same. Inner panel is important as most the mountings will be done on it. Before starting inner panel design, we need to define the tooling direction as a reference for manufacturing team. Then we design the inner panel.

Fig.14 Tooling Direction

While designing the inner panel we need to consider various factors like the mounting point positions, mastic sealant application positions and the cut outs in sheet to make access to tail light assembly easier. As the bottom center part of inner panel is not connected to outer panel, we need to create a surface that comes out and connects to outer panel as shown in the image. This part will be connected to outer panel using mastic sealant. Also, the hinge embosses must be taken by considering proper directions. The emboss surface must be parallel to z-axis to get proper geometry for the latch striker path curve. Then we draw a line connecting the hinges, such that it forms the hinge axis (axis around which the door rotates while opening or closing). Then we can draw a circle with center at the hinge axis and circumference passing somewhere around the emboss endcap. So that when we get the emboss for latch striker mounting, we can take the end cap by creating a plane normal to the path (using On Curve) in the direction normal to the path of striker. Thus, after some trial and error, we can adjust the emboss and the circle dimensions properly.

Fig.15 Latch Striker Path

Fig. 16 Inner Panel

As the inner and outer panel are connected by hemming process, we can complete the hemming after completion of inner panel. Now, as major parts are designed, we can move to reinforcements. It is fairly easy to design reinforcements as shown in image below and detailed images above as we can use the inner panel surfaces using Wave Geometry Linker into the corresponding reinforcement parts.

Fig.17 Reinforcements

In order to make sure the drawing of inner panel body is proper; we do draft analysis. We need to define the drawing direction and select the inner panel body. We take limit angle as 5 degrees. The result of the draft analysis is as follows,

Draft Analysis:

Tooling Direction 1:

Tooling Direction 2:

Draft Analysis is done to analyze the draft angle. This allows us to verify whether the product will be easily removed from the tool.

When designing a product it needs to be slightly angled so that the tool can be pulled away from the part easily, without deformation or damage. This slight angle is called draft.

• In the 1st tooling direction, all components are made
• In the 2nd tooling direction, the parts whic shows negative draft , we will increase the tooling angle to avoid neative draft.

Portions that require different manufacturing method, or are not in the direction of drawing may not be considered in draft analysis results.

Conclusion: Car back door was designed using the given styling surface, and under provided design regulations.