Home » All about the missing S197 2005 Mustang IRS

All about the missing S197 2005 Mustang IRS

by DrivingEnthusiast

Editors note: this article is *not* about the IRS that has been developed for the 2015 Mustang. Instead, this article covers the IRS that was developed for the 2005 Mustang, then abandoned due to all-too-typical Ford cost-cutting. But read on anyway, since the Mustang IRS story is long and tortured – and will make you appreciate the S550 IRS even more.  Read all about the 2015 S550 IRS here.

We’ve written extensively on this topic before, but the October 2011 issue of Motor Trend spurs us into action yet again. Their topic is the upcoming 2015 Mustang, which they say will be an evolution of the current chassis, with an IRS. As we know, an IRS that was developed for the S197 (2005) Mustang was cancelled at the last-minute by Ford executive Phil Martens to save development budget (who was then himself downsized right out the door).

Motor Trend only has a portion of their story correct… where they first go wrong is their image of the car, which is updated from the current Mustang by only a new front bumper. It’s  hard to believe that Ford would reach an important milestone, the 50th anniversary of an American icon, the greatest brand that Ford has today – and only change the front bumper.  C’mon, Motor trend, you can get a bit more imaginative than this. We already know the design direction Ford will take, not only from recent statements by J Mays, but also by the new Ford Evos concept. It’s possible that the Evos front end *is* the 2015 Mustang front end.

But where Motor Trend really gets it wrong is their claim that the IRS design which would be used is the Ford of Australia Falcon “control-blade” suspension (nothing special there, just stamped steel, inexpensive bits, and less than optimal geometry). That’s not at all correct (or desirable), and it’s not even consistent with what Motor Trend has claimed in the past. The Falcon design was evaluated well over ten years ago during the 2005 Mustang development process and dropped from consideration as inadaquate. Ford then designed an all-new IRS from scratch, the production engineering and testing was completed, and it was only shelved at the last moment due to budget cuts. The IRS was never revealed in public, but it was briefly mentioned and it was seen on a production prototype driving around Dearborn during that timeframe. And, better yet, an intact example was recently discovered in a Ford warehouse. It’s a work of art, it will blow you away with numerous beautiful aluminum castings, and the end result is a nearly state-of-the-art bolt-in unit that is lighter and better in every way than “ye olde” solid axle. Imagine a Mustang whose owner would be as proud of what is underneath the car as of the engine and styling. Today we’re going to share actual pictures of the IRS. But before we take a look, lets review some history.

But first, before we get accused again by uninformed naysayers of being “bitter and biased”, we’ll provide our credentials. This particular post was written by the lead author of this site. I’ve owned 20 brand new late-model Mustangs, every significant one from the first FOX Mustangs on up, and open-tracked or autocrossed them all. Each was modified by me to mitigate inherent deficiencies in suspension and braking. I owned two Mustang Cobras with the factory IRS and spent considerable time talking to the IRS designer inside SVT. I’ve been high-speed open-tracking these cars and many others for over 32 years, and currently instruct in one of the best HPDE groups in the country (for the last 12 years). That means driving all kinds of cars around all kinds of tracks at very high speeds, and certainly not just Mustangs. And, especially important, successfully teaching hundreds of students how to drive the same cars they drove to the track. For me, handling dynamics is what its all about. Sure, I’m biased. Now, back to our topic.

Here’s what the current solid rear axle and brakes look like, from the 2005 thru 2014 Mustang: very pedestrian and very very crude. With tremendously high unsprung weight. And, despite engineering “band-aids” to keep it in place, it still tries to redistribute weight to the wrong side of the car in every turn – no matter what you do or how carefully you tune it.  It’s been extensively tuned with all sorts of bushing changes over the years, but it’s still the single worst design element of the car.

Here, on the other hand, is what the rear end of a 2005 Mustang development prototype looked like on the planned and partially announced (magazine interview quotes) 2006 Mustang Cobra: note the IRS (large nut in the middle of the hub) and 4-piston Brembo calipers. This prototype was spotted several times driving around Dearborn.

Cobra, you say? Yes, a new generation SVT Mustang Cobra. However at the last-minute SVT itself was all but eliminated due to quality disasters, the Cobra was cancelled, and the IRS was shelved due to budget cutbacks. Product plans were dumbed down (Cobra eliminated, 4 liter SOHC V-6 extended), options and entire models were cut (3.0 liter DOHC V-6, 7 liter DOHC V-8, both SOHC and DOHC supercharged 4.6 V-8s). The resulting Mustang GT was competitive in a class of 1 since the current Camaro hadn’t yet been introduced. The resulting “Shelby” Mustang had a solid axle, tiny rear brakes, 58.5% of it’s weight up front due to the substitution of an iron engine block, and multiples of poseur scoops and stripes and silly fanged worms (and the only involvement of old man Shelby himself was to sign a licensing agreement to his name). The Shelby has gotten a bit more competent since it first came out, but it still has faults inherent in the crude, tall, narrow, and top-heavy platform. And despite endless tuning of various rear suspension bushings, all solid-axle Mustangs including the newly famous Boss still hop, skip, jump and shift weight in the wrong direction if the pavement in the turns has any bumps whatsoever. And forget about back-roads drives – there couldn’t be a worse choice of car for that.

How do we know that this was an IRS? Lets compare it with two other Ford IRS suspensions: first the ’99 to ’04 SVT Cobra IRS. Notice the nut in the middle the hub.

And here’s how this goes together on the ’99-04 (image from the Ford shop manual) – notice the threaded area on the half-shaft to the upper left, and nut at the lower right. This is typical of all IRS suspensions. The half-shaft is bolted thru the upright or hub.

Part 4B477 is the nut you see above.

The large nut is the giveaway: the red engineering car above has an IRS. A couple of other observations, based on the low-res spy photo:

  • The red mule has 4-piston calipers front and rear. And not just the “baby” Brembos found on the 2000 Cobra R, but much larger “real” Brembos.
  • The rear hub on the ’99-04 IRS has the tie rod attachment point located very close to the rotor – resulting on a lot of heat transfer into the tie rod and also the inability to fit larger rotors. It’s located further inboard on the red mule.
  • It’s not clear how provision is made for rear parking brakes… they aren’t integral in Brembo calipers. There isn’t a second set of small calipers. Therefore, the system must use small drum brakes within the to the rotor (aka the Supra, C5/6, Jaguar S-Type R, etc).
  • Tire size continues to be much taller than SN95s – the tires are 285/40ZR-18s Goodyear F1s.

Unfortunately the spy photos are such low-res that we can’t see any further detail… there appears to be an aluminum lower control arm.

2nd reference: this is a Lincoln LS rear suspension, 2002 vintage. It might be logical that this is what would be used to source some of the parts… however there are some fundamental differences in component location. Note that the LS suspension is state-of-the-art… including coilover shocks. The same rear suspension is also used by Jaguar on the S Type, and also on the Thunderbird (and on the Mustang GT concepts from 2003, which were literally rebodied Thunderbirds using the complete platform including the full SLA/IRS front/rear suspension). Note that on early Jaguar S Type R models, Brembos are used along with rotor-integral drum parking brakes (Lincoln used the cheaper version, with conventional iron calipers and caliper-integral parking brakes).

Conclusion: We know the IRS had been under development from the start, as Ford had been talking about it right from the start as being standard on the future “Cobra”. This time out, given the new platform, it’s an integral part of this new platform rather than an afterthought. And one of the even earlier spy pictures of a development mule appeared to have an IRS underneath as well. The intention was to bring the rear suspension into the 21st century.


History of the Red Mule: A later version of what was likely the same red mule has a solid axle, as shown in these two images.

This happened near the end of 2004 when Martens cut the budget.

Notice the standard brakes, and the standard swaybar hanging down behind the axle. And the lack of the “nut” as shown above. This has now become a solid axle car – once the decision was made to leave the IRS behind, a standard GT axle was simply swapped into the car. This is the same car ole ‘Shel Himself is shown driving, just before it’s debut at the New York Auto Show (2005). Some early Shelby press collateral even referred to the larger rear brakes and 4-piston Brembos although later this oversight was corrected and removed from that material. I spent over a year corresponding with the senior executive inside Ford who was running a project to study the feasibility of the IRS in the Mustang (as well as their original plan, to base the new Mustang on the full DEW98 platform), a few years before the S197 design was finalized. It was his job to figure out how to cost justify it, and it turned into a major political battle against the cost cutters. I won’t mention his name because I was asked to keep it confidential, but from our talks I learned a lot. I’ve also talked to some of the Ford engineers who worked on the S197 IRS project and every single one of them were “mad as hell” that they weren’t allowed to put into production what they worked so hard on, and what they believed should be standard in the S197 across the board. Yes, across the board – every single model. Those folks wanted to build the best car they could – not a low common denominator for customers who (as they said) “didn’t know any better”. Of the people I talked to, only two are still in the company. One was bounced out of the former SVT group and wound up designing a suspension for a worldwide Ranger replacement that little Billy Ford cut out of the budget years ago.

Let’s make sure we all know who to blame inside Ford: both Phil Martens, who canceled the IRS, as well as Hau Thai-Tang (former Director, Advanced Product Creation and Special Vehicle Team:). It was HTT who gave the press a couple of statements that encouragingly revealed the IRS, then later told us we didn’t need it anyway and labelled us all as “snobs”. In progression over the course of several months, he goes from addressing the requirements of his customers and promising they would be met, to outright insulting them:

  1. “Drag racers and Ford’s accountants will be pleased at the choice of a live axle out back. Among our customer groups that know and care what sort of rear suspension their car has, a large number of them want a solid rear axle; they’re primarily the core enthusiast drag racers, and they like the durability, reliability, and ease of modification with it, changing axle ratios, etc.,” says Thai-Tang. “There’s another group that wants the sophistication and cornering advantage of an IRS, and we’re going to offer it on the upcoming SVT Cobra. Unlike the last time, when we kind of shoehorned the IRS in [an older platform]; this time, we’ve designed the rear architecture to accommodate both right from the beginning.”
  2. “Ninety-two percent of (Mustang) Cobra customers wouldn’t have considered a Ford product”
  3. We’ll never appease those IRS snobs.”

And then, thankfully, Hau Thai-Tang was himself “canceled” – sent to South America in exile to build a cheap SUV with no future. This was followed up by SVT itself being cancelled, at least in it’s original form as independent innovators and builders. The reason for that was simple: every product SVT engineered on their own got progressively worse from a quality standpoint. The further away the product got from the “base” production vehicle, the worse the quality became. Their final product, the 2003 Cobra, had terrible engineering problems in several areas (including harmonics, general engine tuning, cold weather warmup, cylinder head casting, and transmission input shaft). Many owners suffered thru engine replacements that Ford didn’t want to perform (including myself, who was amazed to find three identical red Cobras like my own lined up at the dealer for the same purpose – total engine replacement). Finally, the crowning achievement of SVT – the Ford GT – turned into another debacle. What was billed as “the pace car for the entire company” turned into an engineering and warranty disaster. It may well be that the Ford GT debacle was the final straw that forced Coletti into retirement. Great guy as he was, as popular as he was, his products got great publicity but were all flawed from rushed engineering and testing and the resulting quality and warranty issues (and expenses).

The S197 Mustang has not been without issues either, starting on Job 1 with fuel tank pickup issues (and a corresponding refusal by Ford FSEs to acknowledge the issue). We are very disappointed in the current Mustang, and not only for the antique rear suspension but also for the failed MT82 transmission, which has left many owners with broken dreams and an aggravating service experience.

So now it’s time to show what the final IRS design looks like. Here, at last, is the final production-ready IRS. Found in a warehouse of discarded Ford engineering bits outside of Detroit. 1) This is a bolt-in IRS, attaching directly to the frame with welded-on frame tabs (not found with this unit). A subframe is not used; instead, to save weight (and eliminate flex), two structural beams fore and aft provide mounting points and locate the upper and lower control arms and differential. Note the use of aluminum everywhere possible: all structural components, upper and lower control arms, hubs, and differential. This, and careful computer modelling, result in a total weight that is actually less than ye olde solid iron axle. And of course far less unsprung weight.

This is a rear chassis mounting point, rubber isolated.

2) Half-shafts bolt on at the differential. Also shown is the bolt-on mounting point for the upper control arm.

3) This is the aluminum hub, with mounting points for the 4-piston Brembo brakes directly on the bub. Parking brakes are integral to the rotor – note the parking brake cable. The sway bar is a slight compromise, mounted inboard, so it’s not a 1:1 ratio. There is also a wire hanging out for the speed sensor in the hub, used for both traction control and anti-lock braking.

The indentation in the lower arm is the mounting point for the lower end of the shock absorber. The upper shock mounting point is the same as the solid-axle car, again for compatibility. The bracket and bolts to mount the lower end of the shock are missing here.

4) The two large bolts are the mounting rear points for the differential, and note the differential drain at the bottom. The differential is shared with the Explorer, circa 2004 (note speed sensor mount in the picture below that was not present in earlier Explorers without traction control). The Explorer diff uses the familiar 8.8″ ring gear, and of course a limited slip differential would have been standard.

5) Here’s the other side, showing the hub in more detail including the toe link below. Like everything in this IRS (with the exception of the Explorer diff), everything was designed from scratch and owes nothing to any prior Ford design. It’s nice to see the toe link (underneath the lower arm) mounted well away from the rotor so that heat is not transferred. The image also shows the front IRS mounting point on this side.

6) Another view of the same side. The short vertical link maintains the geometry of the hub to the lower control arm and also provides a controlled degree of toe change for stability in turns. We’re again struck by the sheer strength of this unit – there are no compromises in strength at all. The spring and coil are separate in this design, due to having to provide for compatibility with the solid axle car which mounts the spring above the axle.  This isn’t optimal from a geometry standpoint, but it is necessary to provide for compatibility (the option of offering both rear suspensions).  There is a rubber insulator missing on the spring mounting point on this side.

7) Details of the Explorer differential. Note the blocked off speed sensor point on the lower right – this is used in the Explorer but not the Mustang.

8) More details of the differential. Note the holes in the subframe where the half-shafts pass thru. Also note the reinforcing plates for the front mount (top of image) – these look fabricated and may not be production. They help maintain the geometry of the differential as stresses are applied. Also note the scrape on the bottom of the frame, in the lower middle of the image. This was probably a stress test prototype unit, and the scrape may have been from an impact. Which is probably why this unit was disposed of – its integrity may have been compromised.

9) Another view of the complete IRS unit. Yet again we’re struck by the strength of the unit. It’s also nice to see how large the rotors are, and of course that they are ventilated. Note the lower shock mount and the mount for the 4-piston Brembo caliper.

10) This side shows the rubber locating bushing for the spring. The top end of the spring mounts in the standard point on the rear frame, shared with the solid axle.

11) Drivers side suspension, with mounting points. Until this IRS ends up in a production car, we’ll never know how well the geometry works, but it’s a good assumption to say that Ford has designed it with much-needed anti-dive geometry (as in the ’99-04 Cobra IRS, which had far less dive than the standard car). Like the solid-axle SN-95, the current S197 car suffers from severe nose-down braking. Lousy!

12) Front, showing the driveshaft mount. Ford had issues with harmonics in the earlier SVT IRS. The balancer built into the differential pinion here solves that issue.

An interesting question is where the exhaust system would be routed… it’s apparently the lowest point on the car and passes under the IRS (blue circle) to each side of the bolt-on reinforcement bar (yellow).

13) The lower side of the Explorer differential, with the breathing tube. This is an area where racers would need to install a cooler to prevent over-heating. In sold-axle cars, as the diff fluid heats, it expands into the axle tubes and is eventually expelled thru the breather near the right side of the tube (where it ends up spraying onto the tire). In an IRS their isn’t enough room for expansion in the diff casing, so excess fluid is just expelled. If cost were no object, Ford would have developed an entirely new differential casing with extensive cooling fins – reusing the Explorer differential was a concession made for cost.

14) The sway bar follows the shape of the rear structural beam. This also allows for the exhaust pipes to pass underneath the IRS from front to rear.

So here we have it: the S197 IRS sitting on the shelf and ready to go. It was originally designed in the 2002-2004 timeframe, so it’s possible that there might be updates to it for 2015.

But given that the 2015 Mustang will apparently be an evolution of current S197 chassis,  the engineering is done and the main engineering focus can be on reducing the weight of the basic chassis. Weight reduction will be critical to meet the new and far more challenging Federal mileage standards, and it’s the only way that a V-8 engine option will be able to be offered for a few years more. Given the amount of engineering that went into the design of the IRS, the IRS is actually part of the weight reduction program rather than an afterthought. The Camaro will be moved on an all-new platform in the 2015/2016 timeframe (shared with the Cadillac ATS), one which will be lighter than the current Mustang. An enormous amount of engineering has gone into the new ATS program and the result is a very lightweight and nearly state-of-the-art platform. The next Camaro will easily be lighter than the current Mustang and with direct-injected engines across the board (270 HP turbocharged 4 cylinder, 323 HP naturally aspirated six, and a new 5.5 liter V-8) the new Camaro will be well ahead of the current Mustang.

How will Ford compete? With an evolution of the current chassis, the IRS across the board, direct injection on the 3.7 V-6 and 5 liter V-8 (which both are already engineered for), an EcoBoost 4 for the entry level car (most definitively not an “SVO”!!), and a weight loss program. Additional high-strength steel and use of aluminum for further components (doors, trunk, roof) are very likely. To beat Federal standards, compete, and better yet to keep ahead of the Camaro, a loss of 250 pounds might be hoped for. A gram-by-gram examination will be needed – something that former Partner Mazda was very good at (lets hope Mark Fields brought the philosophy back home with him). An entry level V-6 could weigh as little as ~~3273 pounds, and the base model EcoBoost another 75 less.

Unbelievably, it will be 11 long years since the Mustang last offered a modern rear suspension. Lets hope that this time it’s standard, and that we never again see a solid rear axle. There have been too many excuses made, too much outright FUDD delivered, and too many years lost where we could have been enjoying a dynamic drivers car. Couple this with a serious weight reduction, and maybe even the paradigm-breaking EcoBoost V-6, and we might finally have a truly world-class sporting coupe – the likes of which haven’t been seen since the Supra TT left us. Let’s hope this dream comes true, that idiots like Phil Martens don’t get in the way, or that apologists like Hau Thai-Tang don’t try to tell us to be satisfied with obsolete engineering left over from a century ago.

And let’s hope that Ford engineers show us what they are truly capable of, rather than what they can manage with negligible budget and management that doesn’t have the vision to support the them.

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