In general, if the XJ-S
owner wishes to improve the handling and maneuverability of their
car, it is recommended the following items be addressed in order of
importance:
- Steering rack mounts
- Anti-sway bars
- Wheels and tires
- Subframe mounts
- Shock absorbers
- Springs
When searching the various sources for products to improve the
XJ-S suspension, keep in mind that the Jaguar rear suspension has
been largely unchanged since the introduction of the E-Type. This
assembly is very popular with the hotrod and custom car types. As a
result, ads for parts for improving this suspension can occasionally
be found in some decidedly non-Jaguar publications.
Steering
Rack Mounts
It is tempting to describe the replacement of the steering rack
bushings in the Maintenance section, since ALL owners should have it
done. However, it is clearly a modification, not maintenance. So:
even though the term "modifications" may normally imply something you
wouldn't consider, please consider the steering rack bushing
replacement described below strongly.
Even if the original Jaguar steering rack mount bushings in your
car are in fine shape, the steering response is less than ideal. The
stock mounts are very soft, which by itself is not necessarily bad.
However, for some reason Jaguar also chose to position them parallel
to the steering forces (see Figure 18), so they are loaded axially
(shear) rather than radially (compression). There are bumpers (flat
washers with a layer of rubber on one side) on either side of the
mounts to limit the side-to-side motion of the rack. This design
results in a nonlinear response: When you turn the wheel gently, the
steering is somewhat unresponsive, since the mounts flex and absorb
some of the steering motion. When you turn hard enough that the
bumpers are reached, the steering suddenly becomes much more
responsive. The inconsistency makes it difficult to steer smoothly,
especially when driving hard; the car seems indecisive about how much
it wants to turn.
Figure 18 - Original vs. Aftermarket Steering Rack Mounts
The solution is to replace the steering rack mount bushings with
ones that are not merely stiffer but of a totally different design.
This modification is so common that virtually every aftermarket
business dealing in Jaguar stuff offers a kit. The
John's Cars design subjects the
elastomers to compression only, and eliminates the inconsistency in
the steering response. They use polyurethane, making the mounts stiff
but not totally rigid. The entire kit is about US$50, and well worth
the money. XK's Unlimited
reportedly offers a kit of slightly different design made of Delrin,
but the function is similar.
Jaguar Sports Pack
Jaguar itself offers a stiffer mount scheme in its "Sports Pack"
using one normal mount (CAC 1635) and two special ones (CBC 5928 or
CBC 9107). While an improvement, this scheme still maintains the
lousy shear-loaded design, and it is still a better idea to get the
aftermarket kits instead; they are easier to install (no press fit
required, since there is no shear loading) and probably a good bit
cheaper.
To present a contrary opinion: Chad Bolles reports failures in the
aftermarket mounts (this author has had no problems in several
years). Bolles suggests that instead of replacing the original
mounts, you merely insert washers on either side of them to reduce
the side-to-side travel; washers from mag wheel lug nuts work well.
This makes the original mounts last indefinitely, since they hardly
move.
Another idea may be to cut out some hard rubber pieces (old tires
would work well) to insert on either side of the original mounts,
providing a compression member to restrain the side-to-side motion.
Also, since the three mount bushings work in unison, the stiffness of
the rack mount system can be controlled by adding rubber spacers to
just one, two, or all three mounts. The same treatment should be used
on both sides of any given mount, however.
Anti-Sway Bars
An anti-sway bar (also known as a sway bar or anti-roll bar) is
simply a spring that resists one wheel from traveling up or down
without the wheel on the opposite side moving likewise. For a car to
lean while cornering, one wheel must go up (relative to the car)
while the other goes down, so the anti-sway bar is a spring that
resists leaning. This has many benefits, including reducing the shift
of the center of gravity caused by the leaning, and better traction
and tread wear due to keeping the car, and therefore the tires,
upright in a corner.
It is a good idea for the roll stiffness to be balanced between
the front and rear end of a car. If one end is stiff and the other
isn't, then when the car tries to lean in a corner the outside tire
on the stiff end takes all the load while the outside tire on the
other end takes it easy. Typically, this will cause the tires on the
stiff end to behave poorly and wear rapidly. In general, adding roll
stiffness to one end of a car helps the handling of the other end.
If the front end is stiffer than the rear, then the car tends to
understeer, or plow its way through a turn. This is the accepted norm
here in the U.S. In this case, when the car doesn't seem to be going
where the driver wants it to go, he merely turns the steering wheel
farther -- a design considered defensible in this age of litigation.
If the car has balanced anti-sway stiffness or is stiffer at the
rear, there is the possibility that the car will oversteer in a
corner -- the rear end will swing out. The correction is to turn the
steering wheel in the direction of the slide -- a design considered
indefensible in court, and possibly exposing auto manufacturers to
liability.
For those who prefer to drive a car rather than have the car drive
them, adding a rear anti-sway bar to an understeering car is a good
idea. The car will have more neutral handling characteristics. Also,
there is a more "precise" feel to the steering, even when driving
slowly. In general, the car will feel more stable. And the front
tires will usually wear much longer, and wear less on the corners and
more uniformly across the tire.
Ah, but what about the ride? When driving over two-wheel bumps
(such as speed bumps), the anti-sway bar merely turns on its pivots,
and the ride is unaffected. When driving over one-wheel bumps,
however, the motion of the wheel over the bump will have a more
pronounced effect on tilting the car. This effect can be quite
noticeable. In cases where a series of one-wheel bumps are
encountered, the occupant can feel downright jostled, especially if
the added anti-sway bar is very stiff.
The above is a very brief discussion of anti-sway bars. A more
complete discussion would include the effects on particular types of
suspensions, most notably suspensions with shortcomings that an
anti-sway bar tends to compound. Also, some cars have suspensions
with an inherent anti-roll stiffness, and don't require a separate
bar. However, the Jaguar suspension has no such situations, so there
is no need for further discussion here.
It should be noted that anti-sway bars are customarily described
by their thickness, this being an indication of their stiffness.
However, the thickness of an anti-sway bar can only be used for
comparison against the thickness of another anti-sway bar that mounts
the same way on the same end of the same type car. Because of
differences in the leverage of the anti-sway bar linkage as well as
in suspension geometry and center of gravity, a 1/2" bar may be quite
stiff on one car while a 1" bar may be barely adequate on another.
The common U.S. XJ-S has a front anti-sway bar only, and
understeers accordingly. However, early models (prior to VIN 106452;
1979-1/2) came with a rear anti-sway bar as well. These parts can be
added to the later XJ-S, and were designed by Jaguar to provide
balanced handling when used with the stock front bar. The radius arm
assemblies must be replaced to provide attachment points for the
anti-sway bar, or attachment points can be welded onto the existing
radius arms. The mounts on the underside of the car were still being
provided as late as 1988 and possibly later: a reinforced area with
two studs on each side of the car, just above the radius arms.
For the pre-1979-1/2 anti-sway bar assembly, the part numbers and
quantities are as follows:
|
Item
|
Part Number
|
Quantity
|
|
Radius arm assembly
|
C41831
|
2
|
|
Rear Anti-sway bar (14.5 mm)
|
C42178/2
|
1
|
|
Bushing
|
C44931/2
|
2
|
|
Bracket
|
C42114
|
2
|
|
Link
|
C42907
|
2
|
This is a really light bar, and the change in ride from adding it
is minimal. It does help the handling considerably, however.
While adding a light rear anti-sway bar can do wonders for the
balance and handling of a street car, it by no means eliminates body
roll entirely, and certainly will not make an autocross champion out
of the XJ-S. For those who wish to get serious about cornering, an
excellent plan is to replace the 7/8" front bar with a stiffer one,
and to install a suitably heavy-duty item in the rear to match.
"Sportspack" models come with a stiffer front anti-sway bar, and a
rear anti-sway bar that is stiffer than the pre-1979-1/2 model
described above. The part numbers are as follows:
|
Item
|
Part Number
|
Quantity
|
|
Front Anti-sway bar
|
CBC5579
|
1
|
|
Front bushing
|
CBC5580
|
2
|
|
Radius arm assembly
|
C41831
|
2
|
|
Rear Anti-sway bar (16 mm)
|
C42178/3
|
1
|
|
Bushing
|
CBC4901
|
2
|
|
Bracket
|
C3054
|
2
|
|
Link
|
C42907
|
2
|
Addco offers a 7/8" rear bar,
along with a 1" front bar. Note that a 7/8" rear bar would be almost
four times as stiff as the 5/8" bar that comes on the Sportspack! It
is clear that the two Addco units are intended to be used together;
Addco does not offer a rear bar suitable for use with the stock front
bar, evidently under the mistaken assumption that all XJ-S cars come
with a stock rear bar as the pre-1979-1/2 models did.
Unfortunately, since their kit assumes the existence of a rear
bar, it doesn't include the links and other parts needed to connect
the rear bar to the rear suspension of later cars. To use their kit
it will be necessary to purchase the links from Jaguar, and the
radius arm assemblies will need to be either modified or replaced.
When ordering, check that the Addco kit comes with a bracket (the
bushing will be included, since it is larger than stock); if not,
Addco makes a suitable bracket, or you can order a 7/8" Polyurethane
Bushing Kit from J. C. Whitney
-- see below.
It has been reported that Gran Turismo Jaguar (see page Error!
Bookmark not defined.) offers an anti-sway bar kit for the XJ-S.
Polyurethane Anti-Sway Bar Bushings
Polyurethane is much stiffer than the original rubber mounts, and
therefore applies more of the anti-sway bar's effects directly to the
chassis. It is also more durable. And, due to the nature of
aftermarket supply vs. original Jaguar parts, considerably cheaper.
If you are adding the stock 14.5mm bar C42178/2 to the rear of a
later model or simply wish to replace the bushings on an earlier
model, in place of the stock bushing C44931/2 and bracket C42114 a
"Polyurethane Bushing Kit" can be ordered from
J. C. Whitney in a 9/16" size.
This bushing must be modified slightly for the XJ-S by cutting
notches along the bottom edges, but polyurethane is easy to cut with
a razor knife or hacksaw so it only takes a minute.
If you are adding a heavier anti-sway bar, a Polyurethane Bushing
Kit can be ordered from J. C. Whitney in larger sizes as well.
Addco also offers polyurethane
bushing kits in sizes 5/8" and up, but the J. C. Whitney item is more
substantial and fits the XJ-S better for about the same price.
Rear Suspension
Reinforcement
Among those who have developed more torque at the rear wheels of a
Jaguar, the rear suspension cage mounts are a known weak spot. If
your car can smoke the back tires, it apparently can also rip these
suspension mounts fairly readily. According to Mark McChesney there
are hard rubber mounts available from
SNG Barratt.
However, the problem isn't really with the stock mounts; it's a
result of the way in which the rear cage is supported. The cage must
be free to tilt slightly forward and back (flexing around the mounts,
which are at the top of the cage) in order for the suspension to work
properly. The trailing arms are supposed to prevent excessive tilt --
they attach to the lower swingarms, but those in turn are rigidly
pivoted at the diff to only move up and down and not forward or back,
so cage rocking should be effectively limited. However, a lot of
torque apparently causes the entire cage to flex and twist, and the
mounts seem to suffer.
One popular solution is to add a link to help hold the cage more
securely. Typically, a link -- with rubber mounts -- is connected to
the bottom center area of the cage near the driveshaft coupling. The
link extends forward parallel to the driveshaft and is connected to a
bracket added to the floorpan. Such a link helps prevent the cage
from rocking excessively, and helps the cage mounts survive severe
acceleration. Several of the aftermarket outfits reportedly offer
some sort of kit along these lines.
Diagonal Radius Link
With the stock suspension, as the rear wheel travels up and down,
it is traveling around an axis that is at a rather severe angle to
the centerline of the car. In other words, as the wheel moves away
from level, it also moves forward and turns in a little, providing a
little rear-wheel steering. The effect of the stock design is to
provide a lot of stability on the freeway, making the car impervious
to crosswinds and the like. (And you thought it was just because the
car is heavy? Get real.)
In racing, such stability due to rear wheel steering is also
desirable, but to a much lesser extent -- a lot of it tends to make
the car corner poorly when fitted with racing tires. So, the trailing
arm is often altered to move the forward pivot closer to the
centerline of the car. This reduces the rear wheel steering effect
and allows the driver more control over how the car drives at the
limit of adhesion. According to Mark McChesney,
...Terry's Jaguar is now
selling a full kit to convert to a diagonal radius link (with diff
cage supports). I'm not sure the kit will work on an XJ-S, I think
it's intended for E-types.
Rigid Differential Mount
The Jaguar IRS is commonly used on hot rods, Cobra replicas, and
other custom applications. In these applications, it is typical to
mount the differential rigidly to the chassis of the car and
eliminate the trailing arms altogether, in order to provide the
cleanest and most visually attractive chassis. Some people advocate
making the same change to the stock Jaguar; the components are
certainly strong enough to do this. However, this is a radical
geometry change from the stock IRS in the XJ-S resulting in no
rear-steering stability whatsoever and will make a quite noticeable
change in the handling of the car. Also, having the diff rigidly
mounted will result in more gear noise and road noise transmitted
into the car.
In both the hot rod and Cobra applications, the suspension is
normally sprung so stiffly as to not move much anyway, you're driving
a go-kart rather than a car, suspension geometry is not an issue. The
Jaguar XJ-S is an excellent handling car even though it is softly
sprung. If you wish to maintain this, you should not consider
eliminating the radius arms.
Wheels & Tires
Older XJ-S's came with the 215/70VR-15 Pirelli P5 tires; newer
ones were fitted with V-rated 235/60R-15 Pirellis or Goodyears. When
the 70-series tires wear out, replace them with 60-series tires,
which are wider but about the same height. V-rated 235/60R-15 tires
are offered by Goodyear, Yokohama, Bridgestone, Dunlop, and perhaps
others. See the comments on tires in the section on
Suspension & Steering. V-rated
tires wider than 235 mm seem to be unavailable without going to 16"
wheels.
Vanded Plas Wheels
The stock XJ-S wheels are 6-1/2 inches wide. However, the wheels
on some Vanden Plas models are 7 inches wide, and will bolt onto the
XJ-S. While this may not sound like much, a wider wheel holds the
casing of a tire with more stability. The difference is noticeable,
even if you use the same tires.
Aftermarket Wheels
JaguarSport and
D. C. Cook offer some snazzy
7.5x16" and 8x16" wheels for Jaguars. In fact, if you go shopping for
aftermarket wheels, you may find that 16" is the way to go. Late
model XJ-S's and XJ12's came from the factory with 16" wheels. The
aftermarket shops seem to offer more in 16" than 15." But besides the
wheel availability, it also may be easier -- and cheaper! -- to find
V-rated tires in 16" sizes to fit the Jaguar. Specifically, the
255/50R-16 size is suitable, and is widely available (in a Z rating!)
for less than V-rated 15" tires.
Tyre Size Calculator
If you have access to the WWW, there is an online tire size
calculator for determining which size tires will replace stock sizes.
The URL is
http://www.dsm.org/Fun/TireSize.html
Chevvy Wheels
The bolt pattern on Jaguar wheels is the same as on some Chevys.
However, the offset (the distance from the mounting surface to the
centerline of the wheel) is about 1.5", or about 1" greater than
stock Chevy wheels. If a set of stock Chevy wheels is mounted on an
XJ-S, they will stick out the sides of the car, since they will be
located 1" farther outward.
The world of Chevy wheels is not that simple, though. Some Chevys
have different offsets than others; some cars even have different
offsets in front than in back. And aftermarket Chevy wheels can be
anything, since sometimes additional width is added entirely on the
inside, increasing the offset by 1/2 the additional width. With a
little care, it is possible to find aftermarket Chevy wheels that
will work on a Jaguar. If the wheel has enough meat to it, it may
also be possible to machine off some of the mounting surface, thereby
increasing the offset.
Centre Hole Sizes
When fitting non-Jaguar wheels, it is highly recommended that the
center hole fit snugly around the pilot diameter on the hub. While
some wheel manufacturers claim the wheel will be held satisfactorily
by the lugs themselves, this is a much looser tolerance arrangement
and has been known to cause problems.
Lower Ball Joint Replacement
The lower ball joint from the XJ40 (the 1988-on boxy XJ6) will
replace the ball joint on the XJ-S. The part number is CAC9937.
According to XK's Unlimited
this will provide more clearance for wider wheels.
Fitting Wider Tires
If, after reading the
warnings on using different size tires you still feel compelled
to fit the rear of the XJ-S with some 15-inch-wide tires, it probably
can be done through fender flaring, suspension narrowing, etc. The
suspension subframe mounts (not the individual A-arm bushings but the
mounts that connect the entire assembly to the car), front and rear,
should be replaced with hard mounts to avoid dangerous instabilities.
Be advised that the ride will suffer considerably, and some areas or
the car's structure (notably right around the suspension subframe
mounts) may even be overstressed and require reinforcing.
On to the Brake
Modifications
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