Club Cobra - jag rear spring rate

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jag rear spring rate

Could anyone pass along rough idea of what your using for rear spring rates for Jag independent rear in cobra replica with 351W engine for primarily street use? THANKS

Dear (Please people, put a name in your message!) Whoever,

I went through a lot of trial and error with Jag springs. The XKE springs are way too soft. The XJ6 springs are a little too soft, but I made a bracket at the top with several holes so I could adjust the angle of the springs. With KYB coilovers, I could not get a good compromise, they were too soft (with a full 22 gallon tank and two people) or too firm (with a low tank and just me). So, I bought some stiffer springs from Butler racing (805-649-6000). I'm now running two XJ6 springs and two Butler springs and it's about right. Sometimes, with a full tank, lots of rally equipment and two big guys, we scrape bottom on hard dips at speed. Sometimes, with a low tank and just me, it feels a little rough. But the overall compromise is good most of the time. If I were going to spend more time on the track, I would put four of the Butler springs on the car. My original MGB front springs just wore out and drooped on one corner. The new ones started off a little high, but have sagged now to just the right point (were not designed for a V8 car). Should be good for a couple more years.

Paul

FWIW, spring rates on the rear of my car are 275lbs each (two coilovers per side).

Recommendations for spring rates are probably best sourced from your kit supplier - they will know what works best for that specific make.

hth

Hi jdeuel87,

You might want to start by calling Unique :o

Reason I say that is the springs depend mightily on the locations of the mounting points, both on the frame (kit builder does this) and the wishbones at the lower end.

Wishbones are commonly used in different widths: XKE which are the narrowest, the 3.8S/420 which are a bit wider, and custom! Some folks shorten the XJ6/XJS units (must be done correctly %/ or can be disastrous) or even make custom wishbones, e.g. Petrus or Concours West.

Where this all matters in spring selection is in considering the angle that the shocks make with the vertical - all the above elements can change the mounting angle and length of the shocks, which in turn will affect the how the stiffness of the springs is transmitted to the wishbones.

On the Everett-Morrison cars, generally a 300 lbs / inch spring (all four) is about average for their production lately. I'm using a set of 350 lbs / inch springs and am quite happy with them, using a 3.8S set of wishbones.

The ERA web site also probably has a good writeup on this too (have to admit I didn't look, but their manual is quite good :LOL: ).

In your shoes, I'd start with Unique's recommendation and tailor from there as you see fit!

Tom

jdeuel87, Tom is right on the angle mounts, the more angle you have the more spring rate you will need. I change the upper mounting points on my jag rear coil overs and ended up with 27 degrees, after going through three sets of springs, 4/600 lb springs worked for me.
I highly recommend 'PRO SHOCKS', they swapped springs each time at no charge while I worked this out.

http://www.proshocks.com/