for setting up a
matching pinion angle and transmission angle:
The following info was posted on the NSRA
and foremost a universal joint is NOT a constant velocity joint. (This
is where the concern lies) This means if iu-joint is on a shaft at any
angle other than zero, the u-joint is NOT rotating at a constant
velocity. It is
actually constantly accelerating and decelerating on each revolution as
the joint is working itself around the angles. At one point it
further to travel because of the angle and so speeds up and at another
point it has less distance to travel and so slows down (relative to the
transmission speed). This means the drive shaft is rotating in a
'movement constantly slowing down and speeding up. Now down at
the diff end this "jerky" rotation
will be transferred to the diff u-joint which will be transferred to
the wheels so what we would have is the trans trying to drive the
wheels at a smooth constant speed but the u joint is trying to slow
down and speed up the wheels. This means they are in conflict together
causing stress and vibration. The more the angle the worse this
movement becomes. Now to get over this if the diff pinion is set
parallel with the transmission output shaft the u-joint at the
differential is also rotating with this 'jerky'
movement, slowing down and speeding up each revolution but its equal
and opposite to the transmission u-joint so they cancel each other out
result is the diff pinion and trans are rotating together at constant
speed however the drive shaft is still slowing down and speeding up.
Twice per rev in fact .if the diff pinion is not set parallel with the
trans then this canceling effect does not take place and so the
will not be rotating at a constant speed in relation to the trans.
need to set up the correct pinion angle in any car, but when you point
this out to half the individuals who have put an IRS in their car,
truck, or hot rod you get one or all of the following replies, with the
first one being the most common:
doesn't apply because the
Jaguar differential is hard mounted where as a live axle differential
moves up and down with suspension travel"
In my opinion this first
is nothing more than something that "sounds good" to try and justify
doing it, but really has no bearing on the issue. As the pinion on a
live axle moves up and down the angle (in theory) is not changing and
the pinion is remaining parallel to the transmission, further more if
the road is smooth and the live axle springs are stiff enough the
amount of differential movement will be minimal, and you still need the
correct pinion angle to prolong u-joint life.
didn't do it that way"
response, I believe, is the main reason this debate has two divided
camps. “Jaguar did it that way so it must be correct.” To a point I tend
to agree with this sort of thinking. Personally having no formal training in
engineering I must trust the designs of those that do, however this
argument is in my opinion only completely valid IF AND ONLY IF you set
the system up completely as Jaguar designed it, with a cage, with
Jaguar cage mounts that have a specific amount of flex designed in to
them, with Jaguar trailing arms, and with the transmission angle at 3
degrees down. The more of these things you change, the argument
the way Jaguar did it” becomes less and less valid as you are
setting it up the way Jaguar did.
lower control arms (wishbones) on a Jaguar XJ6 IRS are perpendicular to the
pinion flange so mounting the diff at 0 degrees pinion angle allows the
suspension to move perpendicular to the road. If you set the pinion at
an angle the wishbones will also be at an angle and move backwards as
the suspension compresses.”
third argument IMHO has the most validity, and I speculate
is the reason that Jaguar did things the way they did. If the pinion is
perpendicular to the ground the wheels will stay centered in the wheel
well, and all forces will be applied vertically. All that said I do not
see, with only being 5 degrees off of perpendicular, that there would
be that big of an issue. I did a little math and with a 5-degree angle
on the wishbones 3” of suspension travel would only result in .262” of
backward movement at the wheel. A leaf spring set up moves the wheel
backwards at least that much as the spring compresses so I fail to see
With all this in mind I my priority was to match the pinion angle to
the angle of the transmission. I felt that u-joint life and
a reduction in vibration was more important than the wishbones moving
perfectly vertical to the road.
and Leveling the Car:
With the issue of pinion angle sorted out (or so I though, this comes up again on page VII) we can
finaly get some actual work done. Prior
to removing the old system, it is a really good idea to take
measurements of how all the OEM suspension parts are positioned. Also
taking many digital photos is an added point of reference, should you
find you need them during the IRS install. This will give you a
relatively good idea of where the IRS components need to be placed in
relationship to the rest of the car based on the location of the
A tape measure, level, angle
finder, and note pad and pencil will be needed to take and then record
all the measurements. The basic measurements like distance from the top
of the differential to the underside of the car, distance from the axle
tubes to the under side of the car, wheels in relationship to the
fenders, axle tube in relation to the front leaf spring bolts and any
other dimensional measurements can be taken using a tape measure.
measurements are taken based on a fixed location in the car that will
remain intact after the rear suspension has been removed.
record all these measurements it is a good idea to draw out a picture
and then write down all these measurements on said picture. You may
find that there is a slight variation from one side of the car to the
other, and if this is the case determine if it is a result of the
components being misaligned or if it is something on the actual car
that would need to be addressed prior to the install of the IRS unit,
such as a bent frame rail.
to accurately take measurement of things like pinion angle and
transmission angle, it is a really good idea to level up the car. This
step is not totally necessary as you can figure out pinion angle by
said angle and then measuring the angle of the cars stance than using
the two angles to do a simple calculation, however, by leveling up the
car you eliminate the need for calculations. With the car level, you
can simply measure at the pinion or transmission. This is not only
important during the initial measuring but will also need to be kept in
mind once the actual install begins. Having a level car as you
fabricate pieces will make it easier to get said pieces correctly
Prior to being
able to level your car you must find a place on the car that would
be level if the ride height was the same at both the front and rear
wheels. On a mustang the rocker panel offers several locations that
this criteria applies to: the pinch welds just under the rocker panel,
the flat area next to the pinch welds on the underside of the rocker
panel as well as the top of the rocker panel when the door is open and
the scuff plate is removed. Keep in mind that any surface imperfections
can make all of these places less accurate. There may be adhesive on
the top of the rocker panel where the scuff plate was, or the spot
welds on the pinch weld may make the lower edge uneven.
imperfections in mind and eliminate theses issues as best as possible.
A good example would be to scrape the adhesive off of the rocker panel.
Another way to help eliminate issues caused by imperfections in the
rocker panel is take lots of measurements from lots of different areas,
using several different measuring tools. The more information you
collect the better opportunity you have to make sure you have the car
as close to level as possible. I used several levels and my angle
finder to get my car correctly orientated. After each
actually adjust the cars stance and level it up I simply let a
little air out of the tires, took more measurements and then repeated
the process until I had the car level. NOTE Keep
in mind the car needs
to be level in all directions, not just front to back.
Once I had
the car level I was able to use the angle finder to check
pinion angle at the differential and more importantly the angle of the
transmission. As talked about in great detail above, the transmission
angle is critically important to achieve the correct pinion angle on
the IRS unit for the purpose of insuring long u-joint life.
In order to
measure my transmission angle I used an old slip yoke that had an inner
surface that was perpendicular to the mane shaft of the yoke. To get it
truly perpendicular I had to lightly run a file over the surface to
remove a bur. With the yoke in place I was able to take the measurement
and found that the T5 transmission in my car hangs down at an angle of
the OEM parts
not a lot of technical skill needed to remove the OEM parts.
There are however a few things to keep in mind prior to the removal of
the parts. First penetrating oil is your friend!!! Several days before
I removed the rear end, shocks, and leaf springs from my car, I applied
liberal amounts of penetrating oil to all the bolts and nuts that would
need to be removed. This step was done many times and made the removal
easier, but I still had some difficulty removing the nuts from the
u-bolts on the underside of the leaf springs.
consider is make sure the car is solidly supported so that the rear end
parts can be safely removed. To accomplish this I placed jack stands
under the rear pinch weld at the jacking notch as well as a cross
support under my sub frame connectors. Supporting the car at just the
pinch weld or at just the sub frame connectors probably would have
worked, but there is no reason to support the car in just one location
if you can support it in several. This will add extra stability, and in
turn make working under the car safer. With these things in mind
I was able to easily get all the old parts out and take a good hard
look at what I would need to fabricate to install this unit. My plan
has always been to make this install a complete bolt in and with all
the original parts out of the car I was able to confirm that this plan
will be easily attainable.
make it a bolt in?
has been said that it might be a lot easier to cut and weld the
underside of my car to accommodate the IRS unit rather than
make this a bolt in so why go through the trouble? There are
lots of reasons. Will I ever remove this set up and go back
to OEM equipment?? Probably not, than why make it a bolt on?? Well
there are 4 reasons:
1. Some day the car will no longer be mine.
Hopefully that won't be for another 50-70 years, but the reality is
some day I will be dead and gone and the car will hopefully belong to
my daughter or one of her kids. With that in mind, even though I love
to modify things I try to make all my modifications easily undoable.
This way if I change my mind in the future or some future owner wants
to do something different they easily can.
2. Ford did it that
way. The IRS Ford designed for the Mustang was built with the idea of
it being a dealer added bolt on and even though my install will not be
exactly the same, in many ways I am trying to emulate
that original design
3. STRENGTH. The rear
frame rails on a Mustang are way to thin IMHO. If I weld mounting
brackets on to the frame that would put a lot of stress on the middle
section of the frame, and more specifically on the thin frame walls. I
know the weight of the, back half of the car can easily be supported by
the leaf spring mounting locations so I want my set up to attach there
4. Making it a bolt in gives me the opportunity to use mounting
bushings. As you know Jaguar put all the parts in a
self contained cage and then used rubber bushings to mount said cage to
the car. By using the stock leaf spring mounting locations I can easily
use leaf spring bushings to accomplish the same thing. Leaf spring
bushings are durable, easy to find, availably in several different
materials depending on desired stiffness, and will work well in
isolating the mounting frame from the chassis.
NOTE prior to beginning the
fabrication phase of the install make sure
the car is still level. Changes in the weight of the rear half of the
car may cause the front suspension to shift and the car may need to be
re-leveled. In fact it is a really good idea to make sure the car is
still level at the beginning of each and every work session.
Building a pattern for the
make it a bolt in I had to come up with a suport structure to that
utilized the factory leaf springs and run along the curves of the
original frame rails. Rather
than start with the actual material I will be using for the supports,
I decided to make a pattern using some .75" X 1.5" tubing with 18 gage
walls as it was scrap I had laying around, was easy to work with and
started to build the pattern, I relied on my tape measure, and
trial and error to come up with the correct angles. After about an hour
I realized that I could easily get the angles I needed by using my
angle finder in the mustangs frame rails. With an accurate way to
measure the frame the piece came together quickly.
As you can
see it fits well and the design provides a nice level center
section to weld in tubes that will connect the two brackets together.
From there I can hang the Jaguar parts from the cross tubes.
pattern will be used to make the final units out of 1.5" X 2"
rectangular tubing with .120 walls. As I was building the pattern I was
trying to decide weather to notch the tubing, bend it, then weld it
back together my self, or have it bent, at a local shop. Even though
the bent tube would probably be cleaner I decided I want to keep this a
DIY as possible so I will use the notch and weld method, besides then
it will match my sub frame connectors.
the bushings for the OEM leaf spring mounts:
The front mount is a custom bushings and has a sleeve that will be
to the front of my side side rails and bolt up in the leaf
spring mounts. To build the bushings I started with Mustang shackle
bushings and some
1 & 5/8" mechanical tubing. I used my bench grinder to grind the
domes off of the shackle bushings.
machined the outer sleeve and inner sleeve to be a tight fit in the
spring mount. The hole in the shackle bushings is 9/16" and the inner
tube is 5/8" so by pressing the inner tube in to the bushing it expands
making it a tight fit in the outer sleeve. I am really pleased
with the final install. They fit tight and once the tubes are welded to
the end of the side bracket I will have a solid front mount.
the front mounting sleeve to the side rails seams like a simple enough
process; notch the tube and weld it in to place. While that IS
the basic concept its a little more complicated than that. Being
one of the mounting locations where the IRS subframe connects to the
car this connection is an important one and it needs to not only be
but also perfectly lined up.
Making the notch was
easy enough I used the sleeve to mark the notch and then ground out the
material. From there I took a long piece of the same tubing I
used for sleeve and placed it in the notch. This gave me
something to put a square up against to make sure that my notch was
In fabricating my side rails, I
started with the mount end because this end had the most chance of
being different, one rail compared to the other. The reason for
this is if I started with shaping the rails and then installed the
sleeve, if one notch is deeper than the other than it would change the
length of rail. However, by starting with this end, if one notch
is a little different than the other it doesn't mater because I now can
measure from the inner sleeve bushing and make sure both rails are
identical. As we go on to Page VI we will look at the process and
problem solving that was required to build the subframe.