Hi Everyone,

By way of introduction, I've been a Jaguar owner for many years and still drive the one I bought in 1985 when about ten months old (the car, not me) - I guess you could say I know it well

Maybe not well enough, however. as I have recently refurbished the suspension (front and rear) as it was getting very tired - saggy with loose ball joints (sound familiar?). I decided to reuse the original springs because that's the kind of guy I am and needed to pack the front ones and change the seating position on the rears

Although I did many careful calculations, the ride height is about 27mm too high at the rear now. I think the problem is probably one of two things:

1. Incorrect calculation of the ratio between ride height and spring seat height. I calculated this at close enough to 1.25:1 (meaning a 10mm difference in spring seat height results in a 12.5mm change in ride height) Does anyone have an opinion on this?
2. *After* the rear shocks were removed I realised the knuckles fitted would make a difference to the measurements I had previously taken. (For those unfamiliar with knuckles, they are neoprene inserts that go between two coils and stiffen the spring somewhat) The fellow setting up the new Koni shocks (who is very competent) did a calculation to allow for the removal of the knuckle, but I always thought he was over-estimating its effect

The suspension man is yet to do a final alignment including shimming for castor and camber and I'm hoping he will drop the spring seats for me, but I'd like to be sure of my facts before I ask

So, does anyone know that exact ratio and is anyone familiar with the the change in ride height that is achieved when the neoprene knuckles are used?

(By the way, the front was pretty successful - now about 7.5mm too low which is OK - in fact if we can lower the rear by 27mm that should lift the front by roughly that much, funnily enough)

Thanks in advance for any help

Kind regards

Keith

 

Keith
What did you actually change at the rear from what was there prior to your rebuild? Just the new shocks or what?
Secondly, the ratio of change on spring perch height to ride height measured at the wheel to wheelarch gap, is damn near 1:1. Placing inserts (packing pieces) under or above the rear spring (analogously to the ride height adjustment at the front) will directly affect the measurement as the spring itself will always extend to the same length for a given load.
I am pretty sure therefore, assuming new rear shocks and removing any spring perch packing pieces, that your problem is caused by the fact that the Koni shocks' lengths outside the sprung length are different from the OEM shocks you replaced. In other words, the length of the new shocks above and below the spring seats (the distance from the spring seats to the eye fixings) is longer than the originals ones.
This fault is very common with aftermarket shocks a,d the best remedy is to fit Gaz or AVO adjustable height shocks.

 

Hi Greg

Thanks for taking the trouble to respond and sorry about the length of this but it takes a bit of explaining

You ask what has actually changed at the rear. In fundamental terms, what has changed is that the distance between the lower shock mount(s) and the base of the spring(s) has been increased to raise the car using the original worn springs

I have fitted new Koni Classic shocks front and back which were an expensive option but worth it for the performance - the car now handles significantly better than it previously has with new shocks

Whilst they are, technically, aftermarket shocks, they were designed and engineered for the Series III, have the same geometry and use the same original spring and seat. They have been around for a long time and have a strong following - note they have adjustable rebound damping and can be rebuilt

The collets for the spring seat sit on a circlip which fits into a groove machined into the shock wall. There are three of these grooves and the suspension man machined two more to facilitate our calculations. The incorrect height was not a fault with the shock but a fault with our measuring/calculation. The seat can be lowered but at this stage I don't know by how much and it takes time. I'd like to get it right and also learn why it was wrong

That ratio (spring seat to ride height) is definitely more than 1:1 - we've already proven that as the ride height has increased by 43mm - more if you consider we removed the neoprene insert (knuckle) - and we didn't raise the spring height by anything like that

The shocks are mounted well inboard so any up and down movement at the shock is multiplied at the wheel. There is one limiting factor to this and that is that the shock is not perpendicular to the axle (about 61°) so that reduces it again

My calculations suggested 1.41 for the lever effect and 0.89 for the 61° resulting in a factor of about 1.25

So again I'm sorry about the length of this post, but still chasing any concrete info on that ratio or an opinion on how much effect that knuckle might have had

Thanks again
Keith

 

Keith
What i meant was, something you have done must be different if the ride height has changed. If the only thing that has changed is the position of the spring perch, then use another of your available ones. It is not that hard to change the position if your guy has a suitable spring compressor. One or at the most two goes should do it.