Taking my time to rebuild the IRS suspension and need opinions as to the most likely reason the rear ground clearance is so low before any more money is spent. Car has always handled great and has always set low for these past 13 years. Clearance measurement was just 5.5 inches with an empty boot and full tank of gas. Kirby has stated several time that if the rear ground clearance is too low then it can is most likely corrected by just replacing the shocks. Others have stated they replaced the springs and others have stated they replaced both.
To reuse the current springs will save as much as $400 depending on where you buy them.
Looks like the shock of choice is the Bilstien B6 and best price found on those is $99. Current shocks are KYB. Is there a way test spring and shock units when off the car?
Thanks for your responses in advance.
Shocks per se wonât alter ride height but if they have adjustable speing seats they can. New springs will, or fitting alloy collars bellow the springs will. You can take the bare shocks to a good motorbike place with a suspension dynamometer
Nice to see a few posts from you, Pete.
Wouldnât shocks increase ride height, at least marginally, were plain or gas-depleted shocks replaced with fresh gas-filled ones? BTW I have Koni reds with a choice of three spring seat positions, and the height adjustment turns out to be very small.
Ever so slightly, but really not by much.
Thanks for your input. Looks like my gut feeling was correct and both new shocks and springs are required. Now the question is what differences there are between spring vendors. They range in cost from $34 each to $168. One vendor has most probably an error in the listing showing all four springs for under $50. Also the shocks vary from $57 for KYB to over $178 for the Bilstein B6.
Cannot directly speak about an XJS, but over the years, Ive been pleased with KYBs.
At one timeâmaybe no longer trueâ they had a lifetime warranty.
How much the gas pressure impacts the ride height depends on how soft the suspension is. On the XJ-S, the suspension is very soft indeed. Think about how much force it takes to compress a gas shock in good condition, and go apply about twice that much force downward on the rear bumper. Why twice? Because you have 4 rear shocks, not 1, but the leverage is roughly 2:1 for each shock. Anyhow, this will give you a rough idea how much your suspension is liable to sag if all four shocks lose their gas pressure.
âThink about how much force it takes to compress a gas shock in good condition, and go apply about twice that much force downward on the rear bumper.â
Good idea but rather hard to do since the IRS unit is out of the car for nowâș
Iâve had problems with KYBâs âLifetime warrantyâ on another car, The strut leaked after two years,about 20K miles. Long story short, KYB wouldnât replace it. Others have said they had a different luck. I never had a problem with Monroe or Gabriel or Bilstein.
RI have argued that same case in times gone by because itâs true. However, it only became a key factor in those discussions AFTER it was agreed that the coil spring spec and spring seat geometry etc. are the key determinants. You may recall Alex Canaraâs painful insistence that there was no compressible gas in a telescopic damperâŠ
I would take issue with Kirbyâs âdepends how softâ comment in so far as the query applies to static measurement of ride height. Soft suspension allows more extreme suspension articulation over a given bump or hollow, but will have no effect on ride height, asuuming it is measured when stationary.
The original red Koni âadjustabilityâ is somewhat different to what people expect today, since it requires removal from the machine. It was originally also touted as a wear compensation design, assuming one had fitted them at the initially supplied (firm) setting. Indeed the softest setting was still a bit harsh for some.
It was quite a big deal in the bike world at the time, as you could easily/quickly remove the units for adjustment to suit a touring holiday two-up or a fast ride solo etc. Still needed to dismantle the spring though, to fully compress the unit and catch the piston on inside the bottom of the body to allow the rod and top eye to turn and vary the piston orifice setting.
My â92 XJ40 needs new shocks so I managed to source a set of either Boge or Bilstiens, canât remember now 
but when fitted the front ride height raised by 20mm. Shows the old shocks were duff.