The Passenger Safety Cell or safety cage is the area of a vehicle's body that surrounds the passenger compartment and is
strengthened or stiffened to provide extra protection for the occupants.
The passenger safety cell is a passive safety feature and works together with the crumple
zones to protect the occupants of a vehicle in the event of an accident.
As the crumple zones absorb the forces of an impact, the safety cage is designed to remain rigid and not crumple around the
The safety cell is necessary because the passengers would be in serious danger if the entire vehicle crumples upon an impact.
So part of the vehicle crumples and the part directly surrounding the passenger compartment acts as a cage to protect the
persons inside of it.
The first passenger safety cell was patented in a car body by Daimler-Benz AG on January 23rd 1951 and the safety cell is now
widely used throughout the automotive industry.
The strengthened areas that form a safety cage are usually the pillars (from floor to roof), the frame of the roof and the
floor pan which will form a cage like structure.
Other elements such as the side impact bars in the doors also add to the overall safety
of the passenger safety cell.
The method used to strengthen these areas is by using stronger materials for instance by using a different material from other
parts or by using stronger grade of alloys.
If the vehicle is using a monocoque bodyshell, these strengthened areas can serve
dual purposes as these areas are usually strengthened as part of the chassis.
The monocoque chassis of the BMW X6 (except floorpan) with the passenger safety cell
The usual monocoque materials are used in safety cages. Special selected alloys of steel and
aluminium are used. Even carbon fiber can offer varying levels of stiffness.
Even the passenger safety cell which is entirely stiff can have varying levels of stiffness for example the B pillar may be
stronger than the other pillars.
In the unlikely event of a collision or roll over the passenger safety cell is designed to protect the occupants but they can be
injured if thrown on these stiff areas.
This means that many passive features also serve to protect passengers from the impact and the cage. Features such as
side airbags and pretensioners can protect in two ways.
A passenger safety cell is one of the reasons that allow the interior of a vehicle to barely have small amounts of damage while
the exterior can be badly damaged.