What are the Types of Automobile Chassis ?

Types of Automobile Chassis

1. Body-On-Frame Chassis:

  

This is also known as the conventional chassis or the frame-full chassis. In this type of vehicle chassis, the body is made as a separate unit, then joined with a ladder frame. It is used in trucks, SUVs, and bigger vehicles. This chassis is often built so solidly that it sustains little or no damage in an accident. 

A body-on-frame chassis is easier to modify or repair in case of an accident. This type of chassis makes the vehicle slightly harder to handle and steer. This kind of chassis structure is more durable than a unibody structure. The ladder frame chassis is a robust metal framework consisting of two large metal beams joined together by shorter support beams in between. 

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With a ladder frame chassis, the body and engine of the vehicle is mounted on the top of the ladder frame making it a robust structure ideal for Commercial trucks, SUVs, and 4 x 4 Utes that are meant to carry heavy loads for extended periods.

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Examples of Body-on-Frame chassis vehicles:

      1.    Jeep Wrangler

      2.    Land Rover Defender

      3.    Toyota Land Cruisers

      4.    Nissan Patrol

      5.    Toyota Tacoma & Hilux

      6.    Ford Ranger

      7.    VW Amarok

  

Advantages of Body-On-Frame Chassis:

      1.    Higher load-carrying capacity and strength.

      2.    Easier to repair after accidents

      3.    The detachable bodywork allows for quicker, easier repairs.

      4.    Easy to design, build and modify.

      5.    It has off-road superiority.

      6. These chassis have a robust framework & rigid structure.

Disadvantages of Body-On-Frame Chassis:

      1.    The body tends to vibrate easily and the overall vehicle handling and refinement is lower.

      2.    Performance figures are low and fuel consumption is higher due to the weight.

      3.    Heavier vehicle mass makes turning in an emergency is gets difficult.

      4.    The high center of gravity is negatively impacts the vehicle cornering ability as there is excessive torsional flexing.

      5.    Braking is weak due to the vehicle size and weight.

      6.    Body roll can be reduced by upgrading the suspension to a stiffer setup.

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2. Tubular Frame Chassis:

As ladder chassis is not strong enough, motor racing engineers developed a 3- dimensional design. This type of frame chassis employs dozens of circular-section tubes (some may use square cross-section tubes for easier connection to the body panels, though circular section provides the maximum strength), positions in different direction to provide mechanical strength against forces from anywhere. These tubes are welded together and forms a very complex structure as you can see the above picture. 

One of the earliest examples was the post-war Maserati Tipo 61 “Birdcage” racing car. For higher strength required by high-performance sports cars, tubular space frame chassis usually incorporate a strong structure under both doors (see in picture of Lamborghini Countach), hence result in unusually high door sill and difficult access to the cabin.

In the early 50s, Mercedes-Benz created racing car 300SLR using tubular space frame chassis. This also brought the world's first tubular space frame chassis road car, 300SL Gullwing. Since the sill dramatically reduced the accessibility of cabin, 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 frames to enhance the rigidity / weight ratio. However, many of them actually used tubular space frames for the front and rear structure and made the cabin out of monocoque to cut cost.

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Examples of Tubular Frame Chassis vehicles:

       1.    All Ferrari before the 360M

       2.    Lamborghini Diablo

       3.    Jaguar XJ220

       4.    Caterham Cars

       5.    TVR Tuscan

Advantages of Tubular Frame Chassis:

       1.    Tubular frame chassis has high strength.

       2.    A tubular chassis car lighter in weight.

       3.    Tubular chassis cars are stronger and stiffer than Body on Frame chassis cars.

       4.    This kind of chassis has high torsional rigidity.

Disadvantages of Tubular Frame Chassis:

       1.    Tubular frame chassis have very complex structure.

       2.    Tubular frame chassis is costly and time consuming to be built.

       3.    Not possible for robotize the production.

      4.    It engages lot of space, raise the door sill and result in difficulty to access the cabin.

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3. Backbone Frame Chassis:

Backbone tube frame chassis is the type of automobile construction chassis that is similar to the Body-On-Frame design. Instead of two- dimensional ladder-type structure, it consists of a strong tubular backbone that connects the front and rear suspension attachment areas. 

A body is then placed on this structure. Colin Chapman, the founder of Lotus, invented the 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 all the mechanical 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 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 adopted from this design instead of a rigid backbone, it uses lattice backbone made of tubular space frames. That’s lighter and stronger (mainly because the transmission tunnel is wider and higher).

Examples of Backbone Frame Chassis vehicles:

       1.    Lotus Espirit

       2.    Elan MK II

       3.    TVR

       4.    Marcos

Advantages of Backbone Frame Chassis:

       1.    Strong enough for smaller sports cars.

       2.    Easy to be made by hand thus cheap for low-volume production.

       3.    Simple structure benefit cost.

       4.    The most space-saving other than monocoque chassis.

Disadvantages of Backbone Frame Chassis:

        1.    Not strong enough for high-end sports cars.

        2.    The backbone does not provide protection against side impact or off-set crash.

        3.    It needs compensation means in the body.

        4.    Cost ineffective for mass production.

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4. Monocoque or Unibody Frame Chassis:

  

Today, 99% cars produced in this planet are made of steel monocoque chassis, thanks to its low production cost and suitability to robotized production. Monocoque 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, monocoque chassis is already incorporated with the body in a single piece, as you can see the above picture showing a Volvo V70.

In fact, the one-piece chassis is actually made by welding several pieces together. The floorplan, which is the largest piece, and other pieces are press-made by big stamping machines. They are spot welded together by robotic arms (some even use laser welding) in a stream production line. The whole process takes just a few 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 rollover bar etc. Obviously, this is very attractive to mass production cars. There are many disadvantages as well. Its very heavy, thanks to amount of metal used. 

As 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 aluminum 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 tooling is too expensive mouldings. I believe Porsche is the only sports car specialist has the production volume to afford that.

Examples of Monocoque or Unibody Frame Chassis vehicles:

1.    All current Porsche

2.    Nearly all mass-production cars.

Advantages of Monocoque or Unibody Frame Chassis:

           1.    Cheap for mass production.

           2.    Inherently good crash protection.

           3.    This chassis is space-efficient.

Disadvantages of Monocoque or Unibody Frame Chassis:

           1.    This kind of chassis is heavy in structure.

           2.    Impossible for small-volume production.

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5. ULSAB Monocoque Frame Chassis:

Enter the 90s, as tougher safety regulations ask for more rigid chassis, traditional steel monocoque becomes heavier than ever. As a result, carmakers turned to alternative materials to replace steel, most notable is aluminium. Although there is 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, an alternative to pressing. Conventional pressing use a heavyweight machine to press a sheet metal into a die, this inevitably creates inhomogeneous thickness – the edges and corners are always thinner than surfaces. 

To maintain a minimum thickness there for the benefit of stiffness, car designer have to choose thicker sheet metal than originally needed. The hydroformed technique is very different. Instead of using sheet metal, it forms thin steel tubes. 

The steel tubes placed in a die which defines the desired shape, then fluid of very high pressure will be pumped into the tubes and then expands the latter into inner surface of the die. Since the pressure of the fluid is uniform, the thickness of steel made is also uniform. As a result, designers can use the minimum thickness steel to reduce the weight.

Sandwich steel is made from a thermoplastic (Polypropylene) core in between two very thin steel skins. This combination is up to 50% lighter compare with a piece of homogeneous 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 announced 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.

Examples of ULSAB Monocoque Frame Chassis vehicles:

         1.    Opel Astra.

         2.    BMW 3 – Series.

Advantages of ULSAB Monocoque Frame Chassis:

        1. Stronger and lighter than conventional monocoque without increasing production cost.

Disadvantages of ULSAB Monocoque Frame Chassis:

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

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6. Hybrid Design for Chassis Frame:

The safety cell is made through monocoque chassis construction. The rest of the chassis is made through space frame design. Many unibody cars utilize some sort of front and rear sub-frame that bolts to the chassis. 

The sub-frames serves 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 downside to this design is that the chassis itself may twist between the sub-frames, but this is easily remedied by installing an X- brace sub-frame connector under the car. It has some of the advantages of each one. Another advantage is that is simpler and cheaper to produce the Monocoque alone.