Independent Rear Suspension for Factory Five Cobras

The current generation Factory Five Cobras have an independent rear suspension option. The earlier versions of these fine cars utilize a solid  axle ( sourced from Ford Mustangs ).  The Factory-Five specific rear suspension uses the same control arms, spindles, brakes, etc. as our Universal IRS design, but differs from the Universal design in the location of the coilovers and the overall design of the center section.

Similarly to the Universal IRS design, the IRS for the Factory Five Cobras feature the following:

The images below show how the frame is modified to accept this our new Independent Rear Suspension design.

Prior to cutting any frame member it is advisable to reinforce the existing 2x3 frame rail. The reinforcement is necessary to add strength to the member after notching. Welding the reinforcement in first will help prevent any movement or “relaxing” of the frame as a result of cutting out the gusset below. The reinforcement is the same width as the frame rail and should be installed on the top surface of the rail so that the aft end of the reinforcement is aligned with the extreme rear of the shock mount. Tack in position and completely weld the reinforcement to the frame on all four sides of the contact area.

The image below shows the Factory Five frame.

Here is an image of the frame reinforcement piece.

The original upper spring seat is no longer required. This spring seat may or may not interfere with the upper control arm. This IRS is adjustable between three different track widths - why three different track widths?  Since a wide variety of wheel widths and offsets are available, the IRS allows for adjustable track widths. The upper and lower control arms bolt to the IRS center section... Each center section piece has a series of three holes spaced 1" apart. This allows for the builder to choose the track width that is appropriate for their specific application. It should be noted that axles must be spec'd for a specific track width - narrow, medium or wide.

The point of this track width discussion is to point out that for the narrow track version, the upper spring seat must be removed as it will interfere with the hub carrier. The images below do not include the removal of the upper spring seat - indicating that this was NOT a narrow track width application.

Gusset Removal - The gusset between the vertical 2x3 frame member (which runs behind the seat) and the upper 2x3 frame rail must be removed. A hacksaw will work but a power reciprocating saw makes the job easier. Replace this gusset with the round tube replacement gusset supplied in the kit. The new gusset is cut so that it does not sit as far toward the rear of the chassis as the original, allowing more room for the travel of the rocker arm. For the best effect of the new gusset, position it so that it intersects the chassis vertical and horizontal members as close to their outboard edges as possible and weld.

A 3" diameter notch must be made in the original frame - for clearance. This hole is made in the frame member directly below the reinforcment piece that was welded into position in an earlier step.

Mark the position of the 3” notch on the side of the frame rail. The center is  1 5/8” forward of the shock mount bracket (front surface), “A” and 2 ½” down from the top surface of the original frame rail “B”.

Center punch at the crosshairs made by the "A" line and "B" line... Using a 3" hole saw will give the necessary clearance.

Using a 3” hole saw, drill through the frame. Regular standard length hole saws are not deep enough to do the job in one shot so be ready to drill from both sides of the frame rail. Try to set the centering drill sufficiently protruding from the arbor so that the centering hole is transferred to the inside wall of the frame tube. This will allow the two 3” holes to connect in proper alignment making a tidy notch in the bottom of the frame.

The resulting "cut-out" will look like this:

The 3" hole in the frame must be Deburr'd to assure a proper fit of the supplied 3" OD  semicircular insert provided in the kit.

Slide the insert into the fresh cut to close the opening and restore integrity to the frame rail. Slightly  squeeze (spring) the insert with a large pair of slip joint pliers ( if available ) as you slide it into the notch.

Weld all the seams surrounding the semicircular insert

Left Side Frame notch is complete... repeat these steps for the right side.

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