IRS Springs

[StuBaker]

I'm just fitting my new diff so I thought I'd address the ride height problem I have - it's too high.
I have the as supplied Gaz shocks with 8" 400lb springs.
The shocks have 9" travel so at 'correct' ride height the springs rattle off the locating rings. (I've tried helper springs, not convinced)
So the spring adjusters are set high to take out the slack.
They compress to 6-3/8", so loaded by 650lb.
So I reckon a 9" spring is required.
To compress to 6-3/8" @ 650lb load requires a 250lb spring.
Seems a lot different but original E-Types were ~10" springs @ around 260lb so I don't think I'm too far out.
And the spring locators were further apart hence longer spring?
As usual I've scoured the forum for some firm numbers but once again my 'man look' has failed to find any.
Does anyone have detailed experience of the specs?

[peterc]

Sorry I do not have the length or poundage of my rear springs. They were supplied by Ward’s to Gerry’s spec. I purchased a complete assy ready to bolt in.
When the car is jacked up and the suspension is at full extension, the helper springs should be holding the main spring in position. I have not had any issues over all the years since fitting the flimsy helper springs. Certainly no comments from the MOT boys.
Don’t forget that the Hawk IRS has been made narrower than an E Type so spring rates would be different.
Peter C

[Migge]

I'm not 100% sure but the E-Type is the narrowest version you can get and that will fit any replica without modification.

[peterc]

I bow to your better knowledge Migge but why do they always say that the axle has been narrowed to suit the Hawk ( and others) .
Is it merely that there are more others available for adjusting rather than specifically sourcing an E type unit.
Peter C

[simonjrwinter]

Interesting…..I was told by someone the Hawk had a custom length wishbone, is this not the case?

[StuBaker]

Possibly Simon, but the intent really is a more general comparison of spring rate v lower link length.
So if the Etype is similar to the Hawk, why such a difference in spring rate?
BUT, I'd really like a spring that fits and allows the correct ride height set up.
As I mentioned earlier I have failed to find any info on the forum so was just asking for more precise numbers.
Most posts just say they are happy with theirs etc. All my parts are from Hawk too.
I understand not everyone cares about detail so long as the car drives well.
Cheers,
StuB

[simonjrwinter]

How much of a part does the weight of the vehicle play in determining this. I would imagine the Hawk is lighter than an E type….

[Migge]

The main reason for just not mounting the E-Type wishbones? They are still not available. So 99% of all Jaguar wishbones that found a way in a Cobra replica are narrowed XJ6/12 wishbones. The springrate depends on the pickup points of the coilovers at the wishbone and the chassis. The more verical the coilover is mounted the rate is less against a coilover e.g. TVR Griffith approx. 45 °. Sorry, can't post any pics.

[Michaels Garage]

I’ve been trying to sort the same issue with mine.
I have the hawk shocks and springs but set as low as it goes the springs are too short when the cars jacked up.

What was the issue you had with helper springs? As this was something I was considering.

[StuBaker]

As far as weight goes, the 289 is about 1000kg whereas the E is around 1500kg. +/-
I'm still trying to compare actual spring locating positions on the E with the 289 but not managed to do yet full scale.
As for helper springs, I took them off after IVA as I felt the car was still too high. And I didn't like using them. It reminded me of when I had blocks on my bike pedals when I was a kid.
When lowered, the camber was off by over 1.0 degree so I raised the back up again. Visually too high and also the drive shaft is no longer horizontal at the raised level.
I have an E type workshop manual I use for guidance when rebuilding the IRS and the info seems different to what I have in physically.
I'm sorting the camber now, whilst swapping the diff and thought I'd try to sort the spring thing out too.
I'm looking for objective support, or otherwise, for what I'm trying to achieve to try and avoid spending on springs for trial and error.
My calculations for a 9" spring give me 250 lb spring where the 8" 400 lb is currently fitted. It seems a long way off.
I.E. the 400 is compressed to 6-3/8" (by 1-5/8") = 400 lb x 1.624" =650 lb.
For 9" at 6-3/8" compressed 2-5/8". 650 / 2.625 = 247 lb - which is somewhere near E type numbers...
Once I have it all together I'll do some more measuring, should be next week. Just waiting for some shims and bolts.
The springs are easy to change even with the IRS mounted so not too worried.
The only other thing it may be is that the rear is over stiffened to compensate for understeer at the front with MGB setup but things get a bit more serious when trying to understand all that.
Just checked:
E Type lower is 20.469" (52cm), 289 ( Hawk) is 15.75" (40cm) centre to centre
Lower spring location is 5.937" (15.08cm) inboard of outer pivot on both,