We're installing the new RCC Ford 9-inch IRS on a Cobra-replica frame, but any kit with enough room for a Jag IRS unit will work. Here is the assembled rear, complete with inboard mounted brakes, upper control arms, Toronado front hubs, halfshafts, and CV joints. The first step to installation is to level the chassis on jackstands.
In its day, the Jaguar rear suspension so beloved by generations of Cobra replica fans/street rodders was a perfectly good design. Things have changed, though, since 1961, and rear-suspension technology has improved considerably since then. The Jag has the advantage of being compact and easily removable as a unit, which makes it relatively simple to work into a kit-car chassis or under a pair of rod rails.
The downside to the Jag design is the stressed halfshaft, which serves double duty as the upper transverse locating link for the hub carrier. Similar designs were used on a number of front-line racing cars in the early '60s, such as the '60 Lotus 18 Formula One car and the highly successful '58 Lola Mk.I sports racer. However, there were several problems inherent in this design. One is that, if there is any looseness in the side gears or the universals, the suspension geometry can change dimensionally. Also, the radius arm can bind if not properly located.
These problems may not have been so important back in 1961, when tires were about as wide as your hand and hard as a rock, but with a sticky set of Ultra-Wide Terra-Grabbers the limits of suspension compliance are more forcefully tested. The stock Jag unit is also not easily adjustable for rear toe-in. More important, if one of the U-joints breaks, the wheel lies flat or can break off the car. This can seriously ruin your day, especially if it happens as you power-drift your 450hp pseudo-snake around a high-speed bend on open-track day.
As a result of these and other factors, many kit manufacturers are turning to other rear-suspension systems. Some opt for a simple beam axle, which can be made to work very well on smooth surfaces. However, for street use on the rough, bumpy roads most of us have to suffer, an independent suspension offers a better ride and improved road holding. It can also provide a simple way to adjust rear toe-in, which can be used to tune your handling.
One of the newest units to offer these advantages comes from RCC Specialty Products in Fort Worth, Texas. Designed by chief engineer Warren Webster, the latest IRS setup uses a custom-fabricated, forged-steel differential carrier that fits the famed Ford 9-inch pumpkin. This unit is preferred by racers in most forms of motorsport (even Chevy stock cars use them). The Ford diff is virtually bulletproof, readily available, and has a selection of ratios to please everyone from eighth-milers to Bonneville types. It is considerably tougher than the Salisbury diff used in the Jag, and there are a number of limited-slip differentials available for it.
Rather than use the axle as the upper link, RCC added an upper A-arm (similar to that on the 427 Cobra) that takes the suspension load and connects to Olds Toronado front hubs and halfshafts. These use constant-velocity joints instead of U-joints, thus providing smoother operation. Brakes are custom 11-inch vented rotors with Buick Riviera calipers mounted directly to the frame (this loads braking forces directly into the frame instead of the diff carrier). Outboard brakes can be specified for those who prefer ease of maintenance and better ventilation. Suspension geometry has been adjusted to eliminate bumpsteer (toe-in change with suspension deflection--see "Bump Steer," May '95).
Shocks are AVO coilover units with 12-position adjustability, and ride height can be changed by screwing the collars up or down the shocks. Carrera shocks are optional. Since kit cars come in all sizes, any spring rate can be specified to match the weight/use of your car. The weight of the entire system is the same as a Ford live-axle system, but it has less unsprung weight for better handling. RCC uses an aluminum link that connects the Olds steering arms to the A-arm, which can be easily adjusted for whatever toe-in setting is needed.
The Deluxe kit with an open rearend and buyer's selection of gears and spring rates runs about $4,500. Options include limited-slip diffs and Carrera shocks. For scroungers with an ample parts supply, a basic kit containing everything except the Olds and Ford parts lists for $1,400. Warren Webster was kind enough to show us how to install an ndependent-rear-suspension unit when we dropped into RCC. Please bear in mind that each installation will be a little different, but these are the basics for a common Cobra-replica chassis. KC
