The Ford IRS in the result of extensive playoffs between Dearborn (FoMoCo) and Russelhiem (and Fishermans Bend Australia. The benchmark for it was the 1978 Opel Senator/Monza semi trailing arm IRS, which Peter Brock, the Australian race car identity who ran HDT fitted up for Australian conditions with a Corvette C3 IRS center, and then Holden set it up with a 9 Bolt BTR 78/Camaro/Firbird G52 axle center in 1989 for the Vauxhall Lotus Charlton and the first HSV Holden Commodore VP Senator. Although just a Caddillac Cartera underneath, the IRS was extensively reworked with Wissach Geometery and, eventually, lower links just like the 190 E Meceades Benz.
The Mustang awaited 19 years to get a proper Jaguar style upper and lower wishbone IRS. Ford Australia developed its IRS about the same time for the 1998 AU Falcon. Ford developes better mouse traps because it looks at the quality/price tradeoff, and for the IRS, its note something most Americans would notice. They don't notice...Just how good it is. The Fox started off with an IFS and steering system the benchmark in the industry. They cheaped out on the four link rear end because its really just a 1970 Taunus/Cortina copy with a little more refinement, which was based on the GM V and F cars, the Commodre A and FD Victor, benchmark four lnk rar end cars dating from 1966. The Fox Four bar live axle creates the problems the team from Heidts notes....reduce roll stiffness, suden under to over steer transitions, and the induced roll over steer common on V car Oples, Holdens and Vauxhalls without the IRS. Back to back comparisons were done by Wheels Editor Peter Robinson, Modern Motors Barry Lake, and then GM's Milbrock engineers with the Opel Senator/Monza IRS fitted. The result is always the same....IRS reduces body roll, redueces transitional roll stear (normally reducing over steer while keeping or reducing understeer on entry. Under racing conditions, you avoid curbing, as it the rear end causes what feels like rampant bump steer, but on an IRS car, its able to cope with a stunning amount of rear end oval track curb contact.
The only negative with IRS is the Hummer style driveline shunt that occurs with a stick shift Fox in very hard traffic conditions, and in some situations where there is construction join contact. This is called impact harshness, bit its actually drivelije shunt. If a car has a De Dion axle, or a live axle, the drive line shunt can be very moderate. In a bad example, that of the first C3 Corvette IRS, driveline shunt can break axles and make drag strip launches hard. Some Aston Martions had the Corveete Salisbury 8.5 axle center with a De Dion axle assmbly, a half IRS combination with a Live rear axle, but separte CV jointed half shafts. They constanly break under loads. Control Blade IRS from the Focus, BA -FG Falcon, are probably better, but they don't dovetail into the Fox frame rails like the 1999-2004 Cobra style IRS does
The Ford IRS is very well regarded, but , like the progression to four wheel disc brakes and integrated headlamps, the progression to IRS in a series production Rear drive car was always very low on the priority list.
Remember, Fords first IRS was supposed to be the 1965 Mustang IRS option, using the Jaguars Spicer/Salisbury axle center as per the Stingray, but with a few changes to this American built axle center.
Insteead, the first proper Ford Rear drive IRS was the in UK in 1966 with the Mark IV Ford Zephyr Zodiac (not a good one at all), and then with the Granada Mark I of 1971 (Much better), and for pure rear drivers, the US 1989 MN12 Tbird and Cougar twins. The GT40 was a Lola IRS with a Coloti /ZF DS or KarKraft T 44 derived transaxle. Mid engined, so its system wasn't really in the hunt for rear drive front engine cars.
Ford had already picked up Jaguar, (XJ40 rear ended XJ-S) and Aston Martin (XJ-S based DB7), and the cumulative expertese poured into the Fox 4 unibody IRS modification was compared to the very best. Actually, the MN12 was the best IRS to date on those, the Jag IRS has some other issues. The Cobra IRS was very much case of linking the Fox chassis hard points to the best geometric solutions to getting IRS at a reasonable cost.
Every Fox should have this IRS, its highly under rated because its not normal for US cars to have to deal with European or Australian style mid corner bumps, or have to tow a 1.5 ton brakes trailer hithc like the Aussies did when re-developing the German Opel and Ford Falcon IRS back in the late 80's. The Mustang IRS is exceptional.
To have it removed from the options list for the S197, well, that was a budget matter, and the S shell was colsely associated with the Jaguar X and S and LATER European Mondeo underneath. As the enormity of General Motors debt hit home before the Sub prime crisis started bleeding money out of the US economy before the 2008 GFC, Ford Dearborn axed plans to use the Falcon IRS platform, and the Fox 4/SN98 IRS is still the best around for the dollars.
