Chassis Frame

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INTRODUCTION

 

 

Chassis is a major component of a vehicle system. It consists of internal framework that supports man-made object. It is the underpart of the vehicle which consists of frame and running gear like engine, transmission system, suspension system etc. The automotive chassis is tasked with keeping all components together while driving and transferring vertical and lateral loads, caused by acceleration, on the chassis through suspension and the wheels. The key to good chassis design is that further the mass is away from the neutral axis the more rigid it is.

In this project, Unigraphics is the software used for the modelling of the chassis. It is an advanced CAD/CAM/CAE software and is used for analysis (static, dynamic, electro-magnetic, thermal) using Finite element method . Stress anaylsis is a key characteristics of a chassis . The design and analysis of chassis is done by identifying the location of high stress areas. The chassis design used in this project is the ladder frame chassis.

 

Ladder frame chassis is the simplest and oldest of the chassis design used in modern vehicular construction . It is originally adapted from the “horse and buggy”style carriages. as it provides sufficient strength for holding the weight of the components. Ladder frame has several members that cross link to hold frame rails together. A simple design of two rails connected by a simple span and simulated provides a very good indication of how ladder frame is useful in regards to performance auto design. Using aluminium, a simple ladder frame chassis is weighed only 13. 85 kg and had a torsional rigidity of 522. 6 Nm/deg, steel at 39. 25 kg had a torsional rigidity of 1424 Nm/deg. Essentially all the ladder frame chassis is good in today’s world of automotive design in trucks and transport trailer vehicles.

The reason for ladder frame type of chassis is that here it is easier to change the design without having to change the chassis thereby saving overall design time. It also provides a good beam resistance because of its continuous rail from front to rear. The disadvantage with using this type of chassis is that it has poor torsion stiffness , higher fuel consumption and also heavier than a unibody.

 

 

       ABSTRACT

 

 

Chassis is a major component of a vehicle system. It consists of internal framework that supports man-made object. It is the underpart of the vehicle which consists of frame and
running gear like engine, transmission system, suspension system etc. The type of chassis used in the project is ladder frame chassis. Modelling of the chassis is done through a advanced CAD/CAM/CAE software Unigraphics. The design and analysis of the chassis is done by identifying the location of high stress areas.

 

Ladder frame chassis consists of two symmetrical rails and cross members connecting them to provide strength to different components of vehicle system. This type of chassis is mostly used in heavy duty vehicles like trucks. It provides a good beam resistance because of its continuous rail from front to rear. The reason for using this type of chassis is that here it is easier to change the design without having to change the chassis thereby saving overall design time. The disadvantage with using this type of chassis is that it has poor torsion stiffness , higher fuel consumption and also heavier than a unibody . As a result chassis has been designed in a way to reduce vibration, increase strength and optimize the weight of the chassis.

Different Types of Chassis


Chassis have to be stiff enough so that they withstand the forces applied to them. This is point is really important in the suspension settings. If the chassis bends a little the car in not going to behave as expected (as linear) because the ride is being modified, in short, the suspension settings are modified. However, you can not make the chassis completely stiff. That would cause it to be brittle. There will start to appear weak points and it would end breaking throw the weakest. So you need to reach a point where it is neither too stiff nor too weak.
As said before, the car needs to withstand various forces, so which are these forces:

  • Lateral G (Cornering speed)
  • Longitudinal G (acceleration and braking)
  • Load (Passenger and goods)
  • Road irregularities (bumps, pot holes etc)

The main materials used to build chassis are steel alloys , aluminium alloys, titanium alloys composites etc. Every one of those has different properties and applications. Prices vary vastly. These materials are joined in various ways: riveted, bolted, welded, glued together to make chassis.
There are several types of chassis :

  • Ladder frame
  • Space frame
  • Torsion box
  • Monococque
  • Combination

 


1. Ladder Chassis / Body on chassis
Chassis Frame Full Seminar Report and PPTAC Cobra's chassis.

Chassis Frame Full Seminar Report and PPT
This is the earliest kind of chassis. From the earliest cars until the early 60s, nearly all cars in the world used it as standard. Even in today, most SUVs still employ it. Its construction, indicated by its name, looks like a ladder - two longitudinal rails interconnected by several lateral and cross braces. The longitude members are the main stress member. They deal with the load and also the longitudinal forces caused by acceleration and braking. The lateral and cross members provide resistance to lateral forces and further increase torsional rigidity.
 


Advantage:

Well, it has no much advantage in these days ... it is easy and cheap for hand build, that's all. 

Disadvantage:

Since it is 2 dimensional structures, torsional rigidity is very much lower than other chassis, especially when dealing with vertical load or bumps.

Who use it ?

Most SUVs, classic cars, Lincoln Town Car, Ford Crown Victoria etc.

 



2. Tubular Space Frame 


Chassis Frame Full Seminar Report
TVR Tuscan

Chassis Frame Full Seminar Report and PPT
Lamborghini Countach

 
As ladder chassis is not strong enough, motor racing engineers developed a 3 dimensional design - Tubular space frame. One of the earliest examples was the post-war Maserati Tipo 61 "Birdcage" racing car. Tubular space frame chassis employs dozens of circular-section tubes (some may use square-section tubes for easier connection to the body panels, though circular section provides the maximum strength), position in different directions to provide mechanical strength against forces from anywhere. These tubes are welded together and forms a very complex structure, as you can see in the above pictures.
For higher strength required by high performance sports cars, tubular space frame chassis usually incorporate a strong structure under both doors (see the picture of Lamborghini Countach), hence result in unusually high door sill and difficult access to the cabin.
In the early 50s, Mercedes-Benz created a racing car 300SLR using tubular space frame. This also brought the world the first tubular space frame road car, 300SL Gullwing. Since the sill dramatically reduced the accessibility of carbin, Mercedes had to extend the doors to the roof so that created the "Gullwings".
Since the mid 60s, many high-end sports cars also adopted tubular space frame to enhance the rigidity / weight ratio. However, many of them actually used space frames for the front and rear structure and made the cabin out of monocoque to cut cost.  


Advantage:

Very strong in any direction. (compare with ladder chassis and monocoque chassis of the same weight)

Disadvantage:

Very complex, costly and time consuming to be built. Impossible for robotised production. Besides, it engages a lot of space, raise the door sill and result in difficult access to the cabin. 

Who use it ?

All Ferrari before the 360M, Lamborghini Diablo, Jaguar XJ220, Caterham, TVR etc.

3. Monocoque / Unibody
Today, 99% cars produced in this planet are made of steel monocoque chassis, thanks to its low production cost and suitability to robotised production.
Chassis Frame Full SeminarMonocoque is a one-piece structure which defines the overall shape of the car. While ladder, tubular space frame and backbone chassis provides only the stress members and need to build the body around them,  monoque chassis is already incoporated with the body in a single piece, as you can see in the above picture showing a Volvo V70.
In fact, the "one-piece" chassis is actually made by welding several pieces together. The floorpan, which is the largest piece, and other pieces are press-made by big stamping machines. They are spot welded together by robot arms (some even use laser welding) in a stream production line. The whole process just takes minutes. After that, some accessories like doors, bonnet, boot lid, side panels and roof are added.
Monocoque chassis also benefit crash protection. Because it uses a lot of metal, crumple zone can be built into the structure.
Another advantage is space efficiency. The whole structure is actually an outer shell, unlike other kinds of chassis, therefore there is no large transmission tunnel, high door sills, large roll over bar etc. Obviously, this is very attractive to mass production cars.
There are many disadvantages as well. It's very heavy, thanks to the amount of metal used. Chassis Frame Full Seminar PPTAs the shell is shaped to benefit space efficiency rather than strength, and the pressed sheet metal is not as strong as metal tubes or extruded metal, the rigidity-to-weight ratio is also the lowest among all kinds of chassis bar the ancient ladder chassis. Moreover, as the whole monocoque is made of steel, unlike some other chassis which combine steel chassis and a body made of aluminium or glass-fiber, monocoque is hopelessly heavier than others.
Although monocoque is suitable for mass production by robots, it is nearly impossible for small-scale production. The setup cost for the tooling is too expensive - big stamping machines and expensive mouldings. I believe Porsche is the only sports car specialist has the production volume to afford that.
 


Advantage:

Cheap for mass production. Inherently good crash protection. Space efficient.

Disadvantage:

Heavy. Impossible for small-volume production.

Who use it ?

Nearly all mass production cars, all current Porsche.

ULSAB Monocoque
Enter the 90s, as tougher safety regulations ask for more rigid chassis, traditional steel monocoque becomes heavier than ever. As a result, car makers turned to alternative materials to replace steel, most notable is aluminium. Although there is still no mass production car other than Audi A8 and A2 to completely eliminate steel in chassis construction, more and more cars use aluminium in body panels like bonnet and boot lid, suspension arms and mounting sub-frames. Unquestionably, this is not what the steel industry willing to see.
Therefore, American's steel manufacturers hired Porsche Engineering Services to develop a new kind of steel monocoque technology calls Ultra Light Steel Auto Body (ULSAB). As shown in the picture, basically it has the same structure as a conventional monocoque. What it differs from its donor is in minor details - the use of "Hydroform" parts, sandwich steel and laser beam welding.
Hydroform is a new technique for shaping metal to desired shape, alternative to pressing. Conventional pressing use a heavy-weight machine to press a sheet metal into a die, this inevitably creates inhomogenous thickness - the edges and corners are always thinner than surfaces. To maintain a minimum thickness there for the benefit of stiffness, car designers have to choose thicker sheet metal than originally needed. Hydroform technique is very different. Instead of using sheet metal, it forms thin steel tubes. The steel tube is placed in a die which defines the desired shape, then fluid of very high pressure will be pumped into the tube and then expands the latter to the inner surface of the die. Since the pressure of fluid is uniformal, thickness of the steel made is also uniformal. As a result, designers can use the minimum thickness steel to reduce weight.
Sandwich steel is made from a thermoplastic (polypropylene) core in between two very thin steel skins. This combination is up to 50 percent lighter compared with a piece of homogenous steel without a penalty in performance. Because it shows excellent rigidity, it is applied in areas that call for high bending stiffness. However, it cannot be used in everywhere because it needs adhesive bonding or riveting instead of welding.
Compare with conventional monocoque, Porsche Engineering claimed it is 36% lighter yet over 50% stiffer. Although ULSAB was just annouced in early 1998, the new Opel Astra and BMW 3-Series have already used it in some parts. I believe it will eventually replace conventional monocoque.  


Advantage:

Stronger and lighter then conventional monocoque without increasing production cost. 

Disadvantage:

Still not strong or light enough for the best sports cars.

Who use it ?

Opel Astra, BMW 3-series



5. Backbone Chassis
Chassis Frame Full Seminar Report and PPTChassis Frame Full Seminar Report and PPTKia's version Lotus Elan Mk II
Colin Chapman, the founder of Lotus, invented backbone chassis in his original Elan roadster. After failed in his experiment of glass-fibre monocoque, Chapman discovered a strong yet cheap chassis which had been existing for millions of years - backbone.
Backbone chassis is very simple: a strong tubular backbone (usually in rectangular section) connects the front and rear axle and provides nearly all the mechnical strength. Inside which there is space for the drive shaft in case of front-engine, rear-wheel drive layout like the Elan. The whole drivetrain, engine and suspensions are connected to both ends of the backbone. The body is built on the backbone, usually made of glass-fibre.
It's strong enough for smaller sports cars but not up to the job for high-end ones. In fact, the original De Tomaso Mangusta employed chassis supplied by Lotus and experienced chassis flex.
TVR's chassis is adapted from this design - instead of a rigid backbone, it uses a lattice backbone made of tubular space frames. That's lighter and stronger (mainly because the transmission tunnel is wider and higher).
 


Advantage:

Stong enough for smaller sports cars. Easy to be made by hand thus cheap for low-volume production. Simple structure benefit cost. The most space-saving other than monocoque chassis.

Disadvantage:

Not strong enough for high-end sports cars. The backbone does not provide protection against side impact or off-set crash. Therefore it need other compensation means in the body. Cost ineffective for mass production. 

Who use it ?

Lotus Esprit, Elan Mk II, TVR, Marcos.


6. Hybrid design

The safety cell is made through Monocoque chassis construction.  The rest of the chassis is made trough space frame design. Many unibody cars utilize some sort of front and rear sub-frame that bolts to the chassis. The sub-frames serve as mounting points for the suspension, engine, transmission and other mechanical components -- essentially all of the car's moving components. Sub-frame designs are far more adaptable for use on different chassis, and are especially useful for designs that are identical but for their wheelbase (like coupe and sedan versions of the same car). The only down side to this design is that the chassis itself may twist between the subframes, but this is easily remedied by installing an X-brace subframe connector under the Car.

It has some of the advantages of each one. Another advantage is that is simpler and cheaper to produce than Monocoque alone.

 

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