Roll cage

A roll cage is a specially designed protective frame built in (or sometimes around) the passenger compartment of a vehicle. Generally made of welded steel tubing, a roll cage is a safety feature in the event of a rollover or crash. Roll cages are used mainly in auto racing and off-road sports such as rockcrawling. The roll cage also stiffens and adds strength to the vehicle which improves handling. When the cage is constructed on the outside of a vehicle it is usually called an "exo-cage".
Designs.
There are many different roll cage designs depending on the application. Different racing organizations have differing requirements and regulations for roll cages. A roll bar is a single bar behind the driver that provides some protection. Due to the lack of a protective top, some modern convertibles use a strong windshield frame which acts as a roll bar. Also, a roll hoop may be placed behind both front headrests.
A newer form of rollover protection, pioneered on the Mercedes-Benz R129 in 1989, are hidden roll hoops. When sensors detect the vehicle is about to roll over, the roll hoops quickly extend and lock in place. Cars that have this kind of "deployable rollover protection" system include the Peugeot 307 CC, Volvo C70, Mercedes-Benz SL 500, and Jaguar XK.
Roll bars are also used historically on row crop tractors, and roll cages are common as part of the cab on modern tractors.
Construction.
A roll cage is not just steel tubing welded together. It must be designed to hold the weight of the vehicle should it roll over or land upside down. A basic roll bar usually has 4 points of contact with the vehicle chassis. A typical roll cage may be an 8-point or more design. In many cases a roll cage designed for a particular car or truck may be purchased from a professional roll cage manufacturer. Kits are available which are easier to transport and must be welded together inside the vehicle.