Car chassis and what to know in automotive mold machining

Car chassis structure

In terms of overall structure, today's car chassis comes in two main forms with the name:
  • Body on frame (or ladder frame)
  • Unibody (or Monocoque body)
Car chassis structure

Body on frame – Detached car frame

A body on frame is a body structure placed on a separate chassis. Inspired by horse-drawn carriages, with a sturdy and fixed chassis. The detachable frame structure allows for the chassis design changes to be made easily without impacting the iron-xi chassis system, even creating completely new models right on the old chassis platform, thus saving quite a lot of research and development (R&D) time as well as design costs, Investment in production lines compared to having to build a new chassis system.
These frames are commonly found in SUVs, pickup trucks, cars, specialized vehicles …

Advantages of removable car chassis

  • Reduce the noise level when operating. The body and chassis are separate, only in contact with each other through rubber cushions, attached bolts … No noise when the car is moving.
  • Easy to repair if an accident deforms the car. The positions separated from the "frame" can be easily replaced, even repaired into a completely new car.
  • High load resistance, great helix resistance, strong and durable.
  • Less damage caused by rust caused by dampness, mud, rocks, sand roads, water, snow,…

The disadvantages of body-on-frame

  • The volume is large and the focus is high due to the specific design of the chassis part placed on the undercarriage iron. This leads to poor performance, lack of stability when cornering & higher fuel consumption. Therefore, SUVs and pickups often use oil machines for better traction & more economical than gasoline machines.
  • In the past, the detachable frame left heavy consequences for the person sitting in the car quite heavy. Because the car does not have an impulse absorption zone, when there is a strong impact, the car will slow down, but the person in the car continues to move at a speed just before the collision. This speed only decreases when the person in the car collides with the steering wheel or airbag. However, now, with improvements to new safety braking systems, this disadvantage is gradually being overcome.

Unibody – Instant car frame

Unibody is a contiguous body structure, forming a unified block. This structure appears in most sedans, hatchbacks, some MPV, CUV,… The main reason is because it overcomes the biggest disadvantage of body on frame in terms of weight. From the use of lightweight materials with high hardness such as aluminum alloys, carbon,… Really Unibody makes the car structure a lot lighter. Improved performance, fuel economy and increased stability when cornering are the things that are mentioned when using straight-frame vehicles.
Besides, the car frame has excellent impulse absorption, safer for passengers sitting in the car when an accident occurs.
The disadvantages are difficult to repair, the load capacity is not high, the design is complex,…
Unibody Car Chassis Structure

What you need to know in car mold processing

In this article, the author selects the Unibody seamless frame to analyze the texture of the mold and jigs. In addition to the details that make up the chassis, there are also details that make up the car's shell as shown below.
Car shell structure
The separation of components on the chassis is important because the production stages will rely on it to divide the assembly sequence of the detailed clusters. This is also the flow of work in car assembly plants (Figure 4).
Workflow or automotive manufacturing process in the factory

Jigs for Underbody detail cluster

The first step of assembling the Underbody cluster will include assembling the Assy Front rail (RH/LH) cluster (Figure 5)
In the author's example, only two types of steel are used:
  • FEP04 with low elastic limit (about 200MPa)
  • FEE355 has a higher elastic limit (greater than 355MPa) but the ability to deform plasticize is lower.
Usually the rails here will be made of steel with good plastic deformation, large thickness and stamped with Draw mold to create a deep heart. After stamping out the form, the detail has a high hardness (due to the geometric shape) but still has the ability to deform elasticly on impact to absorb kinetic energy.
Detailed jigsworking for front rail car cluster

Components in the Front rail cluster

  1. Rail boxing asembly
  2. Front rail
  3. Front-end attachment bracket
  4. Front rail extension
  5. Front rail reinforcement
  6. Cross-member to rail gusset
  7. Powertrain support reinforcement
  8. Upper front suspension tower
  9. Lower front suspension tower
  10. Rail box
  11. Tower to cross-member brace
Front Rail cluster jigsworking
This part has a rather complex structure, so many locations are difficult to weld. The design of welding jigs needs to pay attention to the detailed, meticulous plan so as not to be missed any welds. Negligence is extremely dangerous, which can affect the whole chassis. In early May 2022, Toyota recalled more than 200 Raize vehicles in Vietnam due to concerns about the collapse of the same detail cluster.
Machining jigs for automotive frame structure
Front rail RH (1) and LH (2) clusters are welded with Front rail back cross-member (5), Strut tower cross-member assembly (3) and Lower dash panel (4). All of these detailed clusters form the Front frame assembly. This part is quite easy to make jigs for airy texture, not much entanglement (Figure 7).

Underbody frame

The underbody frame consists of front subframe (1) (front subframe), Rear subframe (2) (rear subframe) and two Underbody rails (3R and 3L) (two undercarriage rails) (Figure 8).
Front frame assembly machining

The final step in the assembly of the Underbody cluster (undercarriage) includes the details (Figure 9):

Assembly of underbody car underbody car chassis details assembly
  1. Underbody frame assy
  2. Front floor assy
  3. Rear seat cross-member assy
  4. Rear floor front assy
  5. Rear floor back assy
  6. Dash top panel assy or water box
  7. Back panel assy
  8. Rocker panel assy RH/LH
  9. Inner body side assy front RH/LH
  10. Strut tower reinforcement RH/LH
  11. Upper rail boxing RH/LH
After that, the next step is carried out in body comp jigs, connecting the detailed clusters of the shell frame with SPOT, SPR welding technologies,… (Figure 10):
Jigs and molds in the manufacture of automotive frames
  1. Underbody assy
  2. Upper rail right
  3. Upper rail left
  4. Body side right
  5. Body side left
  6. Cowl top
  7. Header
  8. Roof frame
The undercarriage carries the accessories of the front and rear suspension, the powertrain, the exhaust system, the seat, the control cluster, and moreover, it is the main structure facing collisions from the front, rear and also involved in the absorption of collisions from the sides.

Structural analysis underneath the car

Car chassis structure
It can be noticed that the vertical frame below the undercarriage runs along the length of the car. This structure is obviously improved from the body on frame structure but with higher complexity, meeting the requirements of weight as well as load bearing, absorbing collision dynamics. In addition, on the picture there are 2 positions A and B is where the front and rear suspension of the car is attached. When making body comp jigs, these are also 4 standard holes (hole datum) to position the chassis into the jigs, these positions should be noted when designing.

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