Rear Suspension, ride height

[Series1 Stu]

I think the Jaguar quoted 3.125" of bump and rebound is at the wheel, therefore about half that for damper travel, which seems plausible.

Thanks for the measurements, Peter. I'll do some sketch layouts but can't get onto it until next week.

Take a look at the rear suspension illustration in the service manual. I'll lay odds that the original rendering was perfectly to scale and to design intent. The half shaft appears horizontal.

Regards

[PeterCrespin]

Had that drawing in mind when I posted Stu. Trouble is, I’ve driven too many miles behind many Es andin my mind’s eye I see only wishbones sloping down by varying degrees.

[Series1 Stu]

I understand.

Let's get that drawing done and see how it looks. All of the pivot points are fixed as are the lengths of the wishbones and half shafts so it should be easy enough to do.

Regards

[mgcjag]

Hi Stuart/Peter......What is your idea of the "Mid laden position".......Going by the Service manual it is not the position of the car when just stationary. .....The mid laden reference is made in the section for setting camber..where setting links are used to hold the rear hub in the "mid laden position" and measurements are then taken of the rear camber........the service manual says to Press down on the rear to fit the links...however in practice this is near impossible.....i had two large guys sit in the boot but the suspension was still not compressed enough to fit the links......... (i have standard springs/shocks).........So i removed the shocks and positioned the links....and set camber with shims as per manual.....after refitting and getting the car back on the ground and driven/bounced a bit i had very positive camber........I was advised to set camber with car just sitting with1/2 tank fuel ........so it now has slight neg camber......so beware the "mid laden position" .....Steve

[Series1 Stu]

Ok Steve but that is a sample size of 1.

We are dealing with design intent here and that doesn't necessarily have anything to do with when the car is sitting stationary, as you put it.

It's a pity that Jaguar don't quote spring rates.

Regard

[rfs1957]

Yes, Peter is correct, I was measuring to the centre of the wheel, forgetting the parallelogram, the multiplication factor is not 2 but around 1.50.

Nevertheless, if you fit the same springs, constrained by their seatings to the same fitted length and therefore preload, to shock absorbers with different eye-to-eye lengths, you will get different ride-heights - in a proportion of about 1.50 x the difference in length.

I fear that until the lengths of the shock-absorbers are measured this debate will end up going around in circles.

[PeterCrespin]

If I understand you right, you’re thinking like a competition motorcyclist. We’ve probably both come across race machinery so light that it is just about possible to preload the coil-overs so tightly that they remain fully extended until the rider climbs aboard. Under those circumstances the eye-to-eye length of the damper can be used.

For normal vehicles at rest, however, there is always some sag, even sans rider/driver and fluids. So the damper length fully extended is irrelevant. It is the compressed length of the spring plus the spring-to-eye distance at each end that make up the relevant length, which is smaller than the fully-extended length.

I agree with Steve’s real-world experience and would always check/set IRS camber with the car on its wheels and loaded in the manner it will mostly be driven.

[steve3.8]

I would say the drawing in the manual is period correct , a slight change would have occurred with updates during production to spring poundage ,adding packers then longer springs ,,increasing the angle of droop.

150lb was the rating of first springs.




Graham hills EDC 400 ran with 150lb springs



Edit - note input from wishbone bushes !, another reason to avoid poly bushes !

Looking at notes from last year my car [4.2 fhc] measures 7 1/2" to the diff plate lip , today I checked the angle of the wishbone angle @ 5 - 6 degrees from horizontal , with 1 1/4" showing between wheel rim and arch .
The factory drawing shows just under 4 degrees , so taking into account the spring changes all seems correct.
At a guess the last of the factory springs would be closer to 190 -200lb


Back to the thread's original question , i suppose the owner could have S3 springs ??

[PeterCrespin]

Unless the rear springs are only 75 lbs each (150/pair) the rear end maths is wrong.

[MarkRado]

I'd say its plain physics.
Springforce F=k.x, where k is a given spring parameter (ie 150 lb/in) and x the amount the spring is compressed.
The weight of the car at the rear must be in equilibrium with the springforce. Therefore ride height for a given car weight is only a function of spring rate and free lenght of the spring.
If the weight for a level height is given as 1200lb at the rear, the 4 springs have to carry 300lb each; with a spring rate of 150 lb/in they have to be compressed 2 in (50mm).
BUT the spring is mounted to the body via the damper, so there is an additional parameter: the lenghts between spring seats and damper mounting points. With height adjustable dampers such as Gaz you can do just that - change the lenght between spring seat and damper eyelet.

[Series1 Stu]

Unless the rear springs are only 75 lbs each (150/pair) the rear end maths is wrong.

[Heuer]

By the way I understand Boge shock absorbers are no longer available so we are left with the (unsuitable IMHO) Koni's.

[Gfhug]

David, something to discuss on 30th May?

Geoff

[MarkRado]

I apologize,
Johney already explained much better and in detail here:
http://forum.etypeuk.com/viewtopic.php? ... ght#p50018

Measurement on my car to bottom of tie plate lip gave 20,8cm (8" 3/16?),
but that is definitely too high, since I installed distance pieces under the springs to avoid grounding on a long trip with luggage, passenger and tools on board.
I recon my car to be about 2 cm too high, which would be about 7 ¼” or 185 mm (as stated by Steve mgcjag) without the spacers.

[Heuer]

David, something to discuss on 30th May?

Geoff

Yes, most certainly.

[mgcjag]

Hi All...lots of info here already....I fear we may have scared off the original poster....So to pull this right back for the O/P..this is what we know from him....since fitting new shocks/springs his rear ride height is approx 2in too high....his shocks are GAZ adjustable height......we dont know what springs he has.....he dosnt have his originals as they have been discarded......If your still with us Mark have you sorted the problem yet....all the best...Steve

[Geoff Green]

Interesting discussion. All jaguars with IRS like the E-Type show drawings with the axles horizontal which seems correct as I remember all the cars I worked on. Most of the cars were driven with light loads of one driver and 1/2 tank of fuel on average. A couple of things to add to the computations and measurements: tyre diameter, actually radius will change height as will air pressure mentioned earlier. While not affecting rear height measurement the front height setting will possibly cause setting to be changed on the rear.

Concours next weekend so I will look under the cars.

note: all Jaguars have horizontal door sills front to rear with one exception of the XJR-S due to different tyre size front smaller than the rear causing the feeling of falling forward out of the seat until you get used to it.

[andrewh]

I would love to find the solution to this. I bought the CMC springs and fitted them to the Konis when I restored my 3.8 FHC. However, I believe the car to be about 3/4 of an inch too high at the rear and this has the effect of making the front look low ( I am very peculiar, sorry I mean particular) . The ride height at the rear only looks right when the tank is totally full. Its one of those things that just nags away at me and I would dearly love to change to be correct. If one looks at the period Jaguar photos then the CMC ride height is my opinion too high. I will measure the distance when I get it out of the garage next.

[mgcjag]

Hi Andrew...following on from what i posted above and after talking to SNG the Gaz height adjustable shocks that they sell fitted with the specific springs they have for those shocks give a very adgustable height range....whats even better is they actually have them fitted to their demo car 50 EE.....they currently have the rear height ar 7.25 in from lower diff plate to ground with standard profile tyres.....unlike other suppliers who say their shocks/springs fit SNG actually have them on their own car for anyone to look at..Steve

[andrewh]

Thanks Steve. Yes I am aware of the adjustable suspension set up but its a long way from a standard look which is how I built my car. I guess it may be the only answer to get the desired result at the moment though. I shall give that some more thought. Thanks again.