Being able to take the weight of the body in compression is completely different than being expected to take the weight of the IRS in tension.
The rubber mounts may be quite good in compression, but fail in being used as hsngers for the IRS.
Besides, “if they can’t take the weight anymore, they should be replaced”, then a maintenance item check should be “lift car every 10 years, if IRS remains on the floor/ground, replace IRS mounts”.
In my opinion, if the issue is entirely about safety only, it would suffice to do the following:
When the vehicle is just lifted off the ground with rear wheels removed is an appropriate way to inspect the IRS mounts for any suspicion of excessive age, wear, cracking etc
This should be done as part of preventative maintenance once per year ideally
At this point regular adjustable restraining straps can/should be fitted which would prevent the IRS crashing down under any circs once raised to a higher
It seems like OEM style Metalastik last a long time, I suspect the replacement cycle on them would be 10-20yrs subject to inspection
I would not trust any non-oem style rear IRS mount, I have a box full, some are good OEM, others have delaminated due to age
I would not allow an IRS to hang totally unsupported for any length of time, just get some support under, why invite trouble
Precisely! Why invite trouble.
That leads to this: define “for any length of time” as that is precisely what happens every time our cars are lifted any amount.
Some have said lift the car, then put supports under the IRS … why wait “for any length of time” to support the IRS? Why not have the IRS automatically ‘supported’ “upon being lifted”?
I’m not grasping why some say it is okay to let the IRS hang from the rubber mounts … as the car is being lifted … then support it so the IRS does not hang from the rubber mounts … do they have a special knowledge about the condition of the rubber supports which lets them know how long is too long, and how long is not too long (to hang the IRS from the rubber supports) that the rest of us do not have?
Is it a skill or a ‘touch’ that is teachable to us who do not know how to determine that ‘not too long’ time?
My younger brother can do dowsing with dowsing rods, I don’t have that touch, is this similar?
What about driving fast over a slight hill and “catching air”? What if the IRS drops out then? 
Yes 
not trying to be a smart ass here,maybe I am who knows
Visual; remove wheels and inspect while lifted 1" off ground
If you never do this, you will not be able to acquire the touch and skill you seek
This is especially relevant if the parts are unknown age, quality make etc
In my opinion, if they pass that visual inspection, if good, I would consider it safe to lift the vehicle, then add additional support to IRS
As I said, if in any doubt, or just cautious (which is wise), the IRS can be restrained by ratchet straps, or just plain old telco rope, or thick wire
I use old teco rope for many auto applications, 5/16" 800lb breaking strain
I see … we load it to failure to see if it is going to fail.
I guess I’m just quirky about things like that. If someone says something is rated for … oh, say 800lb … I don’t feel that I have to test it to failure to prove it hasn’t failed. I just accept that it is … was … rated at 800lb.
And if there is no reason to test it to failure, such as putting something on it to hold it, my thinking is “why not?”, especially if it is easier than lifting it to failure yearly to see if it had failed.
The IRS is never going to crash to the floor…the radius arms…propshaft…exhaust etc are all connected…i have never heard of or seen one that completely dropped out
So it drops down 10 inches - depending on lift or jack stands that is a lot of inertia.
If the car jumps the IRS jumps with it, apart from the inertia of the springs and wheels fully extending.
You should always follow safe working practice when lifting a car…no one that i know has ever had a need for a mount failsafe…start to lift the car…check the mounts…continue lifting and fit supports…nothing difficult…Steve
Take one into the shop (say for MOT) and they wouldn’t know…
I imagine that if one mount delaminates which I‘ve seen then the other might give soon as it sees twice the load.
Basic safe working…if you jack up a car you should know what your doing and where to lift it…you can lift the car straight under the irs or under the lower wishbones…
Many two posts only reach the radius arms, and that’s where I jack it up when I need the lift.
Hi David…remember jacking points are not the same as safe lifting points…the former is for working on one corner at a time…
That is true but where would you jack your XJ (S) if not at the jacking points which are reinforced and in the correct position.
I would jack at the jacking points if changeing a wheel or quick work on one corner…in my opinion a 2 post lift is not the ideal tool to raise the car as you need additional transmission jack under the IRS…the correct tool is a 4 post and jacking beams…no fear of it falling or tipping and the type of lift that would be used for an MOT in the UK… Steve
Not trying to argue with you, just, most lifts here are two post. Definitely the ones I have available. Not the ideal tool but more than good enough, I’m certainly grateful for them. If it does come down it will be in the worst possible situation. That makes a simple cheap strap good insurance.
Hi David…no argumemt…its a forum…everyones opinion should be welcome…we all have to make fo with what we have…Steve
Not a bad philosophy; one that I generally use myself.
However, these mounts concern me a bit. Obviously not enough to have actually taken any action, though 
The originals were high quality and long lasting but, obviously, “long lasting” is not the same as “will never fail”. Meanwhile there’s anecdotal evidence that new replacements [at least some of them] are of lower quality and are perhaps more likely fail than even decades-old originals. Thus a dilemma is created and my worry isn’t being assuaged.
I’ll mention here that I’m not so concerned about failure while hoisting the car. I’m concerned about failure while driving the car. A sudden change in rear suspension location while driving could be very dangerous.
I’ve not researched new replacements in some time so I don’t know what’s out there. If true, high-grade OEM parts are available I’d install them and put the matter out of my mind exactly as you say.
Many experiences with replacement parts over the years, however, has made me leery of the quality of any replacement parts…including those sold by Jaguar itself.
One of “the usual Jaguar vendors” is/was selling an upgraded mount. Might be worth researching how it varies from others. Real deal or just marketing fluff? I dunno.
Anyhow…
If it comes down to a modification I’d want something that limits travel so that the rubber never becomes fully extended. Determine/measure the movement between compressed and extended. Divine a strap/hook/something to stop downward extension just shy of the fully extended position.
Ideally, really, you’d do the same thing to prevent extreme compression as well. The idea of course being to prevent the rubber from being tortured in either direction.
Seems to me that such a device would increase the life of the mount and prevent actual separation of the cage from the car should an actual failure occur.
Ramble switch turned “off”
Cheers
DD
I purchased SNG’s uprated rear mounts.
Rear suspension subframe/cradle mounting rubber bushing - uprated, 80 shore rubber hardness compared with original at 60 shore.
I have yet to install them as my oem still look good. Anybody install them? Do they last even longer than oem?
Exactly.
I’m thinking of a simple (but it’s hard to get “simpler” than what I drew above) design that will retain excess movement in any axis, instead of relying on the confluence of the two mount angles, and the two ‘claws’ not allowing a ‘slipping out’ option that would result in the IRS dropping out or even shifting position if it did not drop out.
Being as I haven’t worked on the IRS yet, does anyone know how much ‘clearance space’ there is to each side of the mounts?
One idea I’m considering would need about 5/8" to 3/4" clearance to each side to avoid touching/rubbing whatever might be there. I’m thinking that if there is enough space to fit in a wrench to access the mounting nuts, there should sufficient clearance.
There should be lots of space around the mounts. Well, not where the cage is obviously. But again wire trumps any other solution so far because it is cheap and simple.