Design of backdoor
AIM
BACK DOOR
A back door is a BIW component found in all Automotive car generally used for opening and closing of the so called car luggage that is usually used for storing items inside a car. In general the opening and closing of the door is co-ordinated by the Latch and Stricker position and the Gas stay position that is mounted on the body and back door of the car. These positions play a major role in ensuring smooth opening and closing of the tail gate.
Parts of a Tail Gate:
- Outer Panel
- Outer panel Lower
- Inner panel
- Hinge Reinforcements
- Latch and Stricker Reinforcement
- Wiper Motor Reinforcement
- Gas Stay Reinforcements
All these parts are assembled together to obtain the back door of a car
Types of Back Doors:
There are three types of back door based on the position of the Hinge axis. They are
- Top Mounted
- Side Mounted
- Bottom Mounted.
In our design we follow the Top Mounted Design where the Hinges are mounted on the top.
Design Considerations In Back Door Design:
The following are the Design considerations taken while designing the Back Door. They are,
- Visibility Criteria
- Force Dissipation
- Latch and Stricker Positions
- Hinge Position
- Gas Stay Position
- Material Selection
- Space Availability
- Hemming and Relief Areas
All these design considerations go hand in hand with eachother because these positions are dependant on each other and if position gets altered all the positions has to be altered for effective opening and closing of the back door. The Latch and Stricker, Hinge Position and the Gas stay Positions has to be maintained in a proper way so that they play a major role in the opening and closing as well as the rear Visibility criteria of the car.
Material Selection:
The material selection play a major role in determing the overall weight of the back door. The materials generally used are Steel and Aluminium. The Density of Steel is higher than Aluminium thus it results in adding more weight of the door. Basically the material is used based on the budget of the car, because the cost of steel very cheap than Aluminium. If aluminium is used instead of steel there is no need of many parts like Reinforcements for each mounting and the overall weight geta reduced by 6-10 kg approximately. But the draw back is Aluminium is much costlier than Steel. Hence based on the budget of the car aluminium is used. In higher end models like Benz, Audi, Porshe and BMW we can find aluminium is used instead of steel resulting in reduced weight and increased performance of the vehicle.
Another Factor to make a note is using Casted Aluminium instead of deep drawn Steel components and adding reinforcements to it is the space of the luggage commodity gets increased by a certain amount because the draft angle for Casted aluminium is much lesser (3 degree) than the deep drawn Steel inner panel (7-9 degree). Thus it reduces the space between the inner and outer panel resulting in much larger cabin space than the deep drawn steel inner panel. Thus the material selection is one of the design consideration taken. Since it reduces the space between the inner and the outer panel the rear visibility of the car is also inproved giving more space for the mirror to view.
Hence in this design the material chosen is steel since it is designed for a medium budget car and hence high cost could not be afforded to design the back door.
Hemming and Relief Areas:
Hemming and Seaming are two similar metal working processes in which sheetmetal edge is rolled onto itself.Hemming is the process in which the edge is rolled flush to itself, while a seam joins the edge of two materials.
The Hemming is done on the outer and the inner panel with an clearance of 0.2mm on the either sides. Since at the corners the hemming is difficult to perform, clerances has been given at the ends of panels so that it would for the hemming operstion is performed. Here is the image of the hemming relief at the corners.
INNER PANEL
Mastic Sealants:
The below image is the region where both the inner and outer panel are joined at the centre with the help of Mastic Sealants. There is a gap of 5mm for the mastic sealants to melt and gets fixed at the surface of the panels.
Reinforcements:
Reinforcements are generally used to increase the strength of the mountings so that there is no unnecessary dents or noise occurs at the time of motion. This not only add strength to the part but also increase the overall stiffness of the part so that the part is completely stable. The Reinforcements used are
1) Latch and Stricket Reinforcement
2) Wiper Motor Reinforcement
3) Hinge Reinforcement
4) Gas Stay Reinforcements
The Following Image shows the reinforcement that are used on different areas of the inner panel. They are usually spot welded on the inner panel.
Hinge Reinforcement:
Hinges support the weight of the whole backdoor.Hence, the reinforcements must be provided there too.The emboss is designed in such a way that the hinges should operate freely.The hinge is connected to the rear roof of the car.
Gas Stay Reinforcements:
Gas stay is used to lift the backdoor automatically.There are two types of gas stay one is semi-automated and another one is fully automated.In semi automated type the back boor needs little force to lift after that it lift the backdoor automatically.In fully automated , it has power spindle which lift the backdoor automatically.
Latch and Stricket Reinforcement:
At bottom of the panel , reinforcement and emboss are given to accomodate the latch and striker assembly.Generally key operated or automatic remotly controllable mechanism is used to keep the door open or closed.The actuators , recivers and other electronic devices are mounted on the inner panel only.
Wiper Motor Reinforcement:
Wiper is used to to wipeout the rain water falls on the backdoor window.It is actuated by the motor which is mounted on the inner panel.Due to the vibration and heat created by the motor .the inner panel where the motor is mounted gets deform and the parts attached to the motor starts to vibrate which leads to poor NVH.So we have to provide the reinforcement where the wiper motor is placed.
All Reinforcements assembled with Inner panel,
:
Hemming:
Draft Analysis:
Draft analysis of inner panel is carried out through the tooling direction only.Only the green areas are acceptable and i worst cases yellow regions are accepted but the blue and red regions are never accepted.
The draft angle for aluminium casted panel is 2 to 3 degrees is enough but if it is steel then the angle is more than 7 degrees.
In this project i considered it as steel and used 7 degrees as draft angle and direction is choosen as tooling direction which is the deep drawn direction.
Back Door Assembly:
Thus the top down approach is followed to design the back door assembly. At first from the given styling surface the inner panel was created by Offseting the outrer Panel and then individual parts were designed accordingly by using the Inner panel Surfaces. The Reinforcement and other commodities were designed by using the inner surface as the reference thus following the top down approach. The following image show the assembly model of the Back Door.
Conclusion:
Thus the designing of the Back Door or Tail Gate is done as per Industrial standards and Manufacturing Techniques.