Overview

The stock front springs for our 1972 are no longer available.

Stock XJ6 Series 1 front spring RTC0661 for the 4.2 engine

 

XJ6 4.2L stock spring analysis

The starting point for spring compression analysis is getting the right vehicle weights; official Jaguar records leave something to be desired since they were uncomfortably vague about curb weights and weight distribution in the early 1970s. Still, we can recreate the data we need.

1972 XJ6 Series 1 (4.2L) Weight Data (lbs)
Description Vehicle Front lbs-% Rear lbs-% Comment
Dry weight 3527 1956 – 55.5% 1571 – 44.5% Official Jaguar specification
Fluids 177 7 – 1% 170 – 99% 27.8 gallons gas @ 6.1 lbs/gal
Curb weight 3704 1963 – 53% 1741 – 47% Jaguar S3 spec; no driver
Payload 1058 450 – 43% 608 – 57% 2 front @ 225; luggage 83; 3 rear
GVWR 4762 2413 – 51% 2349 – 49% estimated

The per spring minimum load is therefore 982 lbs without any passengers or luggage, and 1207 lbs fully loaded up.

Our 1972 Series 1 was equipped with RTC0661 front springs which are no longer available. Using basic RTC0661 spring parameters – wire diameter, number of coils, free length, and outer diameter – overall compression can be estimated.

RTC0661 Spring Compression Analysis
Spring Scenario Spring Length Compression Comment
Open spring 12.50" 0%
Loaded spring 10.80" 14% front suspension housing
Fully compressed 3.95" 100% solid spring
Curb weight 982 lbs 8.25" 50% factory specification
GVWR 1207 lbs 8.83" 57% 1.75" lower chassis height

Here is the complete physical analysis of the stock Jaguar 1972 XJ6 spring (RTC0661).

XJ6 stock spring for 4.2 engine

The stock front springs must be modified for two reasons: 1) the LS3 engine is about 150 pounds lighter than the XK inline 6 block and 2) the XJ6 attitude tends to be rather nose-high while a lower front end looks more contemporary and generally improves handling.

Other Jaguar XJ springs

In addition to the springs supplied for our 1972 XJ6 with air conditioning and the larger 4.2L engine, Jaguar provided a range of spring rates matched to different engine weights.

Jaguar Springs Rate vs. Engine Curb Weight
Jaguar Model Part Number Spring Rate Engine Weight
XJ6 3.4L engine no A/C RTC2754 360 lbs/in 576 lbs
XJ6 3.8L engine no A/C RTC2754 360 lbs/in 592 lbs
XJ6 4.2L engine with A/C RTC0661 and RTC2751 385 lbs/in 605 lbs
XJ12 engine RTC2753 420 lbs/in 680 lbs

The relationship appears to be not quite linear.

 

Restomod weight changes

The XJ6 restomod substitutes the Chevy LS3 engine for the Jaguar XK6 with a weight reduction of at least 145 lbs. In addition, the dual side steel fuel tanks will be replaced with a single aluminum tank located in the spare tire hold. The new tank holds 5.5 fewer gallons – a 35 pound saving – and eliminates the spare tire as well. Altogether, the XJ6 restomod curb weight should be about 200 lbs lighter than the stock version.

1972 XJ6 RESTOMOD Weight Data (lbs)
Description Vehicle Front lbs-% Rear lbs-% Comment
Dry weight 3362 1811 – 53.9% 1551 – 46.1% 145 engine; 20 tire
Fluids 142 7 – 1% 135 – 99% 5.7 less gallons @ 6.1 lbs/gal
Curb weight 3504 1818 – 52% 1686 – 48% no driver
Payload 1058 450 – 43% 608 – 57% 2 front @ 225; luggage 83; 3 rear
GVWR 4562 2268 – 49.7% 2294 – 50.3% estimated

For the XJ6 restomod, the per spring minimum load is therefore 909 lbs without any passengers or luggage, and 1134 lbs fully loaded up. Since the restomod puts 73 less pounds on each spring and the stock spring rate is 385 lbs/inch, we can expect that if the same stock springs were re-installed, they were be less compressed by about 0.19″.

How much would this difference impact the ride height? The front suspension geometry is complicated, but the change in spring length can be estimate from the 0.125″ packing rings (Jaguar P/N C41271) that raise or lower the by about 0.375″, or a 3-to-1 ratio. Thus, installing the same springs would lift the front of the XJ6 by about 1/2″. (Note also that the 3-to-1 ratio implies that a fully loaded XJ6 sits about 1.75″ lower than a curb weight without any passengers or luggage.)

New spring parameters

To compensate for the estimated 1/2″ increase due to the lighter LS3 engine and – at the same time – lower the front by 1″ (or a total of 1.5″ lower), the compressed spring height at curb weight should be 0.5″ lower, or 7.75″ instead of 8.25″. In summary, we want a spring that compresses from 10.80″ at it loaded position in the front suspension down to 7.75″, or 3.05″, under a curb weight of 909 lbs. Thus, the implied spring rate requirement is 909/3.05 or 298 in/lbs. Under GVWR of 1134 the spring would compress a further 0.75″ dropping the car down another 2.26″.

The drop of 2.26″ is perhaps too extreme, so a slight higher spring rate of 310 in/lbs might be more prudent.

Spring modification alternatives

There are three primary ways to modify the front springs:

  1. acquire new aftermarket substitutes from either King Springs (Australia direct from factory) or Eibach (SNG Barrett distribution)
  2. shorten the original stock springs
  3. design and fabricate custom springs

Aftermarket springs from King Springs (Australia) and Eibach claim that they lower the XJ6 front by about an inch with the original engine, but these manufacturers do not provide any detailed specifications.

King Springs

King manufacturers three different springs for the Jaguar XJ6 (all series and years) for standard height, 30mm lowered height, and lowered height with a Chevy 350 engine (which at 575 lbs is still heavier than the LS3).

The latter spring, KJFL-05, is the closest to our desired parameters. The net ride height change with the KJFL-05 would be about 3/4″ lower as shown below (note that calculation below is approximation since the spring rate of the King spring is not known and assumed to be the same as the stock spring).
lowered 1.18" with 575 lb Chevy 350 @ 575 lbs vs Jaguar XK @ 605 lbs
raised by 58 lbs [460 lb LS3 engine weight vs 350]/430 spring rate = 0.135" x 3 = 0.4"
net change -1.18 + 0.4 = 0.78"

Eibach

The Eibach SBS1500 lowers the front by 25mm but doesn’t adjust for engine weight, so the net change would be 1/2″ as shown below (note spring rate assumed to be the same as stock spring).
lowered 1.0" with stock Jaguar XK @ 605 lbs
raised by 73 lbs [360 lb LS3 engine vs Jaguar XK]/430 spring rate = 0.170" x 3 = 0.5"
net change -1.0 + 0.5 = 0.50"

Shorten stock spring

There are three basic coil spring designs:

  1. Tangential – the end of the coil continues a spiral shape and cannot stand upright on its end
  2. Square – the last coil is bent/ground down to touch the coil below and the spring stands upright on its end
  3. Pigtail – the last coil is square but with a smaller diameter than the coils in the middle

Basic spring types

In general, only tangential springs can be cut down without re-shaping.

The Jaguar front suspension uses square springs that are engineered to make full contact with the mounting points. To reduce the length, the spring must be re-shaped after cutting to maintain the 360º contact. However, in order to re-shape the steel the spring must be heated or ground. Excessive heat over 400℉ may cause the steel to soften (anneal), thus changing the spring rate and weakening the structure.

Cutting down the spring changes the length (obviously), but also alters the number of active coils since the geometry of the spiral continues shifts the ending point of the spring. The stock spring is exactly symmetrical with 6 total coils and 4 active ones; active coils are counted after complete square end coils. Cutting the spring down without reshaping it would reduce the coil number by a fraction, thus impacting the spring rate (the coil gets stiffer which isn’t necessarily a disadvantage).

As calculated above the stock spring would need to cut by 1/2″ to achieve a 1″ lowering as well as account for the lighter LS3 engine.

Cutting off 1/2″ is about half a coil at the end, so the modified spring would have 3.5 active coils instead of 4 and a length of 12″. This would theoretically increase the spring rate about 14% from 430 to 491 lbs/in. But the stiffer spring then doesn’t lower the front as much – kind of like a cat chasing its tail – so even more must be cut off. Reforming the square end after removing more than 1/2″ is also not an simple procedure.

In the final analysis, modifying the stock Jaguar spring isn’t a practical solution.

Custom spring

A custom fabrication with a spring rate of 310 lbs/in would compensate for the lighter LS3 engine and also lower the front end attitude by about one inch.

Solution

We’ve decided to use custom springs since both aftermarket Eibach and King products were designed for different applications. The XJ6 ride height can only be adjusted via a spring modification so custom springs, although slightly more expensive, make sense.

Installed custom springs