Technology – Vehicle Chassis

11 05 2009

UNIBODY VEHICLE CHASSIS STIFFNESS STUDY USING ABAQUS

Most street driven cars are manufactured using a construction technique known as unibody spaceframe chassis construction. This means that the body itself provides the stiffness and structure of the vehicle. Many older vehicles had separate stiffening structures and bodies, the body being solely designed as an aesthetic exterior and as a safety and environmental housing for the passengers. This technique is both heavier and requires more materials raising costs. Modern vehicle chassis are made entirely out of formed sheet metal sections which are usually spot welded together to form the structure of the vehicle. Designing these chassis is no easy task, the geometries being incredibly complex. Making these designs both cost effective, lightweight, stiff and safe is always a tradeoff.

Production vehicle chassis vary substantially in the degree to which they maximize the variables previously mentioned. Higher performance vehicles are often designed with maximum stiffness and lightweight in mind. A lighter weight chassis promotes an overall lower vehicle weight and better performance. Chassis stiffness though is also important for vehicle performance.

As a vehicle is driven, various forces are applied by the suspension on the chassis. This occurs under braking, cornering, driving surface variations, or any other vehicle movement. In higher these loads are, the more is demanded of a chassis. The chassis obviously has to be strong enough not to fail under these loads, but beyond that it must not deflect appreciably. Suspensions are carefully designed to position the wheels and tires of the vehicle for optimum performance under all conditions of vehicle use. If the chassis deflects when the forces are high, it causes suspension mounts and attachment points to temporarily shift, which destroys the careful suspension design when it is needed most. Chassis stiffness is most important in high performance or racing cars, where suspension loads are at their highest and suspension adjustment is most critical.

Chassis Development

Through out the development of the motor vehicle, many different types of chassis have been designed and tested. Through the years many advances have been made in chassis technology, and many old technologies have been thrown out as inferior. Here are some of the basic types which were or are in common use.

Ladder Chassis

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.Major structure of chassis is supported by central rails connected by cross braces. Still used in trucks and SUVs due to good isolation between passenger cabin and road vibration. Since it is two dimensional it is not very stiff, and needs to be built heavier than a good space frame.

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.




Hummer H1& H2 SUV Ladder chassis


57 Chevy Classic Ladder chassis
http://autoweldchassis.com/56chevy.ivnu

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

Who use it ? Most SUVs and all classic cars. I.e 57 Chevy

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