Modified Mustangs & FordsHow To Chassis Suspension
Factory Five Racing IRS Conversion - Independent Thinking
Factory Five Racing IRS conversion for the 1962 Mercury Comet Wagon Project
Live axles are great for drag racing. The suspension is easy to tune, the parts are strong, plentiful, and inexpensive; for straight-line racing, the live axle is hard to beat. Put the same rearend in an uphill righthand hairpin, however, and your suspension is going to cringe, which can leave you in the gravel. For road racing, there is nothing better than an IRS or independent rear suspension. Giving the drive wheels the ability to articulate to a changing road surface independent of either side gives you an advantage over the traditional live axle. The problem comes in converting a straight axle car to IRS.
Part of what changed the Mustang from a "secretary" car into the most successful performance platform in the world was its success on both straight and twisty tracks. Vintage road racing is full of early Mustangs, making the aftermarket full of great race parts. The proven performance of an IRS swap can be found for the first pony cars, but they definitely are on the pricey side, starting at around $5,000 for a basic kit. For the budget enthusiast who doesn't want to drive a Miata, that cost tends to push the IRS swap out of reach. When we decided to add an IRS to this '62 Mercury Comet Wagon project, we knew this would be a challenge, especially trying to make it happen on a budget of less than $3,000.
The early Falcon/Comet platform is what the Mustang was developed on, but there are a few key dimensions that changed, specifically the width. The Mustang is 1.5 inches wider than the '60-'63 Falcon, so even the aftermarket IRS won't be an exact fit. After about eight months of researching, we found an answer—Factory Five Racing. FFR builds some of the coolest reproduction kits on the market, the MK-series being the Cobra roadster platform, currently in the fourth generation, the MKIV. When you order an MKIV, you have the option of live axle or IRS. The FFR IRS uses a few factory components from the '89-'97 V-8 Thunderbird installed into FFR's tubular IRS cradle that is designed to match up to the MKIV chassis. The Thunderbird IRS is often used for IRS swaps for trucks, but the issue with using it as-is in the Comet is the width. The track width on the Comet is a scant 54.5 inches, where the T-bird is rocking 60.2, which would put the hubs past the edge of the rear quarter-panel. The Factory Five IRS cradle also uses the 60.2-inch width, unless you opt for the pin-drive setup, which is designed for using knock-off adapters. This narrows the overall track width to 54.25 inches, and that we can deal with.
Once we had the basic plan, we called the guys at FFR and discussed it with them. While they thought we were off our rockers, they provided us with the necessary dimensions that we would need to ensure the system maintained the proper geometry. The upper mount is separate from the lower cradle; it has to be installed a specific distance from the lower cradle, centered above it, and at a specific distance to the front of the cradle. This keeps everything from binding up at full articulation. We ordered the pin-drive kit, which comes with new axle-shafts, tubular upper and lower control arms, parking brake cables, and all of the bolts. We also ordered a set of Koni coilover shocks from Factory Five.
The installation required some planning, and this is not a bolt-in by any means. The cradle is a close fit to the Comet's subframe, but not close enough, so each mount was fabricated under the car. We did not need a tubing bender; all of the mounts are made from thick-wall square tubing that we sourced at the local metal supply shop. As with any fabrication project, cut once, measure, cuss like a sailor, and then cut it again (or something like that—Ed.).