Rollcage 101 From Velocity Magazine May 2012

https://porscheclubracing.org/wp-content/uploads/2012/09/VELOCITY-57-2.pdf

ROLLCAGE 101 Y Words: Jim Pierce

Jim Pierce welds a rollcage.

An average cage for a touring car

involves about 260 mitered tube

cuts, 28 tube bends, and over

1600 pulse welds. From

beginning to end, the build takes

about five days. Photo by Mike Carson

Koni Challenge ST Class Boxster with

NASCAR bars on the side.

Photo by Jim Pierce

Pikes Peak Hillclimb

car with continuous-tube

X and massive gussets.

Photo by Jim Pierce

Randy Takaki’s Spec Boxster

illustrating mig welds and

roof spreader-gusset.

Photo by Jim Pierce

car stiffer while minimizing weight?” I always

joke with, “Well, how hard do you plan on

crashing?” It’s an educated guess, but I start

by assessing risk, looking at the car’s overall

speed, and then considering the character of

the racing series.

For instance, the amount of tubing

needed in a 165 mph GT3 is greater than

what the Spec 944 car is going to need. The

GT3 is typically going to go off at a higher

speed than the 944. Now if you’re racing in a

series that’s modeled after a bumper-car race

like the British Touring cars or Grand Am’s

feeder series, Continental Sports Car Challenge,

then you’re going to need massive

amounts of protection. If you’re racing Spec

Miata at Infineon raceway up north where

you can hit a wall and destroy a racecar in

nearly every turn because of the track’s nature,

you’re gonna need more bars. Finally, if

it’s a more complex build where the car has

been acid dipped and cut to shreds to remove

weight, then adding more cage helps

strengthen and stiffen a flimsy shell.

The first freedom we’re given (in most

instances) is the type of material used. In

America you’ll come across three different

types – mild steel, DOM, and chromoly.

“Well, how

hard do

you plan

on crashing?”

14 VELOCITY • MAY 2012

The differences among the three are in the

processes by which they are made, and the

amount of carbon/alloy in the material. Mild

steel – also known as ERW – is typically a flat

sheet which is rolled and then electronically

welded together, leaving a visible seam. DOM

goes through a similar process and then is

cold-drawn over a mandrel giving it more integrity,

and the seam is hidden so the tubing

appears seamless. Chromoly is extruded, and

along with DOM is much truer in thickness

and diameter than the ERW.

Weightwise, the three types are nearly

identical. There is a common belief that

chromoly is lighter – it’s not. Strengthwise,

however, there is a huge difference between

the mild steel and the other two. The mild

steel has a low carbon count, making it more

more of the impact, giving more “cushion”

to the driver. Perhaps that’s why several organizations

don’t allow chromoly anymore

for new builds. We build probably 30 DOM

cages to every one chromoly (for off-road

racing cages the ratio is one-to-one).

The second freedom is tubing size; for

most of us it’s going to be a choice between

1¾” x .095 wall and 1½” x .120 wall tubing.

Generally speaking, the larger 1¾” diameter

is going to be stronger in both tension (trying

to pull a tube apart) and compression

(trying to compress it from the ends). Also

it’s just a tiny bit lighter. However it’s more

difficult to fit into tighter cars because it’s

bent using a larger radius die, and it’s not

as strong in shear, meaning the tubing will

crush easier in side impacts. It’s a tossup for

most of us, and I can tell you it’s much easier

to get the 1½” into tighter, smaller cars,

especially when larger drivers are involved.

Considering that the wild end-over-end flips

that SPEED likes to showcase during commercials

are rare compared to driver-side

door impacts, it makes the decision more

challenging. We build 60% of our road race

cars from 1½” tubing and 40% from 1¾” tubing,

but we build nine out of ten rally cars

with the stronger 1¾” material.

The third freedom given to the constructer

is the choice of welding. For most of

us it’s going to be either MIG or TIG welding.

MIG uses steel wire fed through a “gun”

mixed with argon/CO2 while TIG uses a rod

and “electric” torch with various gases cleansing

the weld area. A TIG welder has a little

bit better control of the heat, which can be

modulated during the process, while the MIG

welder’s temperature is fixed before the process

is started. The MIG joints usually have a

wider weld bead covering more area of the

joint, while the TIG process requires a cleaner

material and more precise notch, leaving a

much smaller weld bead and affected area.

Done correctly, both processes are

equally strong and safe, and both are pleasing

to the eye. I think as racecar drivers we

enjoy looking at pretty weld beads – it’s in

our DNA. Over the years, I’ve discovered it’s

much easier to hide an improper TIG weld

than MIG joint, and consequently we’ve

seen more failures in TIG welds in crashed

racecars. In my shop we primarily construct

our cars using the MIG process (probably 25

to 1) because of the safety and cost savings

that are passed on to our customers.

Although he design of the cage is usually

determined by the sanctioning body,

there’s certainly room for creative freedom.

First off, to maximize the envelope of safety

for the driver, the cage has to be fit snug up

against the car’s shell, against the A-pillars

and the B-pillars, and all around the driver.

You want the most clearance around the

driver’s head and body. Having the cage

up against the shell also helps reduce flex

and stiffens the chassis. I see cars in which

I can stick my fist between the roof and

mainhoop, or worse, between the A-pillar

and mainhoop by the driver’s head. To me

a poor fitting design is a sign of a lazy or

inexperienced fabricator.

The joints have to be mitered correctly

and welded completely all the way around.

This takes some skill. If the cage is touching

the body or roof where it’s supposed to

be, there’s no room to weld! We’ll usually

build either a box under the mainhoop and

A-pillar or drill a hole in the chassis so that

we can lower the cage down, weld the tops,

then raise it back up on the box or slide a

floorplate under the cage, covering up the

hole. Additionally the floorplate or box

needs to be on at least two surfaces (floor

or sill, and rocker panel top or side) and

have adequate surface area to distribute the

force applied in the crash so as to not tear

through the unibody.

From years of seeing roofs crack and

cars roll, we’ve realized that the roof area

up front is the first to fail and crumple, so

we like to use gussets in the roof spread-

Jim Pierce is one of few driver/constructors

in the world to have won

major championships in desert, stadium,

rally, and road racing, which

gives him a unique perspective into

the design and safety of modern race

cars. He owns Advance Automotive

in Torrance, CA (310 542-2977,

piercemotorsports.com), where he

prepares off-road, rally, and roadrace

vehicles.

Top:

Spec Boxster floor

plate welded to

three surfaces for

load distribution.

Photo by

Jim Pierce

Left:

Tight fit with good

notch.

Photo by

Jim Pierce

ROLLCAGE 101 ROLLCAGE 101

er area. A gusset more than triples the

strength of the joint, adding significant

safety for the driver. If allowed, we tie the

A and B-pillars to the chassis; triangulation

throughout the cage is a must to help

spread the applied forces of an impact.

Doorbar design on the driver’s side in a

car that races wheel-to-wheel should have

a NASCAR doorbar or a gusseted X, period.

We prefer the NASCAR design, and when

we do Xs we use two continuous bars rather

than a broken X so the driver doesn’t get

skewered during a hard hit.

Ok, so you’ve decided on a 1¾” x .095

DOM, MIG-welded, triangulated, gusseted

rollcage with a pretty two-tube, GT3 style

boxed-in X in the drivers door going through

the front bulkhead, tying into front and rear

strut towers. That’s a mouthful. So what’s this

thing gonna cost, you ask? I can tell you that

1¾” DOM is between $5 and $6 a foot, and

a typical cage uses about four 20-foot sticks,

while the “fancy” cages use five sticks. At $100

a stick for tubing, plan on a $400-$500 material

cost. When you show up with a gutted car

(interior removed), labor for a bare bones

MIG’d cage at our shop is about 20 hours or

about $1500.

So an entry level cage would be in the

neighborhood of about $2000. The cage debendable,

while the DOM may have more

carbon and has been “trued up” by the mandrel,

making it much stronger; and finally

chromoly has alloy blended in with an even

higher carbon content, giving it a tad more

strength and rigidity than DOM and nearly

twice that of ERW. ERW has not been legal

to construct with for over 10 years now, but

I wanted to include it to show how much

stronger DOM and chromoly are.

So that leaves us with DOM and chromoly.

The moly is a bit stronger and more

rigid than the DOM. This has both benefits

and drawbacks. It does make for a more rigid

chassis, but it has more tendency to crack

during an impact; and more importantly, it

transfers more energy to the driver during

a hard crash. DOM, being less rigid, absorbs

scribed above is about a 32-hour build, or

about $2900 with material. A complete rallycar

FIA cage, which requires an extra stick

of tubing, is about $3500. Add $750-$1000

more for a TIG’d cage, and another couple

hundred if you want chromoly. These prices

are pretty standard throughout the industry.

I’d be nervous about a shop that says they

can do the whole job for $1500 and suspicious

of the shops charging $6000 unless

they’re removing interior pieces and installing

seats and other safety equipment.

The bottom line is, you usually get what

you pay for, and in the end, what’s your safety

really worth?