Overview

The front springs in the XJ6 are housed in a “tower” that is sandwiched between the upper and lower control arms. The top of the spring is fixed within the top of the tower while the bottom of the spring rests on a plate that spans the lower control arm.

Front suspension tower and compression spring

The stock front springs in our 1972, no longer available, need to be replaced to match the revised requirements of our restomod build.

Stock XJ6 Series 1 front spring RTC0661 for the 4.2 engine

The stock front springs must be changed 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.

Vehicle suspension geometry is complex in that very small changes to the compression spring height translate into a multiplied movement of the chassis and this changes the ride height.

As the diagram illustrates, weight applied to the frame compresses the front coil spring causing the control arms to pivot. Since the spring position is fixed within its mounting tower, the control arms move relative to the chassis causing the tire to move up towards the fender, thus reducing the ride height.

In the case of the Jaguar XJ6 suspension, the spring-to-chassis ratio is about 3-to-1 based on published spring shim adjustment specifications; that is, a 1/8″ shim placed between the spring and tower top lowers the car body about 3/8″. However, this is only true for very small incremental changes since the spring in the tower housing does not compress in a linear way. The control arms position the spring at an angle, and the angle isn’t constant since the spring also has a lateral flex.

A more elaborate angle correction factor can be estimated by the different geometric distances as illustrated below.

In the case of the XJ6 front suspension, we estimate the angle correction factor to be about 0.87. This means that the ratio isn’t truly 3-to-1 over larger compressions of the spring, but closer to 2.6-to-1. For example, a 2.0″ spring compression results in a 5.2″ reduction in ride height. In effect, the angle correction means that the apparent spring rate is actually higher than the specified linear spring rate as stated by the manufacturer.

Just how much weight is required to compress the spring in the first place is the analysis problem we now face.

XJ6 4.2L stock spring analysis

The starting point for spring compression analysis is getting accurate 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 to a “good enough” approximation.

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. Using basic RTC0661 parameters – wire diameter, number of coils, free length, and outer diameter – the stock compression spring specifications can be estimated.

Jaguar RTC0661 spring analysis

A detailed mechanical analysis of the Jaguar 1972 XJ6 spring (RTC0661), based on physical measurements, indicates a 385 lbs/in spring rate with a total possible movement of about 6 inches (derived from the free length of 12″ minus the solid height of 3.95″).

We can also calculate that at curb weight (982 lbs per spring), each spring will be compressed about 2.5″ and that a further 0.6″ of compression will take place if the vehicle is loaded up to GVWR.

The 2.5″ spring compression does not directly translate into a 7.5″ drop in the body because the stock spring is preloaded in the mounting tower. About 0.65″ of spring length is compressed into the tower (an estimate since Jaguar does not publish this parameter). Basically this means that about 300 lbs of frame weight per spring must be added before the front suspension begins to move off the bump stop (see small yellow arrow in illustration that highlights the bump stop movement).

Since curb weight is about 982 lbs per spring, the frame must be about 1/3 loaded before any bump stop movement can be detected. The remaining compression when the frame is loaded to specified wet curb weight then drops the car body about 5″ (using the angle corrected ratio described about). At full GVWR, the frame will drop another 1.5″ or so.

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? Using the angle corrected ratio, installing the same springs would lift the front of the XJ6 by about 1/2″ versus the stock position. Since the XJ6 already rides high, what we’d like is a position about 1.0″ below stock.

Spring aftermarket alternatives

Aftermarket springs from King Springs (Australia) and Eibach lower the XJ6 front by about an inch.

King Springs

King manufacturers three different springs for the Jaguar XJ6 (covering Series 1 through 3 and all relevant model years) that provide the standard height, 30mm (1.1″) lowered height, and 30mm lowered height for a swapped lighter weight 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 1″ lower as shown below based on our physical measurements of the King spring.

King KJFL-05 spring analysis

King Springworks has confirmed to us that the KJFL-05 has a spring rate of 400 lbs/in and this is validated by our physical measurements.

Since it is advertised as lowering the XJ6 1.2″ (30mm) with the 575 lb Chevy 350 engine (versus the original Jaguar XK engine at 605 lbs), we can gauge the impact of the still lighter Chevy LS3 that weighs about 460 lbs.

The 58 lb weight reduction, using a 400 lbs.in spring rate will have a modest 1/8″ impact on spring compression and will raise the ride height by less than 1/4″. Thus, these springs should come in about one inch lower ride height than stock even with the lighter LS3 engine, our optimum overall ride height reduction.

Jaguar stock springs (yellow background) versus King (blue highlight)

Compared to the stock Jaguar suspension, the new King springs not only lower the car almost an inch, but also provide a somewhat stiffer ride. Thus, as the car approaches full load capacity it will not sag quite as much, and road handling should become a bit less bouncy.

The approximate drop from bump stop rest to wet curb weight is shown below.

Bump stock position with approximate King spring drop

 

Churchill spring compression tool

The large front suspension springs are extremely difficult to install and remove. The stock springs require a preload with hundreds of pounds of pressure applied to the lower control arm mounting plate. We’ve done this procedure with improvised tools — a threaded rod, anchoring top piece, and other hardware — and each time we swear “never again”. Fortunately, we were able to acquire a rare Churchill original tool designed expressly for the Jaguar front suspension spring. While not the most fun you can have in a garage, this tool makes spring swaps and adjustments much easier.

King spring after installation with the Churchill compression tool