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Front springs
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As a rule of thumb the spring force present at the wheel, oddly enough called the Wheel Rate, is about 100 to 150lb/in for road cars, about 300lb/in for a BTCC touring car, and around 600 for LM specials. Only a rough guideline, just to give the following some context.
The front springs on my XJ-S are nearly half way along the lower wishbone, the outer ball joint being a little inboard of the wheel. Roughly from the inner pivot, the spring centre is 135mm, the ball joint 270mm and the wheel centre 380mm. That means that the mechanical advantage the wheel has over the spring is 380/135=2.8. That in turn means that to get a good wheel rate the spring has to be 2.8 times stronger, and that’s a lot. These are not light springs, they are going to take some compressing to get in and out.
Both the standard V12 and the 3.6 Sports Pack springs seem to be the same rate, about 280lb/in which translates to a wheel rate of only 100lb/in, which is quite soft.
Eibach do an uprated item at 750lb/in which would give a wheel rate of 267, which is quite firm but not horrific.
The Sports Pack spring is, however, shorter. This lowers the car and thus the centre of graffiti. Or something like that. This should help reduce roll, a little bit. The Standard spring has a free length (length when there is no load on it, such as when its rolling down the driveway making a bit for freedom towards some children on bicycles, for instance) of 305mm (12ins) and the SP is 261mm (10.2ins).
I looked up several methods of changing the spring, the official method uses a special tool that goes up the centre of the spring and pulls it down to the lower spring pan which is then unbolted. There are a couple of issues with this including the height needed to get the tool in and the very seized spring pan bolts but my main concern was the rarity and expense of the special tool.
I did toy with the idea of making one but the advice from several forii was to use a different method.
I have subsequently learned that the JEC sometimes have these to hire out, which is a much better idea. Better, that is, than the idea I actually acted on.
There were two methods recommended to me, one was to put a jack under the spring pan, undo the bolts and lower the jack, they said this was the easiest method.
The other was to undo the top ball joint and bring the lower wishbone down as far as it will go, which is just past vertical so there needs to be a bit of space under the car. The method then says that the spring just comes out and the new one can be placed in, easy.
They lied!

Before
After
Doner

Here is the sad story.
Obviously I started by putting a jack under front sill to hold the car up then took the wheel off and put stands under the car. So far so good.

I put the trolley jack under the centre of the spring pan and raised it, unfortunately this does not take all the load off the spring pan! Not surprising really with a 280lb/in spring, even with the front of the car in the air I had to compress the spring about 4 or 5 inches, that’s about half a tonne. I ended up with the front of the car balanced on one jack, the spring pan was still at an angle and it occurred tome that getting it all back together this way would be a nightmare, so I abandoned that method. But not before I had snapped two bolts off!
On to method two, after a nice cup of tea, obviously.
The trolley jack went under the outer edge of the spring pan, this give enough leverage to raise the hub enough to get the top ball joint undone.
I cut the lock wire on the two calliper bolts, which looked suspiciously like electrical wire and not proper strength lock wire. There is a link from the steering rod end that uses the lower calliper bolt and another bolt at the middle of the hub so this bolt has to come off too, then I took the calliper off and tied it up so as not to strain the brake hose. The callipers are surprisingly heavy.
Next off is the anti-roll bar link, I wanted to inspect this anyway as it is renown for wear. As it turned out it was quite worn, the metal as rotted and the rubber bushes are permanently deformed. I feel the need for some polly bushes in the near future.
The top ball joint is all that is holding the hub up now, so I put the jack under the outer edge of the spring pan and take the weight off the top wish bone. The easiest way to undo this is to remove the two bolts holding the joint to the top wishbone. This assembly contains shims for setting the caster angle so I make sure to note where they are. The bolts come out relatively easily and I put a bungee round the top of the hub upright to stop it flopping over and straining the lower ball joint.
At this point is remove the bump stop in case it gets mangled when the spring pops out.
I can now easily remove the unwanted disc shields, these do a great job of protecting the discs and callipers from road grit etc, ever had a small stone stuck in a calliper? Scraping merrily down the road. Anyway, I am less worried about stones and more worried about getting cooling air to the disc, so the shields go in the bin.
Now I can lower the jack until the lower wishbone is nearly vertical. The spring is still firmly seated and a further few inches are required to get it out so in goes a small spring compressor, through the centre hole in the spring pan and on to the inner edge of the spring near the top. Squeezing this gives just enough clearance to ease the top coil out of the upper retaining thingy.
Splendid, time for more tea. After which I put the new spring on the lower spring pan and attach the spring clamp between second from top coil and lower spring pan such that it goes through the hole in the centre and it is clamping the inboard edge of the spring.
Wind it in and watch in stunned amazement as the spring bends inwards towards the upper retainer. Wonder at how this cheap spring compressor has wound in a 280lb/in spring nearly five inches, that equates to a force of about 1400lbs (636kg), the weight of a fiesta! If it lets go, lots of things will go bang and break so I am situated at the front of the vehicle keeping out of the way of any angry flying metal.
The compressor is now jammed tight against the spring pan so I cannot wind any further and am still about a cm off!
At this point it starts raining again so I go in for a nice cup of tea and a short scream.
Returning just as the sun is setting, I sit in a chilly puddle looking into the wheel arch. What a stupid hobby.
Then its time to refit everything which went quite remarkably smoothly apart from a couple of rusty bolts that needed replacing.
So now, with it back on four wheels, I have spent all day creating a car that leans to the right. I suppose tomorrow I will have to do the other side.

Well, the other side was much quicker because I was not mucking around trying different theories. It was all done in 5 hours, which is quite a long time to change a spring but not too bad when you consider I had to keep stopping to take pictures for the article. Well that’s my excuse anyway. I recon I could do it again and get both front springs changed in a day.
The new ride height looks better and gives a hub to arch distance of 344mm which is 40mm lower than before but still 10mm higher than the 3.6 when it had these springs on. Possibly because of tolerance between cars (subframe to wing distance) or maybe my car is a bit lighter than I thought.
As a silly aside, after removing the springs from the 3.6 I left it sitting on its bump stops. It only has 2 inches of ground clearance now but looks fantastic! For those who like data, you know who you are, the arch to hub distance on the bump stops is 297mm.
At thispoint itlooks like it might just work... The inner edge of the spring still has a few more inches to expand, oh dear

So, in conclusion; don’t do it this way, do it properly with the correct tool!