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 tube."
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...

 If a chassis has a structural failure in a main rail, you've got much bigger problems than I can describe here. 

BUT, strength is not a big factor in overall chassis design, STIFFNESS IS.

For an article on stress vs stiffness from Machine Design Magazine, click here.

"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.  

But 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 Torque

So the "four times as much" difference is really a little over 11%.

Please check my math!  Here are the formulae (taken out of a mechanical engineering reference book):

 

Where the variables are calculated as follows:  For a round tube: For a rectangular tube:

"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 the price."
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.  Consider the chassis of the new C-5 Corvette.  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.

III. Body

"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 as options."
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 stress crack.)
"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 out there!"

V. Front Suspension

The Factory Five 427 SC Cobra comes with a fully independent tubular front suspension with adjustable coil-over shocks.

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 traction lock."
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.

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