Great Leap Forward

The 1955 Jaguar 2.4 was overshadowed by its successor, but in many regards, was a more significant car in Jaguar’s evolution as a serious carmaker.

jaguar-mark-one-1
(c) jaglovers

In 1955, Jaguar committed their most ambitious act up to that point with the introduction of the 2.4, an all-new, compact saloon of a sporting mien – every inch a Jaguar, but no hand-down version of its larger sibling. Far from it, because despite the announcement the same year of the revolutionary Citroen DS19, the compact Jaguar was probably as advanced a product as could reasonably be envisaged from what was then a low-volume, specialist carmaker.

Initiated around 1953/4, the Utah compact saloon programme would mark Jaguar’s first departure from traditional body-on-frame construction to a stressed unitary bodyshell. Owing to uncertainty over its strength, two stout chassis legs ran the length of the floorpan, rearmost of which (beneath the rear seatpan) would house the mountings for the unusual inverted cantilever semi-elliptic springs, so devised to avoid feeding stresses into the rear of the floorpan. Additionally, a detachable steel subframe, carefully mounted using rubber vee-shaped blocks supported engine, front suspension and steering.

However, Jaguar were neophytes in this area and were faced with a whole slew of seemingly intractable NVH issues. Fortunately, engineering chief, Bill Heynes had an ace in the pack in the form of Robert J. Knight, upon whose shoulders the task of refining the Utah bodyshell fell. Knight was technically gifted, having studied and grasped Maurice Olley’s pioneering papers on chassis dynamics originally published in the US in 1934. But moreover, not even the mathematically impenetrable vehicle stability papers presented at the I mech E in 1956 by Milliken and his Cornell team were beyond his comprehension, making him possibly unique in the UK industry at the time.

Knight outlined the issues with Utah to chroniclers in 1992, saying, “the noise and vibration from the engine, transmission and road surface was worse than a van’s. It was this car which forced me to treat [these] as a major issue.” His findings, coupled to his understanding of dynamics theory saw Knight make early forays into the hitherto arcane science of longitudinal suspension compliance, which was applied to both front and rear suspension mounting systems.

By early unitary construction standards then, the 2.4 was a refined car. But Mr Knight was not an individual who believed any design within his purview was fully signed off until he had arrived at a conclusion, and in the case of Utah, this was a matter best described as ongoing. With the car already on sale in 1956, and still dissatisfied; especially with the transmission of noise from the rear of the gearbox mounting into the central floor tunnel, Knight devised a new mounting system which incorporated a very soft helical spring, “to convince the body floor that it was not connected to an engine”.

Further investigations saw the front engine mountings carefully retuned to provide a pitch resonance in opposition to inputs arising from wheel bounce, thus attenuating its effects. Sir William drove the prototype, asked why it felt like it was worth £500 more and demanded the changes be incorporated in production immediately. From that point, all Jaguar engines would be mounted this way.

Bob Knight codified the essential philosophy of Jaguar’s NVH pre-eminence with this car, principles which both he and his immediate successor would consistently refine in the years that followed. “Attempts to stiffen or detune the structure are of limited value. What you’ve got to do is stop the forces ever getting into the bodyshell.

Originally intended to employ a four-cylinder version of the twin-cam XK engine, a short-stroke 2483cc version of the existing unit was ultimately developed, partially owing to a lack of refinement from the in-line four, but largely because it proved cheaper for Jaguar to produce a smaller six from existing tooling. Furthermore, thus equipped, Utah could also be sold at a higher price. The 2.4 developed a mildly tuned 112 bhp on twin Solex carburettors, giving a top speed of just over 100 mph – impressive for the time, and dismaying for rivals.

William Lyons was a canny businessman and something of a stylistic genius, but his approach to Utah almost derailed the programme entirely. Having carried out his styling experiments in seclusion, he presented the completed design to Heynes with strict instructions that it must not be altered. The streamlined teardrop shape was undoubtedly elegant, and it would appear, smoother through the air than its 1959 successor, but an imposed rear track width some 4” narrower than the front compromised the car’s roadholding, especially given the relatively unsophisticated live rear axle layout chosen.

Furthermore, Utah’s tight packaging dictated a nose-heavy layout with the longer six-cylinder engine mounted forward of the front axle, placing over 50% of the car’s weight ahead of the driver, with malign effects on steering and handling. This was no way to go about developing a car and the 2.4 would mark a watershed at Browns Lane, Heynes insisting that Lyons never do anything like this again. But it meant that the car was to all intents and purposes, reverse-engineered, engineers having to work around issues dictated by the body hard-points.

The 1957 introduction of the larger 3.4 litre engined version placed these matters into sharper relief, with the 120 mph car now at the outer envelope of roadholding and most notably, braking. The advent of disc brakes, co-developed with Dunlop at MIRA and Le Sarthe salved the latter issue, and prototypes running with a wider rear track (and broader wings to house them), demonstrated what was required to at least mitigate the former. Utah Mark 2 was by then in gestation, but this would prove far from straightforward either.

(c) DTW

With the Mark 2 in production, the retrospectively termed Mark 1 was quickly forgotten amid the flood of accolades lavished upon its successor. They also rusted with alarming alacrity, decimating their numbers. Comparatively few have survived.

But it was a vitally important car for Jaguar and the lessons learned from it underpinned everything which followed. Indeed without these learnings, it is questionable whether Jaguar would have made such strides over the decades that followed. For a small cat, the 2.4 represented very a big leap.

Sources/ quotes : The Automobile – June 2008/ Motor – August 1979/ Developing the Legend – Paul Skilleter.

Author: Eóin Doyle

Founding Editor. Content Provider.

11 thoughts on “Great Leap Forward”

  1. Thanks for sharing this story. Lots of new (to me) information. I’ve heard it said that the smaller rear track was chosen because its components were readily available from a supplier, so no new parts had to be designed. Does this story has any truth or is it just misinformation?

  2. “Attempts to stiffen or detune the structure are of limited value. What you’ve got to do is stop the forces ever getting into the bodyshell.”

    Unfortunately Jaguar seems to have forgotten this and deleted the rubber isolated front subframes that they introduced in 1955.

    Great article by the way.

    1. Also, using the engine mass and motor mounts as a dynamic damper for some specific suspension vibration frequency is clever. One of Bob Knight’s many NVH innovations, and one I didn’t know about.

      Again, unfortunately, it will work much better if the engine is damping the vibrations in a rubber bushed subframe, rather than the entire car – which is what you have when you eliminate the front subframe and attach the motor mounts directly to the car body.

  3. Refinement issues aside there could have been some value to the Jaguar Mark 1 later Mark 2 featuring a 2-litre XK inlin-4 or even a 2-litre XK inline-6. up to the late-1960s, thereby at least giving the company something to challenge both the Rover P6 and Triumph 2000 in the 2-litre segment.

    Sure it is easy to argue such a 2-litre engine would be underpowered in the large Mark 1/2 (with outputs likely roughly ranging from 90 hp up to 125 hp), yet the same can also be said of its potential rivals before they both featured larger engines.

  4. Fascinating stuff, Eóin, thank you. Your related “Catastrophe” piece on the MkX and appended comments is also well worth reading.

    Jaguar’s model designations in the 1960’s really were bizarre, a lucky dip of numbers that must have caused some confusion amongst potential customers. They have certainly confused me!

    Let’s see if I’ve got it right at last:

    The large MkX model replaced the MkIX and was ultimately renamed the 420G before being replaced by the first XJ. Around the same time a smaller model, the Mk1, was launched and was then replaced by the Mk2. A new, longer tail was added to the Mk2 and the resulting model was named S-Type, which sat between the Mk2 and MkX. Then, a new, more upright nose was added to the S-Type to create the 420, not to be confused with the large 420G (grand?). Both 420 and 420G were replaced by the first XJ, while the Mk2 was not replaced.

    I haven’t the energy to investigate whether there was an ancestor to the MkX named (retrospectively) the MkI, not to be confused with the “Arabic” Mk1. I’m sure Eóin will know and enlighten me, and correct any other misunderstandings on my part.

    Richard, you’re right about that blue colour on the Mk1. I think it’s because the contemporary (cellulose?) paints were less glossy than modern flat colours, which gives the bodywork an unusual “softness” of form, if that makes sense.

    1. Well, I’ve now summoned the energy to investigate the MkX and MkIX’s predecessors, and things gets even murkier. Apparently, there was a MkVIII, and a MkVII, but no MkVI. The MkVII’s immediate predecessor was the MkV, and its predecessor only became known retrospectively as the MkIV. In production it was simply known as the Jaguar 2 1/2 or 3 1/2 litre saloon. Its prededessor wasn’t a Jaguar, but an SS, the SS1. The SS marque name was abandoned after WW2 for obvious reasons. There were no MkI, MKII or MkIII models. From the MkV onwards, the cars were always referred to in Jaguar literature by Roman numerals, never Arabic numbers. According to Wikipedia, there was no MkVI because there was a Bentley MkVI on the market at the same time.

      The Mk1 and Mk2 were only referred to by Arabic numbers, the former only retrospectively.

      Time for a lie-down…

  5. Even marque historians steeped in the minutiae were confused by Jaguar’s naming systems – most especially the XJ-project numbers – which to my knowledge were never fully unravelled. Heavens, even the likes of former engineering directors got them muddled from time to time.

    There was also a 1.5 litre version of the SS Jaguar saloon, which (again retrospectively) became the Jaguar MK IV after WW2. This was arguably the true predecessor to Utah Mark One, even if the later car had a far larger displacement. This was a big seller for SS/ Jaguar and following its deletion, it left a sizeable hole in sales and revenue terms – one Lyons was keen to plug.

    Another point is that Jaguar was initially a model name – the immediate pre-war cars being the SS Jaguar Saloon and SS Jaguar 100 Sports. Jaguar as a marque name was a post-War creation (for obvious reasons).

    The question of the smaller capacity engine is one worth asking and certainly from a weight and marketing perspective, could have made sense. I will cover this to some extent later, so will say no more on the subject for now.

    As to the rear suspension – while it was based in principle upon that of the C-Type Le Mans car, albeit employing leaf springs rather than torsion bars, it was an all-new design for the car. In addition, it was the confines of the enveloping bodyshell which dictated the track width and given that it ended up being narrower than a contemporary Morris Minor, I’d struggle to imagine what existing componentry it could have employed. But others may know more…

  6. A most interesting article – wasn’t aware of the two extra locating arms on the rear suspension of the 2.4 before, or that the semi-elliptic springs ran up inside the chassis members. I’d assumed incorrectly from a cursory glance at the real thing that they were quarter elliptics and little else, but obviously they weren’t. Unique layout I think. But what caused the car to look as though the rear was sagging? The styling or the springs relaxing? I always assumed it was the latter. My experience has only been with two 3.8 MkII’s. My uncle drove his like a bat out of hell, but my pal maintaining his original would sweat at the thought of exceeding 3000 rpm, so didn’t despite my entreaties!

    The rear suspension is a bit retrogressive when you study the London Transport Routemaster bus designed in 1954. Besides a state of the art aluminium monocoque chassis – no separate frames need apply – it had coil springs at the back, and independent suspension up front. I was always impressed with the Routemaster in 1969 when I returned to the UK to study. Compared to the standard GM New Look “Fishbowl” city bus of North America of the very late ’50s and ’60s with its deafening howling Jimmy two stroke diesel and clonking rear axle despite air suspension all around, the RM was quiet and solid riding. The single deckers LT deployed in the early 1970s were, however, stinkers that the GM could see off with one hand tied behind its back. It’s also worth noting that the 1958 GM bus was also separate chassis-less, they called it stressed skin. I’ve often wondered if GM copied the Routemaster, from the descriptions in Commercial Motor that the Bedford lorry blokes sent back to Detroit way back in 1954. The time scale fits. All ally monocoque, etc.

    Here’s a link to a Commercial Motor magazine scan of its archives. Note: all they’ve done is optically scan old copies, and the result is a bit rough with all manner of errors. And the app to see the full page is wonky, to say the least. But there are no other sources I’ve ever been able to locate.

    http://archive.commercialmotor.com/article/24th-september-1954/94/new-chassisless-kt-bus

    Nevertheless, in a field where innovation rarely happened, it’s interesting to note the LT boys made a much bigger unitary vehicle than a Jag 2.4, and packed in some solid engineering for the time. I suppose it all depends on how many resources are thrown at a problem. Bob Knight and a couple of draughtsmen versus a big team who probably very carefully “stressed” the unitary chassis of the bus – it had to work and no excuses to be commercially acceptable, and the result was still lighter than its predecessors.

    So far as longitudinal compliance in suspension was concerned (and engine mounts too) certainly Cadillac and GM in the 1950s were up on that with their coil-sprung live axles, and Lincoln/Ford were hard at it by 1965, but years after GM. Up till then, Ford just put huge squishy bushings in the front eye of the semi-elliptic cart springs. As did Chrysler who managed to hoodwink an entire generation that torsion bars in the front suspension were a miracle compared to coils, as if torsion bars were unearthly. You can fool the population quite easily if you repeat nonsense often enough.

    1. “So far as longitudinal compliance in suspension was concerned (and engine mounts too) certainly Cadillac and GM in the 1950s were up on that with their coil-sprung live axles…”

      Of course, a major contributor to longitudinal compliance in a body on frame car are the rubber body mounts (he said, helpfully).

    2. Otto Kässbohrer from Ulm designed his first unibody bus in 1951, calling it SETRA for German SElbstTRAgend (self bearing). It used ribs with a riveted aluminium skin and the (Henschel) engine already was in the rear.

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