The Quintessence : (Part Two)

In this second instalment, we examine the XJ6’s technical package.

Proving engineers, Don Currie and Richard Cresswell with a disguised XJ4 prototype, sporting Lyons’ ‘Studebaker’ grille. (c) Classic Cars

Sanctioned in 1964, XJ4 was intended to launch in 1967, which seems in hindsight to have been a rather optimistic timescale. The project team would be led by Bob Knight, Jaguar’s senior development engineer and one of the finest conceptual minds of his era. The Browns Lane engineering department at the time was something of a collection of minor fiefdoms, most of whom worked somewhat independently, as Jim Randle discovered to his amazement when he joined the project team from Rover in 1965. [As related to this author].

When I came to Jaguar, I couldn’t see how they made a car, there was no bloody programme at all! It gradually dawned on me – you got people who were around the company – people like Tom Jones, Cyril Crouch and whatnot. They took their bit of the car; they didn’t talk to anybody, they’d just get on with it. And they often fell out with the guy who was doing the next bit! But you didn’t need anyone producing a programme.

From his overseeing promontory, engineering director Bill Heynes did not envisage anything particularly radical, XJ4 being more about refining a successful formula. So while the double wishbone front suspension closely resembled that of existing models, it was rubber-mounted upon a box-section subframe and would embody an anti-dive geometry which not only reduced weight-transfer upon braking by 50%, but allowed softer road springs to be used. The Burman power-assisted steering was by rack and pinion – a first for a Jaguar saloon.

At the rear, Bob Knight (in conjunction with Tom Jones) oversaw the design of a new twin-link suspension design, mounted directly to the body and employing the driveshafts as the upper links as before. However, this new design employed a torque tube arrangement for the drive-line, with the aim of channelling stresses through the strongest section of the floorpan. The new system was lighter, and displayed a noted improvement in handling and secondary ride characteristics; a weakness of the older E-Type-derived unit, which had been rather hurriedly created in 1957.

(c) Jag-Lovers

Designed from the outset to utilise the all-alloy V12 engine, then in development, the only version of the in-line XK six intended for the car was a 3.0 litre version, developing a net 185 bhp and believed to have been particularly sweet-running. Additionally, a 60° 3.5 litre V8, to be derived from the twelve cylinder tooling was also being investigated.

With Knight tasked with ensuring the new car exceeded Jaguar’s already impressive standards, Jaguar’s resident chassis guru employed everything he had learned about NVH suppression, employing the car’s major masses – its suspensions, axles, engine and gearbox as attenuators, to cancel out unwanted noise and vibrations.

A double-skinned bulkhead further insulated the passenger compartment, while painstaking attention to suspension bushes and damper settings saw new standards in ride isolation and handling being set, despite the pioneering adoption of purpose-designed 70-series low profile Dunlop 205/15 radial tyres, which appeared colossal at the time. Also in the quest for quietness, large quantities of soundproofing material were fitted throughout the cabin.

However, problems soon beset XJ4 development. The most pressing being a resonance of the torque tube, which led to a good deal of head-scratching, as Jim Randle explained. “The original XJ6 system was totally dependent on the angle of the drive link. That system [the alternative Bob Knight design] had a forward-facing tube with a mounting into the drum. The firing order put this on to resonance, and we had a boom”.

Complicating matters further was not only the multiplicity of projects being handled simultaneously, but also the fact that Jaguar’s engineering resource, although of a very high order, was woefully understaffed for the tasks at hand. Owing to the fact that the XJ body was almost as low slung as that of an E-type, XJ4 proved something of a packaging miracle. But that didn’t mean the job of incorporating all that heavy and space-taking hardware was anything but a massive headache for Jaguar’s overstretched development team.

By 1967, with the programme now well behind schedule, the resonant boom being generated by the experimental torque tube suspension system became a potentially ruinous drag on engineering resources, defying all attempts at a cure. With vast amounts of development time lost, Knight abandoned further development.

This entailed, not only the late adoption of the existing rear suspension and final drive unit into the XJ4 bodyshell, but a hasty redesign of the underbody by Pressed Steel, who somewhat fortuitously were prepared to accommodate this last-minute alteration within a now very tight timescale. A further complication being that a good deal of proving and refinement work would need to be reprised.

Lyons was furious, but there was nothing for it but to defer introduction until the Autumn of 1968. A further setback came with repeated delays to the V12 engine. In early 1968, with launch material and sales brochures already in print, it became evident that the V12 would not be ready for the September launch. In fact, delays would continue to bedevil this engine programme, ultimately deferred until 1972.

(c) jag-lovers

An unintended knock-on was the lack of a larger engined variant. Already the 3.0 litre XK unit had been subsumed into a 2.8 litre version, intended to dip below European tax brackets, so the venerable, if rather compromised 4.2 litre XK unit from the 420 saloon was press ganged as a temporary stop-gap. But while the installation proved relatively trouble-free, the taller power-unit wouldn’t clear the XJ’s low bonnet.

To Lyons’ intense dismay, his exacting bonnet line was sacrificed and a central bulge added. Sir William reportedly detested the addition, expressing his surprise to a subordinate during the model’s launch that nobody had noticed. Ironically, this feature, not to mention the 4.2 litre unit itself, not only lasted the duration of the XJ6’s career, but would become over time an essential component of Jaguar stylistic iconography.

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13 thoughts on “The Quintessence : (Part Two)”

  1. Am intrigued by the 185 hp (net) 3-litre XK6 engine, since some refer to it as a detuned road-going version of the 296 hp 3-litre / 2997cc racing-focused E2A prototype engine while others say the former was a separate engine to the E2A.

    Quite surprised about the 185 hp (net) figure of the 3-litre XK6 engine compared to what the somewhat pathetic 140 hp (net) 2.8-litre version was putting out, though the latter makes more sense when looked at as a slightly enlarged version of the originally envisaged 2.6-litre XK6 prototype unit. Would have been interesting seeing the 3-litre XK6 eventually spawn a slightly enlarged 3.2-litre displacement, along the same lines the 2.6-litre is superseded by a properly developed 2.8-litre unit.

    It is mentioned the 3-litre (not sure which one given the possible overlap) was tested by Normal Dewis in 1965, who found the engine in the E-Type gave good low-speed torque, economy and quiet running though torque was not sufficient compared to the 4.2-litre XK6.

    1. Opinions differ as to the nature of the 3.0 litre XK engine. Some suggest it had an alloy block, others that it simply was a short-stroke version of the existing XK unit. (I suspect the latter). All sources appear to be in agreement that it was a very nicely balanced power unit and would probably have made for a very satisfactory entry-level XJ6 model.

      Why the 2.8 developed considerably less power than its larger sibling? A possible rationale may have been that it was allegedly based upon the 4.2 unit – itself something of a lash up and an engine that was ill-regarded by many in the Browns Lane engineering department – one former powertrain engineer describing it as “a bloody awful engine.” The 4.2 ran into all sorts of cooling problems, not to mention a harshness and unwillingness in the upper ranges. As a basis for all XK-related engine developments from that point onwards, Jaguar couldn’t have made a worse choice.

      In retrospect, the 3.0 litre would have been a better bet, especially given the problems Jaguar ran into with the 2.8, but it would not have given them a car they could sell into high-tax markets. Furthermore, by basing it on the 4.2 block, production costs were slashed and by then, Jaguar had to penny-pinch like never before. BMH were going bust and matters under BLMC (post-’68) would be little better.

      Jaguar got a very poor deal out of its 1966 ‘merger’. But staying independent wasn’t a viable option either. I’ll return to this matter in a later instalment.

    2. It is probably the case that the 2.6-litre XK was originally derived from the planned 185 hp (net) 3-litre XK6 (apparently being developed together as mentioned in Heiner Stertkamp’s book), until the latter was cancelled while the former was enlarged to 2.8-litres by being derived from the 4.2-litre unit in order to presumably save on costs.

      Further embarrassing for the 2.8-litre XK was Harry Weslake in late-1961 being said to have developed an inlet port that resulted in the 2.4-litre XK putting out 170 hp. A bit unfair for the 3-litre XK to be so readily dismissed for not having enough torque compared to the 4.2-litre XK.

      While there would have likely been issues with the following, it is interesting to know that William Lyon’s biography notes the possibility of Jaguar joining Leyland instead of BMC had his friend Sir Henry Spurrier lived a bit longer.

  2. The torque tube drive that Knight worked on has of course many variations in detail design available; Buick had one for a quarter century from 1938 or so but always with a solid rear axle. Mazda has had a lateral thinking one in the Miata from the beginning, a three-sided truss instead of an actual tube. Quite why Knight beavered away at it is not clear to me, particularly in the way it is described here in that somehow the transmission tunnel would take the stresses. A tube from the rear of the gearbox containing the driveshaft to the body-mounted final drive would seem to obviate the role of the transmission tunnel in doing much of anything in that regard. Unless the differential mount to the body was to feature unusually narrow-spaced mounting points, but why would they do that? Furthermore, a long thin driveshaft within the tube but without the usual steadying centre support bearing with two piece driveshaft is going to be the very devil to engineer without flail and torsional oscillation issues in and of itself. Sudden torque variations or shock load changes such as going from full-bore bottom-gear acceleration to standing on the brakes as a child suddenly darts into the roadway ahead only exacerbate the issue. So it is entirely unclear to me what was thought to be its primary advantage in enhancing refinement. Indeed, once abandoned, the usual solution was knocked up in less than a year. Perhaps Knight originally wanted the transmission tunnel itself to perform what Mazda later did with the truss and the torque tube terminology was the nearest thing he could label it as. Is there more information available?

    My first ride in an XJ6 was in late 1969 in London. At the Commonwealth men’s student residence in Mecklenburgh Square, there was a mix of scholarship types and chaps with wealthy parents. One of the latter in particular from Rhodesia given to bouts of boasting about rally competition declared he would have himself an XJ6. My friend and I accompanied him for the test drive in a 2.8 where his tentative prods on the accelerator resulted in his afterwards claiming that 170 bhp was so much one had to be careful not to unleash uncontrollable amounts of pure naked power without a lot of practice. Well he did wear a cravat and sports jacket as standard equipment and was probably buying for Dad really. I, from the back seat, thought the refinement of ride over manhole covers mirrored what I had observed in the streets from watching XJs glide over them with little aural noise like the usual tyre thwacks. And this from Dunlop tyres that resembled knobbies to we crass colonials. The overall operating sound level within was in general nothing to write home about to those used to Detroit pillowmobiles, and the ancient BW auto wasn’t even close to a Chrysler Torqueflite, slurring upshifts instead of crisply cracking them off even on light throttle. No, to me the overriding plus was the level quiet ride with no unseemly clonks from the working of the suspension, or changes in tire whir associated with unloading one rear wheel or the other as happened with solid axles. Very nice. Well, that and real leather.

    My uncle had a 3.8 Mk2 up in Harrogate where I spent three weeks at Christmas that year, and he and my cousin whipped off the head early one Saturday morning, decoked it, replaced it and then proceeded to blast into Leeds that afternoon. He, by coincidence, was also Rhodesian but was addicted to deep lashings of throttle. Not what I exoected from a pathologist. The 3.8 was a bit of blunt instrument and frankly not even close to a 1968 340 Plymouth Barracuda in ride, handling. or 6 second to 60 dashes. It merely seemed old-fashioned with wood to bash your head on in the event of an accident, leather, an abrupt ride at the back, and great gnashing noises from the gearbox. Galaxies and Lotus Cortinas had rather beaten up on 3.8s in UK saloon car racing six years earlier in any case.

    Compared to the Mk 10 blimp with bodywork hulking over the wheels, the XJ6 was a svelte delight to gaze upon. It’s amazing to reflect that 50 years on, it still engenders articles on its creation. It wasn’t a step change in suspension design given that the Mk 10 barge had the same layout. It was finally a modern British overall take on a luxury saloon, wide and low and lithe.

    The Australian doctor down the hall in the aforementioned student residence, in London for extensive training in radiology, bought himself a Mercedes 280 SE 3.5 automatic in the summer of 1970, and that thing was a terrier with a feeling of solidity. It however seemed to suffer from external noise entering the huge glazed area, particularly noticeable in reflected noise when negotiating underpasses or vehicular subways or whatever the UK terminology is. The rear suspension was crude by comparison to the Jaguar, of course, but the eagerness of the engine is what I remember most, not hugely powerful but relentless and smooth., quite unlike the whooshing of a Detroit V8 when the secondary venturies of the four barrel carb suddenly opened. An XJ6 fitted with that engine would have squashed the world in my opinion – that was one of my idle daydreams of the time.

    Of course, later experiences of two XJ6’s here in Canada in the ’70s and ’80s owned by colleagues were tarnished by how quickly they turned tatty and the inability of BL to service them decently. But in a reverse of Billy Shakespeare’s Mark Anthony : “I come here to praise XJ6, not to bury her”.

    1. That was worth an article. Non-engineers may like to know that “…the “torque” that is referred to in the name is not that of the driveshaft, along the axis of the car, but that applied by the wheels. The design problem that the torque tube solves is how to get the traction forces generated by the wheels to the car frame. “

    2. Two magic words: ” Lotus Cortina”. Wouldn´t a Lotus Fiesta be an interesting thing (if and only if it wasn´t made to look like a silly rally car)?

    3. Bill: I wouldn’t profess to understand the nuances of torque tube layouts. I seem to recall the 1977 Porsche 928 employed one and it was no NVH paragon either. Unfortunately, during our conversation with Professor Randle, the subject only moved into serious suspension theory towards the very end. [When I say conversation, I mean he explained and I did my level best to keep up…]

      There is very little definitively written about the Knight/Jones IRS design for XJ4 and no photographs I have seen. Perhaps something lurks within the depths of Jaguar Heritage’s archive. According to Tom Jones, they ran into problems with tracking (which was traced to the front suspension and eliminated) but he was quoted as saying, “that boom issue spoofed us”. Lyons leant on Heynes, who then leant on Knight to the effect that enough time had been lost…

      From what I can glean, the fundamental principles were similar to the design Randle created for the later XJ40 programme, although there was a good deal of difference in execution. One would have to say Randle’s take was the more successful of the two, (and a very clever piece of original thinking) but given Knight’s abilities, there was undoubtedly merit in what he was attempting to achieve.

      There was also a separate proposal employing a two-speed final drive rear axle, which again fell prey to Jaguar’s NVH standards. Little that is meaningful is known about that one either.

    4. Thanks very much for the replies. I think the torque-tube dead end probably isn’t worth pursuing further, since it didn’t happen in production. The Randle-led design for the XJ40 rear suspension was so subtle to look at, anyone not knowing how it worked would likely wonder what kept the rear wheels securely attached. Having been lucky enough to have found an I Mech E paper on it by the actual engineer involved (and lost when the tablet went west), I must say I haven’t found anything better, or at least that appeals to me personally. Subsequent German five-link rear suspensions with conflicting radii of link motions that only work by torturing rubber bushes into odd shapes have to be decried in comparison. Worse than that was Ford’s complete inability to understand the subleties of the bushes and in what order the bottom U -arms should be mounted to the differential suspension flanges, doomed the design to ordinariness following their takeover of Jaguar. At least from my take on things.

      Ah, the Lotus Cortina. Now that had a design flaw in the rear suspension that a larger rubber bushing would have solved. Since it never seemed to occur to Lotus why the triangular layout of the railing arms to a differential mount failed at that point, it was left to Ford to revert to cart springs after two years for the 1965 model year. My study of a teacher’s 1962 Chevrolet rear suspension had taught me how conflicting link motions were resolved by large rubber bushes. Because it was obvious that a road car’s solid axle had to be able to be in full compression on one side while drooping on the other, as when negotiating a potholed parking lot for example, some measure had to be incorporated to allow axle twist without ripping off mounting points as it did so. The poor old Lotus Cortina had a system with tiny bushes that effectively wanted to allow only vertical motion with respect to the body, and no twist. Lean that little beast over in corners often enough and mounting points failed. As for a Lotus Fiesta, it seems that both old and new ST models are probably the best-handling FWD chassis’ around anyway – a restyle wouldn’t hurt though. I find the car confining inside. One thing you could say about the original Cortina was that it was a roomy little tin box for its size and had decent shoulder room. These modern cars encroach on my space with their thick doors and curved side glass.

  3. Pity about the torque tube for Jaguar’s XJ4. Had the time been available to develop it the car would have been an even greater improvement than it was (over the competition). There would have been more rear passenger room and the transient rear steer, which the XJ Series 1 through 3, XJS and XJC all experienced, could have been eliminated at source. It would also have been possible to fit a faster steering rack for reduced turns lock to lock (in the end the steering system specified detracted from the car’s overall excellence somewhat, but it was necessary due to that transient effect at the rear).

    A torque tube has natural frequency proportional to its length and it is easily excited by long stroke engine vibrations and coarseness (as Peugeot, great proponents of the torque tube, were later to learn). The length of the Jaguar tube meant that it resonated in response to excitation from the 4.2 engine at certain rpm. This was due to the rough nature of the 4.2 six cylinder engine with its long stroke and over weight reciprocating masses. The XJ4 prototypes likely would not have experienced the same nasty resonances with the V-12 (short stroke of only 70mm and twice the firing impulses per rev). However, the V-12 was not ready in time and so the expedient of patching up the hoary old XK engine was what was resorted to. This time it was all a little too far….

    As the Jaguar engineers experienced, torque tube resonance is not eliminated by the simple expedient of placing intermediate bearings on the drive shaft inside the torque tube (Peugeot did that and thought they had the problem licked- close but not quite as it later turned out). Primarily it is the length of the torque tube itself that matters, followed by the nature of the engine reciprocating mass and firing order. In effect, Jaguar were building an XJ4 with the equivalent of a long single piece drive-shaft (the natural frequency of that would be about the same as what they encountered with their torque tube). As you know, they ended up ditching the torque tube idea and used a multi-piece drive shaft with intermediate bearings all carefully isolated from the car body instead. The idea was to move the natural frequency higher than would ever be attained by any excitation available from the drive-train. The same result can be achieved with a multi-piece torque tube relatively unrestrained in all axes save one…

    In the technical paper discussing the development of the XJ40 rear suspension system there is a section disclosing some of the experiments and rear suspension prototypes which were tried prior to the decision to design from scratch a new rear suspension system altogether. One of the systems Jaguar tried worked very well but suffered from a geometry problem which drove the decision not to proceed further with it. In certain ways it was superior to what was eventually adopted for the XJ40. In this system longitudinal compliance is independent on each side of the car and could be set extremely soft without fear of axle tramp. As it happens the geometric defect is conveniently remedied with a little careful study. This was not disclosed in the technical paper, nor tried on XJ40, probably because it was not realised until too late!

    XJ series 1 through 3 are final over-steerers. They point. The later XJ40 is a final under-steerer. It deploys a rather large roll bar across the front suspension which the earlier cars do not. The XJ40 body shell is not quite rigid enough for the approach and some effects are noticeable.

    There is a story about the engineers selecting the dampers for XJ40. They were complaining about how long the task took- some three weeks or so. Mundy on hearing that commented that on the original XJ it took months and months (possibly years, I don’t recall exactly) of testing to get the dampers exactly right. Drive the cars back to back and you’ll notice the difference.

    1. What was the proposed rear suspension system that was not proceeded with during the XJ40 project due to a geometry problem, which was potentially resolvable?

  4. Bob

    There is an IMechE paper entitled, “The development of a high comfort, high stability rear suspension.” It was written by A J Cartwright, BSc. The relevant section is “5.3 Compliant Wishbone”. Excerpt follows.

    An early experiment into low-frequency compliance systems was a system for hub compliance using a wishbone with rotational flexibility. The concept was based upon the principle of using the hub carrier as an inverted pendulum. Clearly if the hub carrier was mounted on a shaft and allowed to rotate freely in a transverse axis through the centre of the lower wishbone, then static loads would constrain the system geometry but a very low stiffness would be presented at the hub centre-line in the compliance direction. Furthermore, there would be a progressively increasing stiffness with wheel movement in the compliance direction in either sense and the system would be inherently stable. However, the stiffness by this means alone would be zero in its static condition and would undoubtedly allow wheel movements of excessive amplitude. In order to add stiffness and reduce complexity the pivot axis was provided by a number of tubular rubber elements sandwiched between two coaxial tubes. The refinement with this system was exceptional, maintaining un-sprung mass resonant frequencies in the fore and aft sense lower than those in the vertical sense (wheel bounce). Further analysis and testing, however, showed that this system had an inherent bump steer problem. Since the rotational axis of the hub carrier was carried in the lower wishbone whenever the wishbone was in a position other than horizontal, hub compliance (that is rotation of the hub carrier) caused a corresponding rotational component in the steer plane, proportional to a function of wishbone and hub inclinations. It was with great disappointment that this too was shelved.

    I’d post an illustration but know not how to do (let me know how and I’ll get a picture on here for you to view).

    Note that the geometric issue causing rear steer is solvable. The means is not immediately obvious, but it is mechanically simple.

    1. Hi Bob. Thanks for the information. If you click on the ‘Driven to Explain’ tab above and scroll down to the bottom of the page, you’ll find instructions for uploading photos for whatever device you are using to access DTW.

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