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Rollcage 101 From Velocity Magazine May 2012
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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?