IRS Pros and Cons

An independent rear suspension is absolutely a very trick addition to a car that’s worthy of bragging rights. The problem is that many frames lack the arch in the rear framerails to allow for enough hub carrier clearance with a low ride height. That’s why Corvettes don’t have a real trunk. Speaking of Corvettes, I don’t think anyone is going to convince you that early Corvettes ever rode well, although the C6 seems to be a major improvement. When I visit an SCCA race, all the fast C5-and-older Vettes have a 9-inch rearend with a four-link. That suggests to me that the IRS handling advantages are questionable. In the real world, I think your money is far better spent elsewhere if you’re looking to maximize the handling of a muscle car. That said, if you are building a muscle car to win one of the big national awards, an IRS will likely soon become the way to go.

More Grip, Better Parts

Modern tires offer far more grip than tires from decades past, which has forced aftermarket suspension products to evolve to make the best use of the additional grip. With skinny, hard bias-ply tires, so little grip was available that proper geometry just didn’t matter much. These designs were often as much about being cheap to produce as they were about handling. If you read any of Tom McAhill or Smokey Yunick’s columns in Popular Science and Mechanix Illustrated in that era, they recognized the inadequacies of American cars while they were new. Modern wider tires with better grip impose far greater loads than these cars were designed for. For instance, bumpsteer isn’t an issue when there’s only 4.5 inches of bias-ply tread on the ground. There simply isn’t enough traction to allow a car to stop or turn well in the first place.

To increase the power of these cars without upgrading the brakes or suspension makes them less safe than they were when they couldn’t go so fast. Tires that are already in a partial skid during normal maneuvers cannot deliver on their promise when extra power is added to the equation. Modern aftermarket suspension upgrades are all about maximizing the footprint of the tire, and eliminating tire scrub and bumpsteer. Better camber control, better alignment specs, and improved steering and brakes will all work together to allow the tire to perform to its true potential. As the knowledge of proper geometry and methods of problem solving spreads, and just as importantly are installed by rodders, our favorite cars get better and better.

Autocross vs. Road Course Setup

Driving a car around an autocross compared to driving around a road course requires vastly different setups in order to maximize performance. It’s similar to comparing the differences between a fighter jet and a Cessna. The Cessna is designed to be stable to allow pilots and students with a wide variety of experience and ability to fly it safely. It is especially important that it has no bad habits and that it doesn’t maneuver so radically that it easily gets the pilot in trouble. On the other hand, a dog fighter must be inherently unstable. It can turn best by being on the ragged edge of stability at all times, ready to react instantly. Full-on autocross cars are like a dog fighter. Negative camber, minimal caster, and toe-out are used to make the car want to avoid staying straight. This comes at the expense of tire wear, but on runs that generally last for less than a minute, that just isn’t an issue. Road course cars need to have more stability, so more positive caster and little to no toe-in are common settings. With a longer course, higher speeds, and more laps to be run, tire wear and overheating are very real concerns.

Street cars general do best with minimal positive camber, 3 to 4 degrees of positive caster, and toe-in to produce straight-line stability and minimal tire wear. Turning and stopping ability are well within the needs demanded by the naturally less demanding driving needs on the street. Naturally, compromises can be made with an informed view of the tradeoffs for the performance level desired. When the autocross craze began, it was pretty much a run what you brung series. I think we all knew that the natural progression of competition would edge the faster cars toward purpose-built race cars that can be street driven, rather than street cars that can be raced. Witness the teams, transporters, tire blankets, and trained drivers in specially prepped cars seen winning today. That’s all good, if you can live with a fighter rather than a Cessna every day. As long as your upgraded muscle car can out handle a modern performance production car, as has been repeatedly proven, your car has all the handling it needs for normal use.

Best Bang for Buck

With all the aftermarket suspension components that are available today, a very common question is which parts offer the best dollar-per-dollar improvement in handling. First-gen Camaros, second-gen Novas, and Chevelles first need a dropped yet taller spindle, disc brakes, and better shocks and sway bars. A better, quicker steering box is next on the list. Tubular control arms make modern alignment settings easier to achieve, but provide little functional benefit other than style. Likewise, tubular arms offer little change in strength or weight savings, while better bushings can be used in a stock control arm to reduce deflection. Coilovers make adjusting ride height easier without the higher spring rates common to shorter springs, but in fact have little real advantage over upgraded standard shocks and springs other than bragging rights. It’s simply a luxury to have an easier way of tuning spring rate and ride height. If the budget allows, that can be worth a lot in reduced effort. Second-gen Camaros, Chevelles, and third-gen Novas come factory equipped with excellent tall spindle geometry and good disc brakes. In fact, most of the mods that are made to first-gen Camaros merely seek to emulate the superior second-gen design. Dropped spindles, and better shocks and sway bars should be first on the list, with a blueprinted steering box and tubular control arms next.

Another option is Fatman Fabrications’ Camaro Strut independent front suspension conversion. It provides a simple bolt-in upgrade with a front steer rack to allow use of normal rear-sump oil pans, disc brakes as large as 13 inches, and excellent geometry. It can be installed for well under $3,000 by any decent mechanic with hand tools. Although third-gen Camaro build quality has always been in question, they handled well and the MacPherson strut front suspension design has certainly been proven to perform as well as a twin A-arm system.

Aftermarket Frames

With full-frame cars like A-bodies and Tri-Fives, owners have the option of bolting aftermarket suspension pieces onto the stock frame, or replacing the entire frame with an aftermarket unit. Front subframe replacement kits are available for cars like the Chevy II as well. While it is certainly possible to upgrade an OEM suspension with better brakes, lowering springs, and improved geometry and steering, there are often drawbacks to rack-and-pinion steering conversions. The biggest issue is that they limit how much you can change over to modern alignment specs. By the time you have done a standard rebuild and made your upgrades, you may have spent more time and money than replacing that OEM suspension with a more modern suspension. An entirely new frame, properly designed, is simply a substantially improved design. Both the front and rear suspension come ready-made with new components that will deliver excellent performance. There is no need for scraping undercoating and sandblasting. Far greater torsional rigidity is often achieved by using superior crossmember designs and materials. A more trick appearance is a natural result of all this fine work, and a new frame can save on installation time. An original chassis upgrade can make all the sense in the world for a nice driver built on a budget, while a no-holds-barred show car just about requires a new chassis to be competitive.

Frame Stubs

A very effective alternative to a full aftermarket frame is installing a front frame stub. Front stubs can also be installed in some unibody cars as well. Not only are they relatively easy to install but they offer many of the same performance advantages of a full-frame upgrade. For example, when a big-block installation is planned, the rack-and-pinion used will generally solve the header to steering box clearance issues. A small-block or LS1 swap gets even easier as well. Most chassis need the most help up front, and since the body mount and contours from the firewall back are the most difficult part of a new frame to design, a combination of a frame stub joined to an original rear chassis offers the most improvement for the least cost. It is important to note that weld-in stubs need to have the attachment joint properly constructed. Fatman selects framerail tube sizes that closely match the original rails when possible. Internal gussets are then used to construct a 0.25-inch wide by 0.125-inch deep backed-up butt weld. That channel is then filled with multiple weld passes and ground to provide a joint that is stronger than the original rails yet makes a smooth transition. Our frame stub designs are all specific to the car being built, with radiator core support and bumper holes properly located for remounting of the car’s front clip.