So increasing the rear anti-roll bar thickness and/or strength to our understeering XK8/XKR cars creates more neutral handling and and steering precision even when driving slowly (I'm summarizing others).

Here are our options with questions and debatable items. The best may be the last...

1. factory option R Pack rear bar (part # Mna3500da ) is not available from official unused stock as Gaudin Jaguar NV found nothing in USA or UK although they did locate a few front R Pack bars (I ordered one). No one can identify its thickness so
IF YOU HAVE R PACK on your 2003-2006 XK8/XKR coupe, please measure it and let us know. Dan at Jagbits.com believes he has a used one, but their used department says the diameter is 15.75mm, and that would make it smaller than the 17mm others here have identified as the stock XKR rear anti-roll bar diameter.
Further confusing matters, my 2002 XKR coupe with some Super Sport option pre-dating R Pack (2003-2006) has a 19mm rear bar according to my mechanic although XKRacer doubts it and suggests my guy get a new measuring tool.

2. UltraRacing 20 mm bar. Deuce2000 and others bought these although Deuce2000 later noted that the rear sway bar drop links "are too weak for the strength of this bar and are noisy." He asked about any drop link upgrades, and XKRacer said yes, by him. However, those links are 300 British pounds so nearly twice the price of the UltraRacing bar. They are beautiful though.

3. JaguarSpecialties.com expects to release a 22+mm bar (7/8 inch) in 1-2 months according to Andrew there. He says they will take a more traditional set of endlinks and so may not require XKRacer's drop links. Thanks to XKRacer for pointing me Andrew's way!

I'm holding out for that 22+mm bar, unless one of you R Pack coupes want to sell me yours after you install the JaguarSpecialties bar.

 

Here is a link that might answer some of your questions.

2001 Jaguar XKR Silverstone Program #2023 |*MotorWeek

 

If you want more neutral handling you can increase the tire pressures in the rear as well as running toe-in closer to zero in the rear.

Moving the endlink attachment point on the end arms closer to the main part of the bar reduces the leverage of the end arms and is effectively the same thing as a thicker bar because greater force is required for the same degree of twist.

 

The Jtis also mentions they are smaller in diameter for the performance pack in the rear. They must have had a good reason for it, but I don't know how that would translate in driving.

Antoher option for increasing handling would be to go for stiffer shocks (re-valve them as I just did), although I haven't driven my car hard yet, she feels much improved now.

 

I did not realise you had super sport suspension already, so tell your mechanic to hold fire on buying a new vernier or glasses as he may well be correct, tbh I am unsure if that particular roll bar is 19mm as I have never measured one, the standard without doubt is 17mm....

Just to make things more confusing

 

my advice is to install the bar 20mm UR. noise has been identified in the bushes UR having the hole slightly too big. Just slightly shim the brackets of the bushings and everything will be perfect

 

Here MNA3500DA at David Manners:

Part search results from David Manners Ltd - MNA3500DA

 

The article on the Silverstone 01 has details I had not seen anywhere before and I'll bet it is strongly indicative of 03-06 R Pack details as well.

So Silverstone 01 front bar increased to 26mm (1 lousy mm) but rear bar decreased down 2mm to 15mm for "balance." Humbug.
Understeer was "prevalent" in MotorWeek review Episode 2023. No thanks on that. Lucky for me that Gaudin could not locate that R Pack rear bar after all. XKRacer warned me in a pm that factory priority would not necessarily fit mine.

The rear bar on my XKR coupe with "SuperSport" option is 17mm after all (just like all the other XKR s without R Pack). I had it re-measured twice [so XKRacer, your suspicion was validated, thanks for not being shy :-) ].

CoventryWest once installed a second rear sway bar on an XK8 and that reduced understeer notably they said.

So I await that 22+mm bar, and will try the rear tire pressure adjustment in the meantime.

AVOS. Thanks for reiterating your re-valved shocks post. My front CATS shocks were replaced with new ones less than 2 years and 10,000 miles ago. Before I realized that, Bilstein contacted me and informed me they could not recondition OEM CATS shocks so I'd have to find someone in the USA who could do what you had done. I'd like to have switchable CATS modes and CCFulton provided some suggestions although the easy one to make firm mode come on with the Sport button isn't desirable as I want to keep them them separate of each other. Thanks everyone!

 

Please post the last 6 of your vin as I have never heard of the Super Sports option.

 

waiting on a delivery to show up so ....

let's say that for a given length, the torsional resistance is proportional to diameter, which it is all other things being equal

and that the original bar is 19mm while the new bar is 22mm

the increase in torsional stiffness is

((22/2)^2)*3.14 / ((19/2)^2)*3.14

((22/2)^2) / ((19/2)^2)

121/90.25

= 1.34

now, let's say that the torsional force applied by the end arm is proportional to the length of the end arm, which it is

and that the effective length of the end arm is determined by the placement of the end link, which is also true

then, to reduce the torsional force applied, or equivalently increase the resistance to torsional force by the same factor as an increase in bar diameter from 19mm to 22mm

if the end arms are 5 inches long, then move the end link attachment point on the end arm closer to the main bar axis by 5-(5/1.34) =~ 1.25 inches



it should be possible to fabricate fully adjustable end links from off the shelf parts

delivery guys at the door!

 

last six items to my VIN are A27449
Stevinson Jaguar in Denver told me "DDW" re my car says SuperSport...

Plums, THANKS. There really is math in the real world!
So, have there been several examples where people lived with those changes and they did not have unintended consequences? You suggesting that the changes you mention would be more cost effective than the upcoming 22+mm bar from JaguarSpecialties.com?

Deuce2000 certainly had a few learning items with the UltraRacing bar. Had it been me, I would have been irate. No we all know what to adjust.

 

That's only arithmetic and physics

I had in mind a tubular clamp on the bar end of the end link ... seen a lot on club level competition but I can't find a picture right now.

But, take a look at picture #4 on this page:

AutoSpeed - Anti-Roll Bars and Torsion Beam Rear Suspensions, Part 2

From looking at the arrangement for the XK in JEPC diagrams, if the top part of the link has a spherical joint in it and you want to keep it, then remove the bar, flatten the ends, drill new holes where you want them and reattach. All done.

To keep the temper on the bar, first heat, flatten and drill. Then reheat to cherry red and plunge into bucket of water. An experienced metallurgist/blacksmith will use oil instead of water.

All at your own risk, ymmv and all that

edit:

here's how Racing Beat does it for a Miata

MAZDA MX-5 RACING BEAT ANTI ROLL BAR KIT WITH STRONGER ADJUSTABLE END LINKS

also see the first picture in this thread

anti roll bar

 

This article has some info on the rear sway bar for rwd cars:
AutoSpeed - Sway Bar Shenanigans

So that probably explains why Jaguar chose to go for a thinner sway bar for the performance pack (and stronger on the front).

 

I just checked your vin as well as my 2000 XKR convt & they both have the Supersport Suspension. That must be what Jaguar refer to the standard system on the XKR.

 

Thanks for checking. My front anti-roll bar measures 25mm, my rear measures 17mm. The R spec units are not worth acquiring as a result. Aftermarket it shall be.

So, anyone know of a vendor selling larger front anti-roll bars?

 

Try a set of Paragon Performance's sway bar links they are a work of art. I just installed them today. Thank you once again Tony.

 

jax, you using the the UltraRAcing rear bar with Tony's drop links?

I'm curious if Deuce used those or if his final solution was just the shimming he mentioned.

 

Yes I have the Ultra Racing rear sway bar. I have not driven the car yet but will tomorrow when the weather is better & I finnish installing Tony's strut braces.

 

Thank you Tony.

 

Looking good John.......

You had no probs fitting the strut braces? I did mean to mention, you may have had to elongate the hole going through the scuttle, the original bar is squashed flat and i did not want to relieve mine, it would of made a weak point