"I figured that an IRS suspension upgrade was beyond my means but it still was a persistent thought in my mind, kind of like an itch that wouldn’t go away."

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Why did I decide to retrofit IRS on to my Mustang? ABMIA
To put it simply, I cannot leave anything alone. In my mind, there is always a better way to do things, even if a better way has not been invented yet. Many live by the motto “Ain’t Broke, Don’t Fix It” or ABDFI, but I live by the motto “Ain’t Broke, Modify It Anyway” or ABMIA. Fueled by my desire to modify everything for the sake of improvement, I have always had a fascination with the idea of putting Independent Rear Suspension (IRS) in my 64.5 Mustang coupe. Due to this interest, I had done extensive research involving Corvette IRS, late model Cobra Mustang IRS, late model T-bird IRS and many of the aftermarket IRS options, all of which were big dollars and problematic, especially with the narrower 58” wheel base of the 65-66 Mustangs. I figured that an IRS suspension upgrade was beyond my means but it still was a persistent thought in my mind, kind of like an itch that wouldn’t go away. In a feeble attempt to “scratch the itch”, I searched eBay from time to time with no real consistency, regularity or expectation of actually finding something. In January of 2007, one such search turned up a trick aftermarket IRS unit and I made a post about it on a Mustang forum that I am a regular on. One of the responses told me that basically what I was looking at was an aftermarket suspension set up based on an original Jaguar IRS unit. This new information fueled further research which yielded many results including an original IRS unit designed by Ford for the Mustang, but we can come back to that after we look at the advantages of IRS.
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Why is Independent Rear Suspension better than a solid axle housing?
As with any technical information, it is always best to get our terminology correct. There are two families of rear ends, IRS units and solid housing units, correctly referred to as live axles. An IRS unit is as it states, suspension that is independent between the drivers side and passengers side. The other option, the “live axle”, is a solid unit that ties both the drivers and passengers side suspension together. With a live axle, suspension movement on one side of the car effects the suspension on the other side of the car. This can make tire to road contact a problem, especially on rough roads or during tight high speed cornering. With IRS, as one side of the suspension is moving due to bumps, road imperfections and/or cornering, the tire on the other side can remain unaffected and in proper contact with the road. Overall, IRS vastly improves rubber to road contact, which will not only allow for better handling but as a whole, a smoother more consistent ride. Better handling and a smoother ride is why most front suspensions are of an independent design. Even though IRS units are far superior to live axles, in most situations most manufactures of rear wheel drive cars uses live axle units for the sake of simplicity and, as a direct result of simplicity, reduced costs. Jaguar has been the exception to this school of thought. They developed an IRS for their cars back in the late 50s, with the unit debuting on the 3.8S in 1960, and the vast majority of their cars have had one form or another of IRS ever since. Jaguar did such a phenomenal job designing their original IRS unit that the design remained relatively unmodified until 1988 when it was redesigned.

Live Axle: With a live axle, suspension movement on one side of the car effects the suspension on the other side of the car. This can make tire to road contact a problem, especially on rough roads or during tight high speed cornering.

Independent Rrear Suspension: With IRS, as one side of the suspension is moving due to bumps, road imperfections and/or cornering, the tire on the other side can remain unaffected and in proper contact with the road.

The original Ford IRS design
As mentioned before, Ford originally designed an IRS unit for the Mustang and, interestingly enough, it was loosely based on the Jaguar IRS and even used the Jaguar center section. Yes, for you skeptics and disbelievers, Lee Iacocca’s original vision for the Mustang was a sporty car similar to the Corvette and Corvair Monza, with such similarities including but not limited to a powerful V8 motor nestled under a long sleek hood, 4-speed manual transmission, 4-wheel disc brakes, and independent rear suspension. To assist in achieving Iacocca’s vision, Klaus Arning, one of Ford’s suspension engineers and the one responsible for most of the Mustang suspension design, including the UCA drop AKA Shelby drop, designed an IRS unit for the Mustang that was intended by Ford to be a customer requested upgrade for the average Mustang and standard equipment on the GT and Shelby models. This unit was completely designed and installed on three or four prototype cars, most of which were Falcons. The purpose for designing the Mustang IRS was to improve race times and it was supposed to be an upgrade to stimulate sales in the performance oriented market to help it compete with sports cars, such as the Corvette.  When put to the test on smooth tracks there was little to no change in performance time even though ride quality was significantly improved.  Also the Mustang sold record breaking quantities far superior to any manufacturer  without IRS as a performance upgrade option. So without the need of using the IRS design to win on the track and increase sales, Mustang IRS was scraped to save time and money. The basic design of the original Ford IRS unit is functionally identical to that of a Jaguar. The center section is solidly mounted to the Mustang with a Lower Control Arm (LCA) between it and the hub. The half shafts not only serve to turn the wheels but also as Upper Control Arms (UCAs). The two biggest differences between a 1960-1987 Jaguar IRS rear assembly and the original Ford design are the shape and mounting of the LCAs and a need for both a forward and rear trailing arm to stabilize the LCA/hub, due to a single differential/LCA mounting point. Interestingly enough, the original Ford design used Chevy hubs. With this information about the original Ford design being based off the Jaguar design and using other brand parts even the purest of make, in other words those that only put Ford parts on their Ford cars, do not have to take too far of a leap to put a Jaguar rear setup in their Ford, but  for those that would like to purchase a Mustang IRS system based on the original Ford design contact CTM engineering.

CTM engineering Mustang IRS unit based on the original Ford design

Why use a Jaguar IRS unit in a Mustang?
The Jaguar IRS assembly has been a popular suspension upgrade for hot rodders and car enthusiasts since Jaguar first developed these rear ends in the early 60s. The first thing that makes it so appealing is that the unit is neatly packaged in a self-sufficient cage that allows it to be removed and installed as a complete IRS unit.  This design also makes it possible to simply fabricate mounts under the vehicle that the IRS unit is going into and then the cage can be bolted in as it was originally designed.  Mounting the unit in this way is simple, however it does have a couple of drawbacks.  First, as the mounting bushings wear out, the handling quality of the IRS unit will decrease.  Second, if the Jaguar unit needs to be narrowed, modifications will need to be made to the cage or a new cage will need to be fabricated.

Jaguar IRS cage

The Jaguar IRS unit was developed and built by Spicer here in the states and then shipped over to England as a unit.  This means that most of the parts are of American sources and easy to get.  All the bolts, nuts and threads are SAE.  The half shafts use standard Chevy U-joints and the U-joint flange on the front of the differential uses a standard Ford U-joint. Even the wheel bolt pattern is “American”, all though unfortunately its 5 on 4.75” pattern makes it Chevy.  The differential is basically a Dana 44 so locking carriers, ring and pinion gear sets and bearings are relatively common and come in a wide variety of performance applications. I say that the differential is “basically a Dana 44” because although almost identical to a Dana 44, it is actually Spicers earlier design for this type of differential and is called a Salisbury differential.  Difference between the Dana 44 and the Salisbury differential will be discussed during the information on the differential rebuild.  Not all of the parts on the Jaguar IRS unit are common, especially the brake parts, however, any non common parts can be purchased from Jaguar specific suppliers and/or the host of aftermarket suppliers that specialize in putting Jaguar IRS units in other types of vehicles.

One innovation that makes the Jaguar IRS unit especially appealing is the in-board disc brakes.   By mounting the disc brakes directly to the differential housing, they do not need to move with suspension travel and, thus, their weight does not effect suspension movement or reaction time.  This is more technically referred to as un-sprung weight.  There is, however, one draw back to inboard disc brakes and that is that they make changing pads and rotors and calipers more difficult due to their location and the amount of parts, including the half shafts that need to be removed to change these parts.

Jaguar differential

Notice the disc brakes atached directly to the differential

One of the biggest advantages of using a Jaguar IRS unit in a non-Jaguar vehicle is that it is relatively simple and easy to narrow and optimize for the new application, unlike many of the other donor car IRS options available.  In the case of retrofitting a classic Corvette IRS unit or modern Ford IRS unit into a narrower car, issues arise from the modification of LCAs, drive shafts, UCAs and other moving parts to make it fit.  These units are designed and optimized to be a certain width and are complicated.  On these modern types of IRS, there are so many moving parts related with suspension that when you start changing things it is very difficult to maintain the correct geometry.  This is not true of the Jaguar unit.  To narrow up a Jaguar rear end, you only need to shorten the half shafts and the LCAs.  Any possibly geometry issues that may arise from said shortening will only effect camber and that can easily be adjusted with shims placed between the half shaft mount and the rotor.  Also, Jaguar rear assemblies came in several widths from the factory,  53.125”, 56" and 61.75”.  The only difference in these three different IRS units is half shaft and LCA length, so any width between 53.125" and 61.75" should definitely work and chances are many widths outside this range will also work just as well.
As with any modification, there is no “perfect upgrade” and there are a few negatives involved in retrofitting a Jaguar IRS unit to other cars. The first issue is that in some applications with IRS, there can be excessive wheel hop. Under hard acceleration such as that seen during drag racing, an IRS unit will hop more than a live axle unit, however, when drag racing the track is not rough nor does it curve so there is no reason to have IRS.  It all comes down to application.  If you want a corner carver, you put in IRS, but if you want a drag car you put in a live axle 4-link unit.  An IRS unit adds extra weight.  An IRS unit weighs more than a corresponding live axle unit, only about 30-50% more, so the extreme improvement in handling is far greater than any performance loss, due to weight increases. Also, an IRS unit IS more complicated than a live axle unit, has more moving parts and more pieces that can fail.  Even though this statement is true, it is not much different that an independent front suspension.  Yes, there are more moving parts, however, routine maintenance will ensure that the parts function well for many years to come.  As I have said before, these negatives, in most applications, are far outweighed by the benefits.  What it really comes down to is what do you want to use your car for? And what is important to you when it comes to the way your car drives.  For me, my car is not a drag car but a corner carver.  Horsepower and acceleration are important, but not as important as high-speed turns and ride quality.  This is why I am retrofitting a Jaguar IRS unit into my Mustang.  This article is just the first of many that will document the acquisition, disassembly, modification, rebuild and retrofit of a Jaguar IRS unit into my 64.5 Mustang.

You may be wondering, "Where can I get a Jaguar IRS unit at a reasonable price?"
Even though the actual fabrication process has not yet been discussed yet, if you are already sold on the concept of retrofitting a Jaguar IRS unit into your vehicle and would like to purchase a complete used IRS unit, Email David Boger, or check out his web page, www.everydayXJ.com He has many Jaguar Parts cars and has lots of rear end assemblies to sell with a range of gear ratios from 2.88:1 all the way to 3.51:1.  I have purchased two assemblies from him, the prices were reasonable, and he was very helpful.
Want more information about putting a Jaguar IRS in a classic Mustang??  Check out Page II, Page III, Page IV, Page VPage VI and Page VII of this project

BUT if you can't wait to see what a Jaguar IRS looks like under a classic Mustang check out Mustang IRS Success Stories

Disclaimer on Daze Tech Tips
      I am not an expert in this field. I have performed these modifications myself with very good results. I am passing along restoration and performance tips for the purpose of education.  If you are concerned about reliability or safety issues, I do not recommend that you or any other individual perform these changes or attempt to modify your cars from stock configuration except under your own volition.  I do not assume nor accept any liability for the use of this information or how it is applied.

2013 DazeCars
The words / logos for Ford, Jaguar, Mustang, Galaxie, etc are used for descriptive and reference purposes only. DazeCars is neither affiliated with Jaguar Land Rover North America LLC, Ford Motor Company nor the manufacturers/distributors of Ford or Jaguar automobiles.