- ERA generally does not do direct comparisons with other
companies. We would rather have the customer do that research
- and be sure of the unbiased results. However, Factory
Five has made several
statements which at least indirectly challenge the accuracy of some of my statements about
ERA products. We have also had many calls asking why we don't
make a "superior" round-tube frame like Factory Five. Because
some of FFR's claims are technically beyond most
non-mechanical-engineers to check, ERA feels obligated to set the
record straight. We apologize for the technical stuff, but it
is the only way for us to lay it out for you without making vague
claims that cannot be refuted because they mean very little.
I have quoted some sections of the
literature where I have some significant technical
disagreements. You can make up your own mind on this, and
also refer back to our Design Logic
page. Factory Five's text is in BLACK, ERA comments are in RED or VIOLET depending on
how serious I think Factory Five's errors are.
FACTORY FIVE RACING - 427 SC Cobra Replica
I. Frame Design and Engineering
"The Factory Five 427 SC Replica frame serves
as the base of the kit. It is computer designed to be dimensionally and
functionally the same as Ford and Shelby's original frame. The fit and
finish of this piece is of the highest level."
"There is ongoing discussion and debate regarding the
use of round vs. square tubing in frame design. Factory Five Racing
believes that the original 4" round tube frame with round tube cross
members and vertical cross supports as designed by Carroll Shelby and
the engineers at Ford Motor Company is the best evolution of a Cobra
ladder type frame. The important facts are as follows:"
"1. Round tube weighs less than square
True - for a
given outside dimension there is less material in a round tube.
"2. Round tube is available in stronger steel. There is no rectangular
frame made from 1020 cold rolled steel, which is 37% stronger than 1010
square tubing. "
According to my
references, A500 (rectangular material) has a yield strength of 58
Kpsi, 1020 cold roll @ 78 Kpsi (as far as I can tell, this is not used
in tubing larger than 2"), 4" DOM tubing (the most likely that FF is
using) is 70 Kpsi in sizes above 2.75" OD. So the advantage
in MATERIAL STRENGTH ALONE is from 21% to 36%. Of
course, this does not take into account the shape or size of the tube,
only the material. A 4" x 3" rectangular tube put
in vertical bending mode will support about 11% less before failure
than the higher yield strength round tube. Have you ever
heard of a chassis failing in this mode? Me neither...
a chassis has a structural failure in a main rail, you've got much
bigger problems than I can describe here.
For an article on stress vs stiffness
from Machine Design Magazine, click here.
|BUT, strength is not a big factor in overall
chassis design, STIFFNESS
"3. In Torsion, (exactly the kind of stress
that cross members are subject to) round tubing is more than four times
stronger than square tubing."
First of all, see
the previous answer about stiffness vs strength.
there appears to be an error in their math anyway. My results
for stress/unit torque are:
4" x .125"W
Round tubing > Torsional stress (psi) = .35 x Torque
4" x 3" x
.125"W Rectangular tubing > Torsional stress
(psi) = .39 x
"four times as much" difference is really a little over 11%.
check my math! Here are the formulae (taken out of a
mechanical engineering reference book):
"4. Under vertical bending loads, square tubing made
from the same steel as round is stronger, but go back and read #2,
hence, there is no significant difference between 4" cold rolled 1020
round tube and 4" square 1010 tubing with respect to vertical bending
loads (same as thickness)."
Again, see my
comments above about strength vs stiffness.
"5. The end result of using round tubing is the
lightest ladder frame possible, with superior torsional load rigidity
and vertical load resistance equal to any square tube design."
Again, in a complex
structure, it is the total design that determines rigidity, not
individual pieces. Their claim over "any square tube design"
is taking hyperbole a little too far.
"6. Perhaps the most interesting fact is that square
tubing is easier for the kit car manufacturer to work with and is half
In order to be a
better overall design, our chassis is much more difficult to build than
FF's. The difference in material cost is only about
$40. Our chassis uses rectangular tubing because it works better in this application.
chassis of the new C-5
They start out with a round tube - it's easier to bend. Then
they cap the ends and use hydraulic pressure to form the tube into a
rectangular shape in steel dies. They do this extra step not
for economy, but because
it makes a better chassis. ERA gets the same effect by using
rectangular tubing from the beginning, although we have the added
effort of cutting and welding to get the ideal basic chassis design.
"The Factory Five Racing Cobra comes with a
beautifully crafted non- stressed (which means it is rubber mounted and
not resin bonded to the substructure) fiberglass body with mounting
plates and pre-drilled mounting holes. The standard body is a hand laid
polyester resin shell. Epoxy resin or carbon fiber bodies are available
Just because a body
is mounted in rubber, doesn't mean it is unstressed. In a
poorly designed structure, the weight of the fiberglass itself will
cause stress cracks. The early Corvettes had a rubber-mounted
body - and lots of stress cracks. Cracks are cause by flexing
and vibration. With a stiff chassis and properly braced body,
you don't get stress cracks - and you don't get cowl shake either, a
typical characteristic of rubber-mounted bodies.
"The average weight of a fiberglass replica
body with interior panels can exceed 600 lbs." (ERA's is about 350 lbs)
"The reason for the weight is that fiberglass is commonly used as part
of the structure, requiring thick lay-ups to avoid stress cracking."
(Stress cracks come
from flex, but a thin body won't crack if properly supported.
It's only if large sections are allowed to vibrate, that a body will
"Factory Five Racing realized that
fiberglass is best used to produce an elegant exterior body shell. By
using fiberglass for the exterior body shell only, removed the stress
and significantly reduced the weight. The ultra-lightweight body is
attached via 20 isolated rubber mounting points. The body uses a matrix
of 3/4" tubing for support which is welded to the frame, not molded
into the fiberglass shell. Many builders bond steel to fiberglass
routinely to gain extra rigidity and strength. (Not ERA, of course...)
While this makes it easy to build, any metal bonded to fiberglass will
ultimately distort (due to the different thermal coefficients of
expansion of the materials). Factory Five Racing uses metal plates only
to reinforce body mounting holes, and they are on the lower edge of the
body where they remain unseen.
Most kit manufacturers mold their own bodies. Factory
Five Racing decided to go with the experts. The body is molded by some
of the best composite materials craftsmen in the business. In addition
to molding the Factory Five Racing Cobra body, they build some of the
world's most advanced America's Cup Yachts. The fit and finish of the
fiberglass body on your Cobra will be as good or better than anything
V. Front Suspension
The Factory Five 427 SC Cobra comes with a
fully independent tubular front suspension with adjustable coil-over
VI. Rear Suspension
"The Factory Five Racing frame that is
supplied with the kit has all mounts welded and ready to accept the
entire bolt-on Ford 8.8" rear end, which comes right off the donor
Mustang 5.0. This is a solid axle, four link, quad shock mount with
And you will never
fit a proper Pin-drive wheel on this car without replacing the donor
control arms. The excess hub-to-hub distance prevents
it. You might want to convert to disc brakes on the rear,
too. Inboard disc brakes are standard on the Jag suspension,
and our optional rear suspension mounts the rotors on the hub carriers
for better cooling.
VIII. Roll Bar
"You get standard with the kit a three point
0.120" wall thickness, 1020 cold rolled steel, black competition
Roll-Bar which duplicates the look and critical function of the
original SC Roll-Bar."
Take a close look
at the way the Factory Five bar is mounted. The inside leg is
mounted off the backside of a 2" x 3" rectangular tube (putting the bar
both in torsion and bending loads against its smaller
dimension). As far as I'm concerned, this is one spot where
strength does count.
PLEASE USE YOUR BROWSER'S BACK