Here's a bit of little known gearhead knowledge for hot rodders; when the Mustang was being prepped for its world-changing launch back in 1964, the original plan was for it to come with an independent rear suspension (IRS) as standard equipment on Shelby GT350s and as an option on standard Mustangs. Designed by famous Ford engineer Klaus Arning, the suspension geometry was derived from the Ford GT racing program and would actually have had a slight rear steer effect during suspension travel that would toe the outside wheel slightly inward, and toe the inside wheel slightly out making the steering feel more responsive. That would have helped the rearend rotate into turns, which would have made the handling very nimble and place the Mustang somewhere between ponycar and true sports car. What happened to it? The bean counters killed it, of course.
With that program axed, the only muscle/sports car to be bestowed with an IRS during the 1960s was the Corvette. That's too bad, because the technology was obviously there for both Ford and GM, but neither stepped up to offer the option to the public in their ponycars. Perhaps that's because drag racing was by far the most prevalent sport for American hot rodders, or perhaps it was because the general public just wasn't aware of the benefits enough to demand it.
So what's so great about IRS versus a straight axle? Well, on a purpose-built road course with a perfectly smooth and prepped surface designed for high-speed racing the benefits between a straight axle and an IRS won't be obvious, but that's not where you're going to spend most of your time with your car. Out in the real world the roads are uneven, cracked, and bumpy, and that's where IRS really shines.
Ever gone through an unexpectedly rough section of pavement on a turn in wet weather and had the rearend skip and try to step out on you? While a solid axle has to seesaw a 100-plus pound piece of iron up and down to compensate for road irregularities—reducing the tire's contact patch while doing so—an IRS drastically reduces the unsprung weight per side and allows each tire to adjust to changes quickly on its own without interfering with the other side of the car. That not only means an inherently smoother ride than is possible with a solid axle, it also means more consistent rearend traction. There's really no better option for real-world daily driving and touring, and it's also terrific for autocrossing and handling events, which are often setup on less than optimum surfaces like parking lots.
Of course there aren't too many ways to get an IRS under a Mustang, and none have offered an absolutely easy, home garage bolt-in solution until now. When you're ready to give your Mustang the type of exceptional ride and handling it could have had, Heidts Hot Rod Shop now has you covered with their easy-to-install Pro-G IRS system. You might think that adding a modern, clean-slate independent rear suspension would be difficult, but it's not. We were pleasantly surprised to discover we could rip out the leaf springs and original 8-inch rearend and have the Pro-G hanging in place in any 1964-70 Mustang or Cougar in about two days. Watch how it all goes together in a 1966 Mustang convertible!