Safety: Roll Cage

You may have noticed that all saloon racing cars have lots of round metal tubing inside the car. The common conception is that a roll cage is there to protect the driver (and co-driver in rallying). Well this is only partly true. A roll cage also has the benefit of stiffing the cars chassis which reduces body twist and roll.

The decreased level of twist and roll results in improved handling and the cars suspension, caster, camber and toe settings can be made more precise which in turns makes the cars easier to drive, more consistent and reduces tyre and component wear.

This update will explain how roll cages are classified, the materials used and how they installed.

Classification

There are two types of roll cage, these being bolt-in and weld-in. The difference is in the way the roll cage is fitted to the car. The cage design and fabrication is fundamentally the same. The bolt-in cage as the name implies is bolted into the car (thus making it more easy to remove), the weld-in cage is welded into the car.

A weld-in cage is lighter than a bolt-in cage and makes the car shell more structurally rigid. In addition the world governing body (FIA) and country specific governing bodies (eg: MSA in the UK) mandate race cars have to have weld-in roll cages.

Design

Roll cages are often referred as “point” cages, the points refer to the number of points the cage is fixed to the car. A 4 point cage is fixed to the car at 4 points, a 6 point at 6 points etc.

Material

Roll cages are built out of either high tensile carbon manganese steel (T45) steel or Cold Draw Seamless Carbon Steel (CDS). T45 has a higher tensile and minimum yield strength. Roll cages constructed using T45 will be lighter than the same cage design using CDS. However, T45 is more expensive.

Components

A post gusset

Improves strength as the cage A pillar can be welded to the screen pillar

Dash bar

Improves rigidity and side impact strength. 

Door X bars

side impact protection; improves door aperture stiffness. 

Front cross

Improves lateral stiffness of strut tops and bulkhead. 

Hoop to A pillar tube

Designed to improve support for A pillars at roof level. 

Hoop cross

X section now required by the latest regulations to improve roll over protection.

Lower rear stay

Triangulates loads from the rear struts so improving rigidity. 

Mid rear stay

Adds further stiffness to rear section; improves side impact strength.

Rear X

Stronger than the simple single diagonal tube. Gives additional strength to rear damper mounts. 

Roof cross or roof ‘V’

Increases roll cage rigidity and safety in the event of a roll over; essential in later model rally cars. 

Screen pillar support

Important for latest generation saloons with a steeply raked windscreen to provide additional support to the top of the A pillar/roof section. Now required by the latest FIA regulations. 

Seat harness tubes

Needed for wrap-over seat belts. Safety improved with shorter belt length and better belt angle. 

Single roof diagonal

Lighter than a full roof cross but provides less protection – acceptable for race cars. 

Triangulation tubes to front struts

Allows the front suspension loads to be fed into the whole cage structure; provides protection for occupant’s feet. It also improves side impact protection and increases the load capacity of the ‘A’ pillars/lateral hoops. 

Tunnel brace

Improves side impact protection and supports the main hoop in the event of a roll over. 

Race Car Roll Cage

The race car has a roll cage manufactured by Custom Cages (UK) and has been constructed from Cold Draw Seamless Carbon Steel (CDS). The diameter of the bars varies between 40mm and 45mm, with the wall thickness being between 2mm and 2.5mm, with a minimum tensile strength of 360 N/mm2.

Custom Cages have provided a roll cage certificate and plaque (welded onto the roll cage). The team have to provide  this information when the car is undergoing event technical inspection.

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