"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."
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.
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.
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.
