Rear spring rate Rover V8

I am a bit stuck here.
My B type chassis has the Jaguar IRS quad shock, spring setup.
I have no droop at all, It just sits fully extended. Driving the GT feels its way to stiff also as it seems the springs wont compress at bumps.,
jacking up the car or me just standing on top doesn't make a difference.

Removing 1 shock, each side lowers the car by 3/4 in

I can not find true date about spring rates used on the Jaguar XJ6 & 12 from the '70s - '80s. Not even my local Jaguar specialist can suply the data as Jaguar seems to lost them in the '80s.

I like to see a 1 3/16 droop to get the ride height correct and give me some rebound space.

This calculator with my weights and geometric numbers - -calculates I need 280lb springs per side, so thats in a quadruple setup 140lb / spring.

I like to see some numbers used on a Rover V8 powered GT40 to verifie before I start buying stuff.



Ian Anderson

Lifetime Supporter
DAX 350 front 600 rear.
but it all depends on angle the shock sits at, the fixing point at the hub….how close to centre of wheel etc.

I ran this set up with no anti roll bars front and rear!

Hi, stock Jag IRS springs are approx 250lb/in. While the rate is possibly high for your application, resetting the free height of the springs shorter so that they are only a few mm longer than the shock on full droop will allow you to properly gauge. Stock Jag spring height will likely have far too much preload for you. Cheers, Andrew
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Hi, stock Jag IRS springs are approx 250lb/in. While the rate is possibly high for your application, resetting the free height of the springs shorter so that they are only a few mm longer than the shock on full droop will allow you to properly gauge. Stock Jag spring height will likely have far too much preload for you. Cheers, Andrew
Well, I went to a Dutch automotive spring manufacturer to have the Jag spring weighted. It checked out 233Lb/in.
He suggested to go for 50% less rate and a progressive coil to get the droop & preload needed for my car.

I am now waiting on a quote from him.
I'm sure you've seen the info about ride frequency to suit the application. Assuming 120Lb/in is the right rate for your car, I'm guessing the current damper is going to be too stiff for the new springs.
I'm sure you've seen the info about ride frequency to suit the application. Assuming 120Lb/in is the right rate for your car, I'm guessing the current damper is going to be too stiff for the new springs.
You miss read or miss understand. I have a Jaguar IRS setup, twin shocks + springs each side.
Yes, 120lb, times two equals 240lb each side. (480lb total for the rear).
Thats how KVA did the B type chassis.
Jag IRS, inboard brakes with Jag handbrake, Jag hubs etc.

Yes indeed, twin gas filled (Boge) dampers are twice as stiff as one usual damper setup.

A Jag XJ6 or 12 is a hell of a lot heavier then my 1140kg KVA B.
KVA developed the B type and only three have been build as a prototype. I have one of those three, and probably the only survived.

Mike Pass

In order to calculate the correct spring rate you would need to know the corner weight (or use a typical GT40 value), the leverage ratio of the rear wishbones. The leverage ratio is the distance front inner pivot to outer pivot compared to distance from inner pivot to coil over attachment. Hard to tell from one pic but the angle of the coilovers is so close to vertical as to be unimportant. It would be useful to remove the springs and measure the fully open and closed lengths of the dampers and the gap between the top mount and the spring platform at normal ride height.
There are two issues to resolve. Estimating the spring length will be tricky if the shocks do not have adjustable platforms. If the dampers are not adjustable then achieving the correct damping rate will be impossible.
I would suggest two routes to the correct setup. Change the dampers to ones which have adjustable spring platforms and also adjustable damping rate. From the damper spacing info and corner weight and then end use the spring rate can be calculated. With two springs this would then be divided by two to get the individual spring rate. The stiffness setting would be trial and error to refine the ride required. In speccing the dampers the damping rate would be half the stiffness of a single damper setup.
Mike, its all I did.
Corner weight (actual) 705lb
Unsprung wieght 100
Dimension a 11.4in
Dimension B 14.9in
Spring angle 70'
4in shock travel where I want it to sit on 30% droop.

With the four 233lb jag springs each corner (466lb per wheel) the suspension sits full extended, no droop at all. It wont even compress with me standing on the engine.
The Jag springs have ½in preload while they sit in ther cup.
Removing one shock/spring combo each side lowers the car 3/4in (droop) and lets the suspension behave like a suspension should.
Doing .
220lb springs, one on each side lowers the car to 1in.

in my understanding, if a shock absorber sits fully extended, your suspension isnt working properly. If springs are to stiff you will loose grip and cause bad handling. As I have both issues related to each other causing also a 4x4 look.

Mike Pass

My rough calculation from your info using a suspension frequency of 1.5 (for sports cars) gives an overall spring rate of around 280 pounds. This equates to a spring rate of 140 pounds per inch for each of the two springs.
As your sprung weight is 705 pounds minus 100 pounds = 605 pounds - at your chosen ride height (use about 4.5 inches chassis to ground) the springs must support that 605 pounds. They must therefore be compressed by 605/280 inches = 2.16 inches at the normal rise height from the no load state. I don't know what your damper spacings are - at the correct ride height what is the dimension from the spring platform to the top mount? What is the length between the top and bottom spring platforms? From these dimensions what is the space above the ride height spring platform for the spring when compressed at rest. How much does this leave available for suspension droop?
It may be possible to calculate the spring lengths to suit the dampers you have and any necessary preload but this will still leave the issue of excessive damping which is non adjustable.
I suspect that the way to get the correct setup is to change the dampers to ones with adjustable spring platforms so you can fine tune the ride height and adjustable damping so you can fine tune the ride damping. If the suspension is too stiff it will move so little that the only effective suspension is the tyre itself which is a poorly damped spring - not good as the car will bounce around in avery unpleasant way!

Shocks are NOS Boge, gas filled, Jaguar aftermarket replacement. Some google shows they are recommended for a sporty ride by Jag enthusiasts.
They have quite a bit of pressure on them.
Still the ones when you saw our car at Adrean Beale. Not sure if Mick Ridley had any influence in the chassis before Adrian got it.
Of the record, the V5 paperwork you saw with the car, has now a Dutch registration.

Coilover platforms aint cheap. We have two of those already made by Gaz for the front end. (KVA B uses Jag rear up front also).
The Boge shocks have the same top tube diameter as Capri & Escort mk1-2 front legs. DIY coilover conversion kits are affordable available for those and I have experiance with those kits converting Capri legs to coilover platforms.

Just got a quote from the Dutch spring manufacturer. €65 each spring, 120lb with progressive windings custom made. If they sag to much after a year they wil re-shape them.
So they will be the strenght of one spring each side (what I have experimented), minus 1cm to get the stance, progressive so they get stiffer in bound
So I gave them a thumbs up as it worth to trie.
Its all digital, so if I need stiffer ones in the end, they just push a button.

Mike Pass

Probably not a bad idea to try the springs first. If the damping is excessive you can still use the springs on alternative coilers. If the damping is too much maybe just changing one damper and setting it to very soft may be enough and even with only one adjustable platform damper that would allow some ride height adjustability.
I use Protech shocks and Faulkner springs.

Howard Jones

My guess is this. The rear of the car will like pretty close to effective 350-pound rear springs on a streetcar gt40. Your setup places the springs straight up and down and therefore the effective spring rate is close to the actual spring rate. Angling the spring reduces the actual effective rate. Given that my strings on my GTD are angled about 30 degrees or so and they are currently 450s on the rear I would come up with approx a little less than 400. I set the car up for mostly track use so it is a bit stiff. If I was to change them I would more than likely go to 400 to arrive at about 350. That's my best guess on the rear of a GTD used on the street.

There is more to this than just spring angle but it looks like your setup is similar to a GTD as far as A-Arm length and shock mounting location on the A-Arm.

With effective total springs at 466 and you think one of them at 233 is too soft then my guess is you are about 100 pounds too stiff. Try two 175s on a pure streetcar and two 200s if you like a more aggressive track/street setup. I run 350s on the front with similar spring angles as the rear. So my effective string rates are about F305 R390 now. I think F275 and R350 (effective F250 R300) would be a good comfortable spring setup on my GTD for a pure streetcar.

I know that you don't have a GTD but I bet that total weights and other factors are close enough for GTD data to be useful if you take into consideration differences in powertrain weights between a Rover V8 and an aluminum head SBF. You should be able to come up with a pretty close estimate of what you need using the above GTD data. I have Renault R21 in my GTD by the way.

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Just a thought - before you commit to new springs, what is the free height of the current rear springs?

OE Jag shocks are usually 220-227mm platform to platform when fully extended. If you have the free height of the current springs reset to your fully extended shock platform height so that there is no preload on them but they just hold the platform retainers properly in place, you could find you get a significant reduction in ride height and a suspension in the working range.

If - for example - the spring free height is currently 20mm higher than the fully extended shock, doing this will drop ride height 40-60mm. I've found the Jag IRS gives a 2-3x reduction in ride height when you change spring free height. The stock jag springs are preloaded a couple of inches when installed into the OE shocks - far too much for use in lighter applications. You need to get the suspension off its locked position to properly assess if what you've currently got is actually too stiff.

Cheers, Andrew