What Is An Automobile Chassis And What Are Its Various Types?

A chassis is the primary load-bearing framework of any automobile. It also serves as the mounting point for various components of the vehicle.

If you have ever seen a rally championship, you know that while the car might look similar to its road-going counterparts, it has stronger underpinnings that allow the driver to subject it to some pretty severe abuse.

What is a chassis?

A vehicle experiences a multitude of static and dynamic loads, all of which can be allocated to the categories stated below:

Static loads, such as the weight of mechanical and other components, passengers and luggage, or dynamic loads such as:

  1. Vertical and shear loading, when driving over an uneven surface
  2. Inertial stresses due to acceleration and braking
  3. Centrifugal forces when taking turns, especially at high speeds
  4. Torque from the engine and transmission
  5. Lateral forces in the event of a collision, crosswinds etc.
Ken Block drives a Monster World Rally Team Ford Focus RS WRC 08 car during Rally of Turkey 2010 WRC championship(EvrenKalinbacak)s

A rally car experiences extreme stresses that are borne by the chassis (Photo Credit : EvrenKalinbacak/Shutterstock)

In order to withstand loads like these, it is important for the automobile to have a sturdy load-bearing framework that does not deflect or distort upon impact. This framework is known as a chassis.

Even though we may not be rally racers ourselves, our vehicles also face a multitude of stressors on a regular basis. The chassis of any automobile is therefore a component that involves intensive engineering.

A chassis is designed to suit a certain application, which is achieved by design trade-offs. While a vehicle chassis may occasionally withstand forces, it is not designed for prolonged exposure to such loads, which can often result in catastrophic failures. For example, a truck cannot take turns like a race car without the risk of flipping over on its side. Likewise, a race car cannot be made to haul heavy loads without the appropriate architecture to support it.

Materials used to form the chassis

Traditionally, chassis have been fabricated from carbon steels. Following the need for developing lighter vehicles, there was a shift to aluminum alloys that were as strong as their predecessors, but not as heavy.

Zonda R Rear 1

Carbon composites such as carbotanium are used to make sports car chassis, e.g., the Pagani Zonda (Photo Credit : Pagani Automobili/Wikimedia Commons)

However, with the advent of carbon composites, such as carbon fiber and carbon-titanium alloys, the weight of chassis has reduced significantly while also improving in strength. While carbon composite chassis are commonly seen in sports cars, they have yet to be employed in regular vehicles.

Cross-section of chassis

A chassis framework comprises various links that are joined together, either by forming or welding. In order to maximize resilience and load-bearing capability, while not increasing their own weight, chassis links have cross-sections.

Chassis cross-sections can either be open-ended, such as a C- or U-shape, or closed, such as a box shape or a tube.

industrial material

Various cross-sections of chassis links (Photo Credit : Mathisa & 25krunya & seeshooteatrepeat/Shutterstock)

While open-ended chassis frames can bear heavy loads, they are not as good when it comes to transverse loads. Modern constructions often involve open cross-section reinforcement for closed cross-section chassis for extra support. Tube-type chassis are primarily used in two-wheeled vehicles.

Various types of chassis

While the chassis of double-track vehicles, such as cars and trucks, share commonalities, single-track vehicles like bicycles and motorcycles have a profoundly different chassis design.

1. Single-track vehicle chassis:

Backbone frame

The frame is a spine that simply connects crucial components, but does not support them. The engine is bolted to the frame at one end and remains suspended at the other. This is typically employed in low-cost motorcycles that have a small engine and do not see heavy usage.

Cradle frame

A cradle frame is similar to a backbone frame, except that it uses an additional tube link support for the engine. The ‘cradle’ can either be a single tube or double tube, depending on the size of the engine fitted to it. A single cradle frame is cost-effective to manufacture, but does not exhibit superior load-bearing and handling properties at high speeds. A double-cradle frame comprises two tube links that hold the engine. They exhibit better load-bearing and handling properties due to these two links.

Single cradle frame as seen on a bike

Single cradle frame as seen on a bike (Photo Credit : Khomenko Serhii & Aisyaqilumaranas/Shutterstock)

Perimeter frame

A perimeter frame is fashioned out of box frames, rather than tube links, and connects the steering link to the rest of the body using the shortest possible linkage. The engine is suspended, as in a backbone frame, but the structure makes this style more conducive for high-performance applications.

KTM Duke 390 at KTM exhibition booth at BANGKOK MOTOR SHOW 2018 in Bangkok( Akarat Phasura)S

A trellis frame is a special type of perimeter chassis that employs tubes arranged in triangles (white and orange links, as seen here) (Photo Credit : Akarat Phasura/Shutterstock)

A trellis frame is an iteration of the perimeter frame that employs tubes instead of a box frame to reduce the overall weight of the construction. Like a perimeter frame, it sees usage in high-performance motorcycles.

2. Double-track vehicle chassis:

Ladder frame

This chassis is composed of two parallel beams connected to each other by means of cross members to make a ladder-like structure. All components, including the body, are mounted on the ladder frame. Vehicles with a ladder frame chassis have a higher center of gravity and exhibit superior vertical load-bearing capabilities. However, they are not very resistant to cornering forces that are encountered when taking turns at high speeds. While they were once popular in older cars, they are now commonly found only in trucks and buses.

open truck chassis(multitel)s

Ladder frame on a truck bed (Photo Credit : multitel/Shutterstock)

Unitized body chassis

Instead of fixing a body that does not have load-bearing capabilities on a frame, a unitized body chassis makes the body an integral member of the frame. Such a chassis is inherently safer and has a low center of gravity. It keeps the vehicle stable when cornering at high speeds and responds well to warping loads. Due to the entire body being a load-bearing member, collisions are absorbed effectively, which minimizes the risk of injury during mishaps.

Unitized body chassis are also known as monocoque chassis and are almost ubiquitously employed in cars.

Rimac's carbon fibre monocoque at the Geneva International Motor Show 2019( Kaukola Photography)s

A unitized body or ‘monocoque’ chassis comprises a body that is integral with the frame (Photo Credit : Kaukola Photography/Shutterstock)

Special purpose chassis

These are iterations of either ladder-on-frame or monocoque chassis. They are purpose-built and are not seen in commercial usage. One such example is the carbon fiber monocoque chassis, commonly referred to as a carbon fiber tub, which is used in Formula 1 racing.

Suggested Reading

References

  1. Automotive Engineering: Powertrain, Chassis System and Vehicle Body Hardcover – 16 Sep 2009 by David Crolla Professor
  2. Bieap
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Prashant is a mechanical engineer and MBA from NMIMS University, Mumbai. An auto-fanatic with an insatiable need for speed, he is constantly on the look out for new technology in the field of automobiles. When he is not working, he loves to read, blog about cars, test the latest rides on the market and wash his own vehicles.

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