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Suspension and Steering Modifications

  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:

  1. Steering rack mounts
  2. Anti-roll bars
  3. Wheels and tires
  4. Subframe mounts
  5. Shock absorbers
  6. 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.

 

 

On to Brake Modifications

 

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