I need a new rear spring and I’m not sure if I have standard or sports pack suspension.on the logbook it says xjs convertible auto and underneath that it says body type sports. How can you tell which suspension you have or is it a call to the dealers? Thanks for any assistance.

 

Brinny
Most unlikely to be sportspack in a convertible V12 as they are not that rigid to start with. Have you stood all four up together and checked they are al the same height and all straight?
Greg

 

Thanks Greg. Yes I have compared them .3 are identical one is obviously bent and needs replacing. Not too dear for the normal springs so will add it to the growing shopping list! Cheers

 

I'm not a fan very stiff suspension so when I replaced my rear springs I tried to get stock rate, but these are just not available new here in Aus, so I ended up installing Lovells heavy duty standard ride height and was pleasantly surprised at how well they made the car ride, not harsh at all.

The car still squats when taking off with gusto. Best the rear does not drag on the ground when loaded.

Be aware if you replace the rear shocks with anything other than stock Boge this can affect ride height. I used Bilstein and my ride height dropped 20mm, so if you do replace the rear shocks measure the distance between the spring perches and add a spacer if the perches are further apart than the Boge. I need to remove my springs and add spacers to raise the rear of the car slightly.

 

Hi, I have already bought some new boge shockers ready so should be good there. Thanks for your advice anyway all assistance is appreciated.

 

Sportspack on the later cars gives a bigger front antiroll bar (1" vs 7/8") and adds a rear bar. Note that the resistance to bending of the bar varies by the third power of the radius.

Based on radius, if it was linear you would expect a 1" bar to be 14% stiffer than the 7/8" bar, but in reality it's 50% stiffer.

 

How difficult is it to find a 1" bar? I would like to change mine out when I finally get my rear bar installed.

 

There are a few after market places in the US that sell them. Addco is a brand, Summit sells it.

I had trouble finding the rubber bushes for the bar, as the PO had put on polyurathane and it squeaked badly. The bushes from the X300 XJR fit, as that car had a 1" bar. Has to be the XJR, the XJ6 has a smaller bar.

 

The 1" bar in my case is thicker walled than the OEM, this would account for the additional stiffness. Also the way the ends are made will also affect stiffness.

BTW When I originally installed my 1" front bar with poly bushes and used the supplied grease it squeaked, I pulled it apart and used silicone grease liberally, wiped off the excess and no squeak now for 5 years.

 

Yours are not solid? In any event it doesn't matter much, the inner portion contributes so little to the stiffness that it doesn't matter if it's hollow or solid. That's why driveshafts are hollow, to reduce weight. The wall thickness only needs to be enough to resist buckling and mechanical damage.

 

Old age is creeping in you're correct they are solid. I was thinking about my rear bar that is hollow chromoly with adjustable blade ends. I have not fitted it and probably won't.

 

I’m of a mind to add a rear bar but you don’t seem so keen. Palms book suggests improved handling. What is your opinion on it please.

 

Mine has one and it certainly corners very flatly. It's important to size the bar correctly, as too big a rear bar will cause oversteer. It must always be smaller than the front bar, typically 1/8-1/4" smaller.

 

Do you mean by flatly, no rolling about?

 

Yes. The amount of body roll is noticeably less than some of my other cars.

 

My experience FWIW has been that the V12 pre-facelift with inboard brakes handles, corners and rides better without the rear ARB. Provided, and this is important I found, the ride height is correct and certainly not too high, and more importantly still, there is at least half a degree of negative camber (wheel top sloping inwards) on the rear wheels, and 3/4 is better.
Greg

 

The bar should make zero difference to the ride quality. In a straight line it does nothing.

 

Quite so, on a straight flat road, very few of which exist in the UK. As soon as one wheel moves up or down relative to the other though, the ARB stiffens/affects the relative movement, thus to an extent, nullifying the benefit of an independently sprung rear axle. My point is really that on a properly set up XJS V12 the rear axle does not roll much on corners and that the ARB adds nothing to the car's cornering ability, and does adversely affect the ride and compliance on real-world (European) roads.

 

Can anyone shed any light on why they initially fitted arbs then did away with them and then went back to them? Also my 4.0 facelift very early one, has one fitted? Just thinking why the need on a 4.0 but not on a more powerful 5.3 ? Finally do facelift arbs fit pre facelift? As always much obliged for any info/ comments

 

Yes, facelift and pre facelift ARBs the same fitment. This debate is like the "which oil is best" I reckon. I think we are talking fashion, basically. In the mid 1980s there was a great fetish among motoring journos for "great handling" - for which read rock hard suspensions that are suited to trackday stuff and unsuited to a comfortable ride/handling compromise. So when the 3.6 came out Jaguar stiffened it up a bit, and added a sportspack (= stiffer springs and dampers) to combat the "too soft" journalists love for rock hard competitors.


Some v12s did have the rear ARB, but (and this is my view) the point of an independently sprung rear is better to keep the wheels in contact with the road surface under acceleration, braking or bump/rebound. This latter is achieved by ensuring a road irregularity on one wheel does not affect the opposite one - which in a live rear axle (ie cart-spring solid axle) it always will. The best IRS layout is probably a double wishbone setup which all XJSs have. An ARB restrains one side of an IRS axle's movement relative to the other; so it will reduce the independence of the IRS axle to a degree. The plus side is that it controls body roll and the adverse effect that has on cornering ability. My guess is that with an ARB the grunt from a V12 makes the loss of each side's independence under acceleration, braking and cornering, more than the gains. But this is only my feeling from trying both.
Also, many people are used to driving live axle cars, and the compliance and smoothness of the Jaguar IRS feels very different, and people want to feel the rear end more solid - which the ARB does; even if it does nothing for cornering power or handling, it feels as though it is.
I realise this is a drum I beat constantly, but, if all is well with the car's suspension, front and rear, and the rear ride height and camber is correct, (a) the rear will not roll unduly on cornering and (b) the rear will be stable and have gradual break-away characteristics. If the ride height is too high, or the camber not negative, or if the front is all over the place, neither will be the case.
I have driven both 3.6s and v12s with and without the ARB, and their owners all agree the car is better without. But ultimately, if an owner wants a more solid feeling rear end, then why not add the ARB? But it will not improve the car's cornering power or handling, I believe, unless it is masking another out of spec fault.
Greg