Fueled by my desire to modify everything for the sake of improvement, I have always had a fascination with putting Independent Rear Suspension (IRS) in my 64.5 Mustang coupe. I have known for some time that IRS options included Corvette, modern Mustang Cobra, modern T-bird/Mark series and at least half a dozen aftermarket units. With these options, 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. One such search turned up an aftermarket suspension that I learned was based on an original Jaguar IRS design. I had not considered the Jaguar as a possible donor car for IRS. Further research into Jaguar IRS unit gave me lots of information and I decided it was the right choice for my application. Following is some info on why I chose to use a Jaguar IRS unit in my Mustang.
You may or may not know this but Ford designed a dealer option IRS unit for the first generation mustang. The basic design 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. For more information on the original Ford IRS unit check out
CTM engineering.
The Jaguar IRS assembly has been a popular suspension upgrade for hotrodders and car enthusiasts since Jaguar first put these rear ends in their cars 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. If the Jaguar unit needs to be narrowed such as the case of the Mustang install, modifications will need to be made to the cage or a new cage will need to be fabricated.
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 from 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 of future installments of my web page documenting this IRS project.
Not all of the parts on the Jaguar IRS unit are common us parts, 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, 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.
One of the biggest advantages of using a Jaguar IRS unit in a non-Jaguar vehicle is that it is relatively 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, especially one as narrow as the 65-66 Mustang, 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 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. 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.
There are a few potential negatives that need to be considered before deciding to install an IRS unit in your car. Under hard acceleration such as that seen during drag racing, the wheels of 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, about 30-50% more than a corresponding live axle unit. In my opinion the extreme improvement in handling is far greater than any performance loss, due to weight increases especially sense much of the weight on an IRS unit is un-sprung where as all of the weight of a life axle is tied to the springs.
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 for many years and thousands and thousands of miles.
For me these negatives 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. 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.
Want more information about my Jaguar IRS in a classic Mustang project including technical info on the benefits and my process far with the project, Check out
Page I,
Page II and
Page III of this project.
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Re: Independent Rear Suspension
