Experience
in a Book
Suspension
& Steering Mods
In general, if the XJ-S owner wishes to improve the
handling and maneuverability of his car, it is recommended
he address the following items in order of importance:
- Steering rack mounts
- Anti-roll 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.
A word to the wise: it appears to be a common belief that
a serious performance car should have very rigid springs. If
you consult with the guys who actually win road races,
however, they all usually recommend the same policy: very
stiff anti-roll bars, good shocks, and fairly soft springs.
In fact, one of the most common mistakes in race car setups
is making the springs too stiff. It is easy to spot, even
from the grandstands; the slightest undulation in the track
surface, and the otherwise speedy car must slow to a crawl
to avoid losing control. Even though the stock springs on
the XJ-S are designed for a luxurious ride, there may be
little or no reason to change them, even for relatively
serious competition; the most valid reason for change will
probably be to adjust the ride height!
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 27), 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.
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 $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.
Because there has been some confusion due to inadequate
instructions provided with these kits, one thing needs to be
made very clear: when these aftermarket bushings are
installed, they should fit snugly within the bosses on the
subframe. There should be no gaps or spaces whatsoever. If
the boss is wide enough that the bushing assembly fits with
space left over, some 5/16" fender washers should be
installed to fill the gaps. The mounting bolts should be
tight, compressing the boss (Slightly! That's why you
need to add fender washers if necessary!) onto the ends of
the metal tube within the bushing.
Jaguar itself offers a stiffer mount scheme in its
"SportsPack", 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.
The urethane mounts may present their own durability
problems, however. Chad Bolles reports failures in the
aftermarket mounts (this author has had no problems in
several years). The problems seem to occur in urethane
mounts but not delrin mounts, and may be due to this being a
poor environment for urethane. Jim Cantrell says, "The
melting point is about 350ƒF and the maximum recommended
service temperature is 250ƒF. Above this temperature, the
esters boil out of the matrix and brittleness follows.
However, they rate poor in resistance to hydrocarbon
exposure, acids, synthetic lubricants and hydraulic fluids.
Could be something there in the engine compartment besides
heat that degrades the material."
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 --
preferably with some cord for strength, like old tires or
drive belts -- to install on either side of the original
mounts, providing a compression member to restrain the
side-to-side motion. Make sure to fill the space, leave no
gaps; for the rack to move at all, it should be
compressing material. 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.
POWER STEERING BOOST REDUCTION: Many drivers
object to the inert feel of the overboosted power steering,
and seek ways to reduce the assist. One of the simplest
cures is to install a smaller diameter steering wheel; Momo
and others make aftermarket wheels and adapters. The owner
of a later car, of course, has a driver's side air bag to
contend with.
John Goodman points out that the XJR-S has reduced power
assist, and it may be possible to retrofit the valve body on
the steering rack to the regular XJ-S. "I don't think the
racks have changed much, the valve is probably
interchangeable. Reduces assistance by.... well, you need
two hands to turn the wheel at standstill, normal round town
corners are OK. On the road it is noticeably heavier but
much improved.
"SPC 1034 is the part no. for the rack valve from a'89
parts list.
"SPC 1286 is the part no. for complete rack from '92
parts list.
"SPC 1158 is the part no. for the rack valve again '92
parts list.
"The rest of the steering is std. Front springs/shocks
are unique XJR-S items though."
The XJ-S owner is advised to only consider the boost
reduction mods after replacing the rack mounts with
aftermarket items. The rack mounts are the cheaper mod, they
will usually provide the feel the driver is looking for --
and they make it easier to take the rack in and out to make
further mods like the valve replacement.
ANTI-ROLL BARS -- WHAT THEY DO: An anti-roll bar
(also known as a sway bar or anti-sway 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, one wheel must go up (relative to the car)
while the other goes down, so the anti-roll 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 US 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-roll 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-roll 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-roll 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-roll bar is
very stiff.
The above is a very brief discussion of anti-roll bars. A
more complete discussion would include the effects on
particular types of suspensions, most notably suspensions
with shortcomings that an anti-roll 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-roll bars are customarily
described by their thickness, this being an indication of
their stiffness. However, the thickness of an anti-roll bar
can only be used for comparison against the thickness of
another anti-roll bar that mounts the same way on the same
end of the same type car. Because of differences in the
leverage of the anti-roll 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.
ANTI-ROLL BARS -- WHAT YOU'VE GOT: Really good
question, actually, since apparently there are a lot of
different configurations. Most of the info in the chart
below was provided by Chad Bolles.
|
Front Anti-roll Bar
|
Rear Anti-roll Bar
|
|
P/N
|
dia.
|
Bushing P/N
|
P/N
|
dia.
|
Bushing P/N
|
1975-79 5.3
|
C36887/1
|
7/8"
|
C32925
|
C42178/2
|
9/16"
|
C44931/2
|
1979-87 5.3
|
C36887/1
|
7/8"
|
C32925
|
None
|
n/a
|
n/a
|
87-on 5.3, 6.0
|
C36887/1
|
7/8"
|
CAC4651/7
|
None
|
n/a
|
n/a
|
5.3 SportPack
|
CBC5579
|
1"
|
CBC5580
|
C42178/3
|
5/8"
|
CBC4901
|
6.0 SportPack
|
CCC7198
|
|
|
|
|
|
4.0 Coupe
|
C36887/1
|
7/8"
|
CAC4651/7
|
|
|
|
4.0 Convertible
|
C36887/2
|
|
CAC4651/6
|
|
|
|
4.0 Cpe SportPack
|
CCC7197
|
|
CBC5580
|
|
|
|
XJ6
|
C36887
|
3/4"
|
C30502
|
None
|
n/a
|
n/a
|
Typically, the brackets holding the anti-roll bar
bushings will not change with bar size; the outside of the
bushing remains the same, only the size hole through the
middle varies, so the same bracket can be used throughout.
Note that the brackets for the front anti-roll bar on the
XJ-S were originally C30500 (r) and C30501 (l), but after
VIN 179737 changed to CBC5336 (r) and CBC5337 (l). Some of
the same bushings were used before and after this change, so
either set of brackets is apparently compatible with any
bushing.
ANTI-ROLL BARS -- SUGGESTED CHANGES: As noted
above, the common US-spec XJ-S has a 7/8" front anti-roll
bar only, and understeers accordingly. However, early models
(prior to VIN 106452; 1979‡) came with a 9/16" rear
anti-roll 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-roll 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‡ anti-roll bar assembly, the part
numbers and quantities are as follows:
ITEM
|
P/N
|
QTY
|
Radius arm assy
|
C41831
|
2
|
Rear Anti-roll bar (9/16")
|
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-roll 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-roll
bar, and a rear anti-roll bar that is stiffer than the
pre-1979‡ model described above. The part numbers are as
follows:
ITEM
|
P/N
|
QTY
|
Front Anti-roll bar (1")
|
CBC5579
|
1
|
Front bushing
|
CBC5580
|
2
|
Radius arm assy
|
C41831
|
2
|
Rear Anti-roll bar (5/8")
|
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 under the mistaken assumption that
all XJ-S cars come with a stock rear bar as the pre-1979‡
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 (a
bushing will be included, since it is larger than stock); if
not, the Addco "D" series or "W " series are suitable
brackets -- see below.
It has been reported that Gran Turismo Jaguar offers an
anti-roll bar kit for the XJ-S.
POLYURETHANE ANTI-ROLL BAR BUSHINGS: Polyurethane
(or urethane) is much stiffer than the original rubber
mounts, and therefore absorbs less motion and applies more
of the anti-roll bar's effects to the chassis. It is also
more durable. And, due to the nature of aftermarket supply
vs. original Jaguar parts, considerably cheaper.
"ProThane" bushing/bracket sets (see Dee Engineering),
with the "B" type being a little larger and more
substantial. The "B" 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 and this makes an excellent
installation. The "A" type would probably also work in this
application.
If you are adding a heavier rear anti-roll bar or
replacing the stock bushings on a Sportspack model, Addco
also offers polyurethane bushing kits in sizes 5/8" and up
and in several different styles. Their "B" series is their
"small shoulder style"; their "D" series is the larger,
heavier unit with no shoulders; and they also have a "W"
series that is a large shoulder style. Any of these can be
made to fit the XJ-S rear, although the "B" series requires
that the slotted holes be enlongated a little with a round
file to fit over the studs. The "D" and "W" come only in
urethane, while the "B" normally comes with rubber bushings
but can come fitted with urethane if you ask. The "W", being
similar to the J. C. Whitney "B" type described above, may
require the same minor trimming on the urethane bushing.
Addco's catalog lists their "B" for the rear of the XJ-S,
but the "D" or "W" are actually better choices.
If you wish to replace the front anti-roll bar mounts
with aftermarket units, there are a couple problems to note.
First, the top (bottom? It's mounted upside down) of the
arch of the stock bracket clears the subframe by only about
º", and the subframe moves around a little on its soft
rubber mounts so the º" clearance is minimal to avoid
banging. To be acceptable, the height of the arch of the
bracket must be 1‡" or less.
The second problem is that the bolt holes on the brackets
are staggered, with the forward stud being located outward
from the centerline and the rearward stud being located
inward. As a result, the stock brackets have two different
part numbers; C30500 for the right side, and C30501 for the
left.
The Addco catalog indicates their "D" series fits the
front of the XJ-S, but these brackets definitely will
not work. The bushings themselves might be
usable with the stock brackets, but it will require some
serious trimming on the bushings at least. Addco's catalog
does not seem too reliable.
Just when things are looking difficult, however, a
solution comes along. Addco offers a bushing and bracket set
that will fit the front of the XJ-S perfectly. They
refer to them as "S" series brackets, but that doesn't tell
the whole story; their "S" series designation indicates only
that the brackets have offset holes, but they have many
different types of "S" series brackets to fit different
cars. The set that fits the XJ-S perfectly is the set that
was designed for the 1967-68 Chevrolet Impala, although
(hopefully) you can now merely ask for the ones that fit a
Jaguar XJ-S and get the correct items. The bushings
themselves are the same as the "W" series bushings described
above.
Addco bushings come with no split in them, as though you
are supposed to feed them on from the end of the bar. This
is really difficult, as the urethane is stiff indeed and the
end of the bar has that formed eye; the bend isn't easy to
get around either. It is a simple matter, however, to cut a
split into one side of the bushing with a razor knife so you
can just spread it open and pop it into place around the
bar. Addco includes a small package of special grease with
each set of bushings, and applying a little of this grease
to the razor knife makes cutting even easier. Since the
loads on the front bushings are vertical, the ideal place to
make the split is on one side of the bushing. At the rear,
the loads are still vertical but the bushings are mounted at
an angle, so the cut should ideally be made at an angle so
it is located horizontally when installed. Use of a saw is
not recommended, since you don't want to remove any
material, you just want to cut it. The bracket will hold the
split bushing securely -- in fact, considerably more
securely than the original Jaguar split rubber bushing was
held.
POLYURETHANE ANTI-ROLL END LINK BUSHINGS: J. C.
Whitney, Addco, and many other aftermarket sources also
offer polyurethane end link bushings, again as a stiffer and
more durable replacement for original rubber items. These
end link bushings are applicable to the front of the XJ-S,
replacing C10996; it would be necessary to cut about 1/4"
off the back end of the bushing to make it the correct
length. Addco also offers entire end link assemblies, with
the 3æ" size being the correct length to replace the
parts (C10996, C46186, C33682, and C11045) on the front of
the XJ-S.
Randy Wilson reports problems with using urethane end
link bushings, however; the relative angles between the bar
and the end link and between the end link and the control
arm vary considerably with suspension travel, and the
rigidity of urethane end link bushings apparently results in
the end link itself being bent back and forth, resulting in
it breaking very shortly. Wilson has broken both the
original Jag links and the aftermarket links by using
urethane bushings. He claims some success with modifying the
urethane bushings by machining them to allow more room for
flexing, but the jury is still out on long-term results.
Perhaps sticking with rubber items would be wise here.
Generic rubber end link bushings are available at most auto
parts stores; again, it is necessary to cut part of the
generic bushing off with a razor knife to make it the
correct length.
Both J. C. Whitney and Addco offer adjustable end links,
which have a spring within the assembly that can be adjusted
to absorb some of the motion and soften the effects of a
stiff bar. The spring may also allow the urethane bushings
to back away enough to avoid the end link failure problems
described above.
The rear is another matter; the dogbone link that
connects the rear anti-roll bar to the radius arms seems to
be quite unique, and must be ordered from Jaguar.
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
-- which is in turn a result of the way the rear suspension
works. Since the lower swingarms swing about a pivot
parallel to the centerline of the car, the hub carriers need
to move only vertically and laterally through the suspension
travel range. The radius arms, however, pivot around their
forward mounts and therefore try to pull the hub carrier
axially forward and back a little as the suspension travels.
If all of the mounts and pivots were totally rigid, the
whole suspension would seize up. Since the cage is mounted
at the top with four rubber mounts that are fairly close
together, the entire cage is free to tilt slightly forward
and back to allow the radius arms to pull the hub carriers
forward and back a little. Also, the rubber mounts at both
ends of the radius arms are very soft, so they can give a
little to reduce the hub carriers axial motion.
This all works very well under most conditions, but
doesn't take to drag racing well. The radius arms are
supposed to absorb the forward loading at the hub carriers,
and also prevent excessive tilt of the cage assembly -- they
attach to the lower swingarms, but those in turn are rigidly
pivoted (needle bearings) at the diff to only move up and
down and not forward or back, so cage rocking should be
effectively limited. However, the radius arm mounts are soft
and a lot of torque apparently causes the entire cage to
flex and twist, and the upper cage mounts seem to
suffer.
The best solution appears to be to add some sort of
additional link to accept the torque loads within the cage
while allowing the stock mounts to deal with weight loading
only. Several of the aftermarket outfits reportedly offer
some sort of kit along these lines.
CHEAP REAR SUSPENSION UPGRADE: On the SportsPack,
the bushings at both ends of the radius arms are different.
The bushings at the small end are made with a stiffer
rubber. However, the bushings at the front end are the same
as on the standard XJ-S except that they are rotated 90†.
These bushings have two holes through the rubber, and on
normal cars the holes are aligned with the radius arms so
that push/pull forces apply directly at the holes. On the
SportsPack, the holes are aligned crossways to the radius
arms, so push/pull forces apply to solid rubber. If you'd
like your IRS to be a little stiffer, you could apparently
do some good with a press without even buying any new
parts.
REAR SUSPENSION GEOMETRY CHANGE #1: The
combination of rigid needle bearings and soft rubber mounts
in the rear suspension assembly have a net effect of pulling
the rear wheels forward a little as the suspension travels
either direction from normal ride height; effectively, the
hub carrier rotates about an axis that is roughly a line
from the inner end of the lower swingarms through the
forward mounts of the radius arms -- 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 this 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 and driven near the limits of adhesion. So, the radius
arm is often altered to move the forward pivot closer to the
centerline of the car. The axis of suspension travel is
still a line from the inner end of the swingarms to the
forward mounts of the radius arms, but since the forward
mounts of the radius arms are closer to the center of the
car, this line is much closer to parallel with 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 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."
REAR SUSPENSION GEOMETRY CHANGE #2: 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 radius 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. V-rated tires wider than 235 mm seem to be
unavailable without going to 16" wheels.
The stock XJ-S wheels are 6-1/2" wide. However, the
wheels on some Vanden Plas models are 7" 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.
D. C. Cook offers some snazzy 7.5x16" and 8x16" wheels
for Jaguars, and you can also get 16" wheels direct from
Jaguar; late model XJ-S's and XJ12's came from the factory
with 16" wheels. In fact, if you go shopping for aftermarket
wheels, you may find that 16" is the way to go; 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 money than
many V-rated 15" tires. John C. Palm reports that "Corvettes
used 255/50-16's from '83 to '87. Camaros & Firebirds
have long used 245/50-16's."
Regarding what will actually fit, Scott Horner says: "I
use 255x50xZR16's on the rear of my car & 225x50xZR16's
on the front...The rims I have, do have different offsets (I
can't tell you what tho'). I've used a couple of different
brands...BFG Comp TA's were a good fit (a fairly oval shaped
side wall), but soft, especially on the front, suffered from
real bad camber wear (esp after going to the race
track!).
"I've now changed to Goodyear Eagle GSC's, a fantastic
tyre for grip and lack of sidewall flex...These seem to be a
bit wider & squarer (even tho' they're the same spec)
and occasionally, (esp with a passenger) going over a bumpy
road at speed, the tyres will rub on the wheel arches, but
don't appear to be doing any damage as they're only rubbing
on the edge of the tread.
"The 225's will rub on the front suspension at full lock,
but again don't do any damage."
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
(or try http://www.dsm.org/tools/tiresize.htm).
The bolt pattern on Jaguar wheels is the same as on some
full-size Chevys. However, the offset (the distance from the
mounting surface to the centerline of the wheel) is about
1º", 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.
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.
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.
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.
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