Brace Yourself!

Strengthening an FFR Cobra with additional bracing

Story and Photos by Dan Reiter

While the original Cobra replica was based on the design of a 1950s British sports car, modern technology has been adapted and integrated into replicas offered by various manufacturers. It’s an amazing melding of old and new.

Even so, we still encounter some of the same characteristics and flaws of the original design, which are exacerbated with improvements in power, performance tires and brakes. One of the most basic is chassis flex because of some similarities to the original’s bendy ladder frame.

While racing versions sometimes have a roll cage added to the base chassis design, a benefit both from a safety and rigidity standpoint, it can be a contortionist’s nightmare to get in and out of. So rather than a full roll cage, something else was needed to improve the rigidity of an older Factory Five Cobra replica (MkII).

One of the most beneficial results of this modification was the car now becoming one solid “plank,” minimizing chassis twisting. The car
was corner-weighed and cross-balanced (note the scales under the wheels) for optimal track handling. The car set up remarkable quickly due to the car not flexing and changing.

Note how the rolling chassis of the Factory Five Spec Racer has been beefed up. The passenger area is now fully surrounded by side structures, which tie the chassis into a solid box and give the passenger space full protection from side impact. The additional roll bar structure also provides additional chassis rigidity.

This is the base chassis of a Factory Five Roadster. It employs the traditional design of the two central 4-inch tubes, plus various crossmembers, supports and braces. While the design is stronger than the original Shelby, and is suitable for street use, some additional reinforcement would be welcome for high-performance and competition applications.

One FFR owner, Mike Noyes, who consistently auto-crosses his car, overcame some flex problems by linking the front and rear cockpit bulkhead braces together, creating a box, and greatly reducing the chassis flex.

It worked well in his car, and seemed to be an ideal compromise between a full cage and ease of access. His design was the basis for the car shown here, with the additions of extra braces for safety, strength and appearance.

While not the full side roll cage of FFR’s Spec Racer, it offers the much-needed sides of the box in terms of design and strength. And the “X” design still allows access to the seats. The tubes are 2-inch steel, and are full thickness as specified by all racing organizations.

They wanted to find a middle ground between these two chassis types, tying the two bulkheads together but allowing the doors to still provide relatively easy access to the interior. The first step was to mock up a design from 2-inch plastic tubing to serve as a pattern.

Since the car was already complete, a decision was made to split the body, so the nose could be removed for installation of the chassis reinforcement. This change also allowed for easier engine removal and front end mechanical repair. A location for the split was determined and marked out on both sides of the cockpit.

Once the X-tubes were installed on the car, a test drive revealed an immediate difference in character — much stiffer, bumpier and more responsive.

The next step was to upgrade the springs in the coilover shocks to address body roll. Once all work was finished, the car was brought to the shop of Mark Dougherty, FFR builder extraordinaire, to have the car balanced, corner-weighed and racing-spec aligned. Immediately, another plus was discovered: The chassis work allowed the car to be quickly set up, and very responsive to the coilover shock adjustments.

A bracket was fabricated to secure
the two body halves and provide a solid mounting point. This bracket mounts to the chassis crossmember underneath for greatest strength. The X-tubing was then welded to the chassis while held in a jig to ensure it was straight.

This view is of the passenger side, with the seat removed, shows how the the “X” is welded into the frame. It also indicates the height of the “X” in relation to the door opening. Access to the seat is not impeded very much.

The driver’s side of the car with the side cage installed. The lower “X” ties together from two bulkhead crossmembers of the Factory Five frame, and also ties in the longitudinal chassis crossmember via the vertical tube. Part of the design was to fit between the seat and the
body without interfering with the driver. This structure is permanently welded in for greatest strength, while the upper bar attaches via a clamp on the roll hoop.

The new springs flattened the car out, the chassis mods ended any flexing, and the handling was now in the territory of a true race car. Quick, precise, lightning-quick reflexes and driver inputs now resulted in immediate response from the car.

Interestingly enough, a flaw in the brakes was now realized with the new chassis, as they were now working even harder. Very quickly, lap speeds increased, and the next step was deemed to be a brake upgrade. It is important to note that this design was never intended to be legal for full competition. A full roll cage is required and recommended for any racing use.