[racing] XJS spring specs?

Hello all, does anyone have the front and rear specifications for
the springs on the XJS V12. Factory or aftermarket lowered will do.
The reason I ask is, I have found a supplier with tons of spring
types, rates at good price. I just dont want to pull them off the
car to get the info.

Rate, I.D. and or O.D and free height will get me started. I’m
looking at possibly building a track car out of my parts
car…Any help or suggestions will be great.–
Travis Harper
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Hi, as follows:

stock rear:

wire diameter (WD) 11.5mm
total number of turns (T)(including the flat ground top and bottom coils with the zero pitch gap) 8.2
outside diameter (OD) 97mm
spring rate (SR) 250 lbs/in
free height (FH) not sure

heavy duty rear:

WD 13mm
T 7.5
OD 99mm
SR 428 lbs/in
FH 205mm (sits about 1.5 - 2 inches lower than stock)

stock V12 front:

WD 17.7mm
T 6.1
OD 157mm
SR 466 lbs/in
FH not sure

heavy duty front (civilised fast road spring):

WD 19mm
T 5.75
OD 160mm
SR 652 lbs/in
FH 245mm (sits about 1.5 - 2 inches lower than stock)

super duty front (hard core fast road and track day):

WD 20mm
T 5.75
OD 162mm
SR 800 lbs/in
FH 230mm (sits about 1.5 - 2 inches lower than stock)

be careful with free height - very hard to preduct exactly where ride height will end up car to car. Often requires fine tuning with packers or even getting the spring reset to a new free height.

also, total number of turns assumes that both the top and bottom coils have a zero picth gap and consequently provide no effective contribution to spring rate, and are flat ground for spring seating.

I currently run the SD front, HD rear spring, 27mm front bar and 16mm back bar in my tarmac rally and circuit car, which has about the same weight on each end as an early stock V12 XJS, and it is pretty firm but not bone-shaking.

Hope this helps,

Andrew Robertson, New Zealand
Xj S1 454TT, Xj S1 383

From: “Travis Harper” xjs53v12@yahoo.com
Date: 2006/02/17 Fri PM 02:29:32 GMT+13:00
To: racing@jag-lovers.org
Subject: [racing] XJS spring specs???

Hello all, does anyone have the front and rear specifications for
the springs on the XJS V12.

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In reply to a message from kinrob@xtra.co.nz sent Fri 17 Feb 2006:

If anyone needs springs, I can supply front or rear in just about
any spec you want. Fronts are $350 A SET. Rears are $140.00 A PAIR.
Chadbourn Bolles
803 798 3044–
Dr. Chadbourn Bolles
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In reply to a message from Dr. Chadbourn Bolles sent Sat 18 Feb 2006:

Thanks for the replies, I’ll get to researching this weekend and
post what I find. Thanks again.–
86 XJS
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In reply to a message from Travis Harper sent Sat 18 Feb 2006:

Hello Chad, just wondering who manufactures the springs you have
for sale? I’m lookin to lower the car and improve handling. Maybe
one day hit the track. Heres how it got started… I have a buddy
who is a hotrod builder and I was loving the set of Carrera coil-
over set he was installing on a ‘34 Chevy pick-up. Shortly, he
gave me a www.speedwaymotors.com (dirt track and hotrod) catalog
and it has tons of cool stuff and good prices, even some Jag IRS
stuff. In it there are Carrera, Hyperco and Tru-coil springs in
various rates, but not the right diameter. There all smaller. The
closest for the front is 5.5’’ dia. The rears would be a 2.5’’ dia.
They range from $40 to $50 each, so $300 would respring the car.
Taking into account you would need new shocks, would a retrofit be
worth it? These guys dont have the ‘‘right’’ size springs. Any
thoughts?–
86 XJS
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IMO for a dual purpose street and track day car leave the basic suspension configuration as is and (after ensuring balljoints and A arm bushes etc are all sound) focus on installing better shocks designed specifically for the Jag application (like adjustable konis), higher rate springs of the right physical size, possibly bigger anti-roll bar(s), and then consider getting the best out of that investment by solidly mounting the front subframe, and installing one of the many mods that better controls rear end dynamic toe under accel/braking, if you don’t mind an increase in NVH.

Cheers, Andrew Robertson, New Zealand

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In reply to a message from kinrob@xtra.co.nz sent Tue 21 Feb 2006:

OK, Andrew, what is NVH?–
The original message included these comments:

IMO for a dual purpose street and track day car leave the basic suspension configuration as is and (after ensuring balljoints and A arm bushes etc are all sound) focus on installing better shocks designed specifically for the Jag application (like adjustable konis), higher rate springs of the right physical size, possibly bigger anti-roll bar(s), and then consider getting the best out of that investment by solidly mounting the front subframe, and installing one of the many mods that better controls rear end dynamic toe under accel/braking, if you don’t mind an increase in NVH.


Mike, 1990 5.3 XJS Convertible, ‘Caterwaul’
Lakewood, OH, United States
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In reply to a message from Travis Harper sent Tue 21 Feb 2006:

Travis, I can supply rear springs in weights from 100 up to 700 lb
per in.
The front spring on the Jag is 5 7/8s ID and 6 3/16s OD. These are
custom made springs to your specs. They can be made to lower the
car or keep it right where it is, your choice.
I also can suppy front and rear sway bars. These bars with the
stock springs make one heck of a difference in the way the car
handles, I know as I have them on my car.
Chadbourn Bolles
803 798 3044–
Dr. Chadbourn Bolles
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In reply to a message from Travis Harper sent Tue 21 Feb 2006:

Travis, I forgot to say, I can also supply front subframe solid
mounts. I have these in my car and what a difference in the
steering.
Chadbourn Bolles
803 798 3044–
Dr. Chadbourn Bolles
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Noise, Vibration, Harshness - a general reduction in refinement.

From: “mike90” mcgawma@yahoo.com

OK, Andrew, what is NVH?

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In reply to a message from Travis Harper sent Fri 17 Feb 2006:

FWIW, I installed adjustable (Spaz(?) not Spax)shocks that lowered
the rear end. Went with higher rate springs and ditched the rear
anti sway bar. Gone it is. This helps keep the rear tires planted.

I’ve gone to Delrin bushings on the radius arms at the rear and
Polyurethane on the large horizontal ones.

I can try to get the spring rate for you, but it seems everyone has
an opinion so you may not need mine.

Charles
88 V12 Track
91 V12 Street–
Charles
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In reply to a message from Charles sent Wed 22 Feb 2006:

Charles:

What’s the story on the stiffer rear springs: are you using this
setup with the OEM springs in front, or have you uprated those
springs, too?

The reason I ask: if changing the rear spring rate alone is
sufficient to obviate the need for an anti-roll bar, this may be
preferable for many to contemplate. The potential downside is: is
the car still reasonably balanced with stiffer than OEM springs in
back, combined with OEM springs in front?

Thanks,

Mike–
The original message included these comments:

FWIW, I installed adjustable (Spaz(?) not Spax)shocks that lowered
the rear end. Went with higher rate springs and ditched the rear
anti sway bar. Gone it is. This helps keep the rear tires planted.


Mike, 1990 5.3 XJS Convertible, ‘Caterwaul’
Lakewood, OH, United States
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In reply to a message from Charles sent Wed 22 Feb 2006:

Charles, why would you remove the rear bar? I have seen F1, NASCAR,
IRL, CART, and just about every other race car use it.
Chadbourn Bolles
803 798 3044–
Dr. Chadbourn Bolles
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In reply to a message from Dr. Chadbourn Bolles sent Wed 22 Feb 2006:

Mike and Chad,

My car was set up last my Brian Donovan at Donovan Motorsports in
CT. He is a tuner of very fast E-Types, works with Jaguar on the
Select Edition promotional tours and races all over the US.

The way it was explained to me was that if the car is too stiff in
the rear, as it tries to make the turn, the inside wheel will
actually start to lift. Since the rear of the car is so much ligher
than the front, this leads to major oversteer. Before Brian
confirmed this with me, Bryan Maletsky at Motorcraft in NJ told me
this. He used to wrench for F1 Ferrari years ago. I disconnected my
rear bar at Lime Rock one afternoon with Bryan’s adivce, and help,
and picked up a second or two on my lap times. I have never been
faster as I am now with the new rear set up. Lime Rock is a fairly
short track with 9 turns, lots of work for a big car, made for
small cars. I’m down to 1:06 and my engine is stock, not bad I
think. Watkins Glen is better suited for a big car, and my times
have improved there too, with no engine work over the past 2 years.

I assume for street, a rear bar is fine. There is not nearly the
lateral force on the car as in a road track scenerio. I have not
experienced any mishandling with the stiffer rear springs and with
the anti-sway bar removed. Of course this may also mean I’m getting
better and used to the car this way.

And Chad, to answer your question: the Jaguar rear end is not an
F1, IRL, CART, NASCAR kind of thing. I think it pretty unique. This
rear end is much different than any other car I have seen on the
track.

Charles–
Charles
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Charles, good on you for developing the handling of the car to a new level.

Springs and roll bars dictate where the lateral weight transfer goes when cornering, and to generalise, roll bars are tuning tools used to “assign” that weight transfer between front and rear to maximise the contribution of each tyre contact patch. A front bar couples weight transfer to the outside front wheel while a rear bar couples it to the outside rear. The relative sizes dictate how much goes where. The addition of (or an increase in the size of) a rear bar will reduce an existing understeer tendency or exacerbate an existing overtseer condition. A rear bar is often not a useful addition to a car that is already neutral - your experience bears this out.

My car tends to understeer so removing my rear bar would make that worse - every car is different.

Most rear wheel drive cars with a bit of power are theoretically best set up with a bit of steady state understeer so the rear tyres have enough residual grip to provide accelerative effort out of turns - the objective being a fairly neutral car under power.

Cheers, Andrew Robertson, New Zealand

From: “Charles” maraia@att.net
I disconnected my
rear bar at Lime Rock one afternoon with Bryan’s adivce, >and help, and picked up a second or two on my lap times. >I have never been faster as I am now

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In reply to a message from kinrob@xtra.co.nz sent Thu 23 Feb 2006:

Andrew:

Thanks for the explanation- I have been trying to understand this,
as, my reading on XJS-R’s would seem to indicate that the later -
R’s did not use rear bars, but did use higher rate springs in the
rear (and the front).

My question had to do with the consequences of taking a stock XJS,
and simply changing the rear springs to a bit stiffer set. Would
this a) remove the need for a rear bar, and b) would it upset the
balance of the car (I would expect spring rate choices are made in
the context of both front and rear). I suppose if the answer to a)
is that it would obviate the need for a rear bar, the answer to b)
probably is that the car is better balanced.

It would seem that the results that Charles found are maybe
specific to his car? IIRC, this car was lowered considerably in
front, but uses stock XJS springs in front, am I recalling this
correctly, Charles? But the rear uses higher rate springs, and no
bar. I don’t recall if Charles has stated what front bar he is
using.

Thanks,

Mike–
The original message included these comments:

Charles, good on you for developing the handling of the car to a new level.
Springs and roll bars dictate where the lateral weight transfer goes when cornering, and to generalise, roll bars are tuning tools used to ‘‘assign’’ that weight transfer between front and rear to maximise the contribution of each tyre contact patch. A front bar couples weight transfer to the outside front wheel while a rear bar couples it to the outside rear. The relative sizes dictate how much goes where. The addition of (or an increase in the size of) a rear bar will reduce an existing understeer tendency or exacerbate an existing overtseer condition. A rear bar is often not a useful addition to a car that is already neutral - your experience bears this out.
My car tends to understeer so removing my rear bar would make that worse - every car is different.


Mike, 1990 5.3 XJS Convertible, ‘Caterwaul’
Lakewood, OH, United States
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Mike,

The theoretical answer to your question is yes - stiffer rear springs alone
have a similar impact on roll couple as does the addition of a rear bar,
although as a generalisation, it is probably a more usual approach to do
springs and shocks first, using a combo that is pretty balanced front and
rear and gives the desired compromise between control and comfort (going up
30% both front and rear has worked well for me on my street Jag), and then
tuning out any residual understeer/oversteer bias by fine-tuning with bars.

Cheers, Andrew

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In reply to a message from mike90 sent Thu 23 Feb 2006:

In going back to the work orders, I have 200lbs per inch
springs in the rear. GAZ adjustable shocks that adjust the
ride height. I think the setting are pretty soft.

I have a front bar and I believe it is the 7/8’’??? But I
can’t remember…sorry. It may be the stock one and I now have
poly bushings. The rear bar was one bought form Keisler, which
was a mistake and I sold.

Hope this all helps. Andrew seems to have it down to a science
.

Charles–
Charles
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Hi Charles,

stock rear springs are around 250lbs, so your set-up must be very soft at the rear. I run around 430lbs in my road car. The front bar sounds like (or similar to) a stock v12 peice. As an aside, roll bar stiffness increases to the 4th power of change in diameter, so small changes make a big difference.

Cheers, Andrew

From: “Charles” maraia@att.net
In going back to the work orders, I have 200lbs per inch
springs in the rear.
I have a front bar and I believe it is the 7/8’’???

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In reply to a message from Charles sent Thu 23 Feb 2006:

You should have never installed the Keisler rear bar with the stock
front bar. If anyghing you should have used a 1 in bar in the
front then maybe the Keisler in the rear.
Chadbourn Bolles
803 798 3044–
Dr. Chadbourn Bolles
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