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XJ-S Suspension &Steering Modifications

// JagWeb // XJ-S Help // Contents //

 

XJ-S Suspension & Steering Modifications

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:

  1. Steering rack mounts
  2. Anti-sway 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.

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

 


// JagWeb // XJ-S Help // Contents //

 

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