Most of the IRS units shown...
Most of the IRS units shown here have been photographed from the rear, which is what most people will see once the unit is installed, but this view from the front shows the pinion support rods and radius rods (arrows) that connect to brackets on the chassis 'rails, preventing the IRS from "winding up" or tearing itself from its upper mounts on the crossmember.
The beauty of designing a dedicated suspension system as opposed to adapting one from a production car is that it can be made to look as good as it performs from the outset. The Superide IRS has an aluminum centersection, which can be polished, stainless steel lower control arms are available to replace the standard equipment plain steel versions, and chrome halfshafts are available as well as a range of other appearance options. With aftermarket wheels getting ever larger, and the windows in them increasing in size proportionally, a detailed independent rearend is always going to look good, on even the lowest full-bodied car. Inboard rotors and an IRS will always ensure the best view through such wheels.
We noticed Heidt's offers high-horsepower 3-inch-diameter halfshafts for its IRS, and asked Gary Heidt when he'd recommend installing them. "It's not only a matter of horsepower, as vehicle weight, engine torque, tire size, and other factors will put a high load on the driveline. In general, we recommend them for muscle car applications, as those cars weigh much more than a '32 roadster, which would "launch" quite easily. If someone is in doubt, they should certainly use them."
The "standard" Kugel IRS,...
The "standard" Kugel IRS, if there is such a thing, employs 9-inch Ford gears, Corvette heavy-duty universal joints, and steel halfshafts.
Flat Out Engineering
Flat Out Engineering started out by offering kits to install Corvette IRS units in classic trucks ('47-59 Chevrolets and '41-64 Fords) but now offers kits for '33-40 Ford cars too. The kits bolt on or weld in, depending on the application. Aldan coilovers or Air Ride Technologies ShockWaves are used in place of the monoleaf spring.
The Ford passenger car kits use C4 IRS units that have been narrowed to 57 inches in width, and include all the crossmembers and bracketry required for fitment. The car kits require welding to the existing chassis. Flat Out's Don McNeil says this is because "Most of these chassis have already been boxed, but not all, so fabricating a weld-on kit made more sense."
An IRS system from Kugel Komponents is supplied fully assembled, pre-aligned and with toe-in and camber adjustments pre-set. It is even filled with gear oil, leaving the customer to just weld the crossmember, radius rod, and pinion support rod bracketry to the chassis.
There are a number of options available for both the 9-inch and quick-change units, such as polishing, limited-slip differential, and tubular stainless control arms. Custom made 11-inch Wilwood rotors are standard, with Corvette aluminum calipers supplied on the 9-inch and billet Wilwood calipers on the Winters Champ Car quick-change IRS. Check the company's Website for all the options available.
Units as narrow as 52 inches have been supplied in the past, and are available in 1-inch increments up to 62 inches, while custom four-bar hub uprights are available for high-horsepower applications.
Here's Progressive Automotive's...
Here's Progressive Automotive's polished and painted '88-96 Vette rear installed with the optional Air Ride Technologies ShockWaves in the company's '55-57 Chevy chassis.
Air Ride Technologies
While Air Ride Technologies doesn't manufacture or even sell IRS units, they do offer rear Shockwaves, which means you can run air suspension with an independent rearend. The rear Shockwaves are specially designed, as the name suggests, for the rear of a vehicle, with more travel and a softer spring rate than those intended for use on the front. The 4-inch diameter 7000 series is designed for vehicles weighing 1500 lbs or less on the rear axle, while the 5-inch-diameter 9000 series is recommended for vehicles weighing 1,500-2,400 lbs on the rear axle.
Progressive Automotive offers its Sweet Ryde C4-based IRS kits for numerous early trucks from many manufacturers, as well as '35-40, '41-48, and all Model A Ford cars; '31-39 and '55-57 Chevy cars; and '37-41 Willys cars. Usually a narrowed rear frame section is required on the cars, and all kits are supplied with such, with all bracketry already attached, as well as Aldan coilovers and mounting hardware. Customer-supplied Corvette parts are narrowed as part of the kit, while 1-inch rear sway bars are also available, as are optional Air Ride Technologies ShockWaves.
And now for something completely different. Well, not completely, but certainly different enough for us to take notice when we saw it for the first time. This design differs from every-thing else shown here on two counts. First, it doesn't use the halfshaft as a control arm, but actually has an upper and lower arm, and secondly, it employs inboard coilovers. According to Team321's Dave Held, this IRS has been installed in a '35 Ford, a T-bucket, and other custom frames. "The modular design allows for adaptation to a wide variety of vehicles. Team321 can even create a 3D CAD image of your chassis to virtually fit the IRS before any cutting takes place."
The IRS uses an 8.8-inch Ford differential mounted in a subassembly, the wide control arm mount base controls fore and aft wheel deflection without the need for radius arms, and a range of track widths is available from 56-inches. Bolt patterns of 5-on-4 1/2 and 5-on-4 3/4 inches are available and brake options include 11-, 12-, or 13-inch cross-drilled and slotted rotors and Wilwood calipers.
This is Progressive's '62...
This is Progressive's '62 Chevy chassis, complete with an '88-96 C4 Dana 44, chromed and polished, and suspended on ShockWaves.
The 321 IRS features fully...
The 321 IRS features fully adjustable upper and lower control arms and a Ford 8.8-inch third member in a subassembly the customer welds into their chassis.
You can run air suspension...
You can run air suspension with an IRS thanks to Air Ride Technologies ShockWaves 7000 and 9000 series, depending on the weight of your vehicle.