Cavitation

Cavitation (pronounced kav-i-tey-shuhn)

(1) The formation of pits on a surface.

(2) In fluid dynamics, the rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure (associated especially with devices such as rotating marine propellers or the impellers used in pumps.

(3) Such a pocket formed in a flowing liquid; the formation of cavities in a structure.

(4) In biology, the formation of cavities in an organ (used originally to describe those appearing in lung tissue as a result of consumption (tuberculosis)).

1868: The construct was cavit(y) + -ation.  Cavity was a mid-sixteenth century borrowing from Middle French cavité or the Late Latin cavitās, from the Classical Latin cavus (hollow, excavated, concave), the construct being cav +‎-ity (the nominal suffix).  The suffix -ation was from the Middle English -acioun & -acion, from the Old French acion & -ation, from the Latin -ātiō, an alternative form of -tiō (thus the eventual English form -tion).  It was appended to words to indicate (1) an action or process, (2) the result of an action or process or (3) a state or quality.  Cavitation is a noun, cavitate, cavitated & cavitating are verbs and cavitatory & cavitatory are adjectives; the noun plural is cavitations.

The original use of cavitation dates from 1868 and appeared in the literature of human pathology, describing “the formation of cavities in the body”, especially those appearing in lung tissue as a result of consumption (tuberculosis).  The use in fluid dynamics (particularly pumps and marine engineering) emerged in circa 1895 although oral use may have predated this: the verb cavitate (to form cavities or bubbles (in a fluid)) documented since 1892 so it was either a back-formation from cavitation or the construct was cavit(y) + -ate.  The related verbs were cavitated & cavitating.  The suffix -ate was a word-forming element used in forming nouns from Latin words ending in -ātus, -āta, & -ātum (such as estate, primate & senate).  Those that came to English via French often began with -at, but an -e was added in the fifteenth century or later to indicate the long vowel.  It can also mark adjectives formed from Latin perfect passive participle suffixes of first conjugation verbs -ātus, -āta, & -ātum (such as desolate, moderate & separate).  Again, often they were adopted in Middle English with an –at suffix, the -e appended after circa 1400; a doublet of –ee.  The noun supercavitation was a creation of plasma physics and described an extreme form of cavitation in which a single bubble of gas forms around an object moving through a liquid, significantly reducing drag.  As observational technology & techniques improved, the form ultracavitation also appeared to describe instances where instances of the phenomenon meant drag tended as close to zero as was possible.

Cavitation is an interesting aspect of fluid dynamics but it’s studied because it’s something which can cause component failure in devices like the pumps used for liquid, fluid & gas which can have catastrophic consequences for both connected equipment and people in the vicinity and beyond.  Such components typically feature robust construction but cavitation is a function of sustained operation (often 24/7) at high speeds and some vulnerable parts may be heavy and the fragmentation at high velocity of a heavy, reciprocating mass is obviously a serious problem.  Technically, it’s the formation of vapour- or gas-filled cavities in a flowing liquid when tensile stress is superimposed on the ambient pressure and one novelty in the science of cavitation was in 2021 noted by researchers in an oncology laboratory.  Using a gassy, explosive bacteria to destroy cancer cells by bombardment, the strikes were observed to produce a brief sonoluminescence (in physics, the emission of short bursts of light from imploding bubbles in a liquid when excited by sound), the cavitation bubbles producing a brief flash of light as they collapsed.

In the specific case of “pump cavitation”, the problem typically occurs when a hydraulic pumps which pumps liquids suffers a partial pressure drop.  What the change in pressure can induce is the formation of air bubbles, leading to cavity creation.  Inside the pump, the pressure shift transforms the liquid into a vapor which is then converted back to liquid by the spinning impellers.  The air bubbles thus are constantly moving inside the housing and as they implode during pressure changes, the surfaces of the impeller are eroded and it’s the creation of these tiny cavities which can accumulate sufficiently to weaken the structure to the point of failure.  The issue particularly affects centrifugal pumps but can occur in submersible devices.

Lindsay Lohan enjoying the effects of fluid dynamics.

Although something identified by engineers in the nineteenth century, the exact nature of cavitation wasn’t fully understood until the application in the 1950s of high-speed photography and the mathematical models developed then were later confirmed as close to exactly correct by computer simulations later in the century.  What was found was two causes of cavitation : (1) Inertial Cavitation in which a shock wave is produced by the collapse of bubble or void present in a liquid and (2) Non-inertial Cavitation which is initiated when a bubble in a fluid undergoes shape alterations due to an acoustic field or some other form of energy input.  Also observed were two behaviors of cavitation: (1) Suction Cavitation induced by high vacuum or low-pressure conditions which reduce the flow of fluid, bubbles forming near the eye of an impeller eye; as these bubbles move towards the pump’s discharge end, they are compressed into liquid, and they will implode against the impeller’s edge and (2) Discharge Cavitation which occurs when the pump’s discharge pressure becomes abnormally high, altering the flow of fluid, leading to internal recirculation, the liquid becoming “stuck” in a pattern between the housing (and the impeller) thereby creating a vacuum which in turn creates the air bubbles which will collapse and cavitate the impeller.

Representation of fluid dynamics under specific resonant conditions.

In fluid dynamics, a flow becoming “stuck” is often something to avoid but an aspect of the behavior can be exploited and it was a specific instance of certain “resonant conditions” Chrysler’s engineers exploited in 1959 when designing their Sonoramic induction system.  The idea wasn’t new, the math explained as early as 1863 and in racing cars it had been used for years but what Chrysler did was make it a focal point.  Sonoramic was an implementation of Sir Isaac Newton's (1642–1727) first law of motion, more commonly known as the law of inertia: “An object at rest tends to stay at rest and an object in motion tends to stay in motion” and it’s the second part which was exploited.  During the intake cycle of an engine, the fuel-air mix flows through the intake manifold, past the intake valve, and into the cylinder, then the intake valve shuts.  At that point, the law of inertia comes into play: Because the air was in motion, it wants to stay in motion but can’t because the valve is shut so it piles up against the valve with something of a concertina effect.  With one piece of air piling up on the next, the air becomes compressed and this compressed air has to go somewhere so it turns around and flows back through the intake manifold in the form of a pressure wave.  This pressure wave bounces back and forth in the runner and if it arrives back at the intake valve when the valve opens, it’s drawn into the engine.  This bouncing pressure wave of air and the proper arrival time at the intake valve creates a low-pressure form of supercharging but for this to be achieved all variables have to be aligned so the pressure wave arrives at the intake valve at the right time.  This combination of synchronized events is known as the “resonant conditions”.

Representation of cavitation in mechanical gears.

The behavior in pumps is now well understood and both design parameters and maintenance schedules are usually cognizant of cavitation and its potential consequences.  However, instances remain not infrequent, especially when pumps are fitted into systems by non-specialists, the most common causes being (1) low fluid pressure, (2) insufficient internal diameter of suction pipes, (3) excessive distances between a fluid source and a pump’s impeller(s), (4) pumps being run at too high a speed (which may be within a manufacturer’s recommendations but inappropriate for the system in which it’s installed), (5) too many fittings added to a suction pipe and (6) debris intrusion (often a consequence of inadequate filter cleaning & maintenance).  Cavitation is a function of speed and in devices such as slow-speed propellers (such as those in many marine applications), cavitation is not an issue, thus the frequent use of light, efficient, thin blades.

Propeller

Propeller (pronounced pruh-pel-er)

(1) A person or thing that propels.

(2) A device with a hub to which are attached evenly spaced & shaped radiating blades, rotating on a shaft to pitch against air or water to propel an aircraft, ship etc.

(3) A wind-driven (usually three-bladed) device that provides mechanical energy, as for driving an electric alternator in wind plants (not a universal use).

(4) A steamboat thus propelled; a screw steamer (now rare).

(5) In fishing, a spinnerbait.

1780: The construct was propel + -er and the original sense was “one who or that which that propels”, an agent noun from the verb propel.  The verb propel was a mid-fifteenth century form from the Middle English propellen (to drive away, expel), from the Latin propellere (push forward, drive forward, drive forth; move, impel), the construct being pro- (the prefix here use in the sense of “forward direction, forward movement”) + pellere (to push, drive), from the primitive Indo-European root pel- (to thrust, strike, drive).  The meaning “to drive onward, cause to move forward” emerged in the 1650s.  The –er suffix was from the Middle English –er & -ere, from the Old English -ere, from the Proto-Germanic -ārijaz, thought most likely to have been borrowed from the Latin –ārius where, as a suffix, it was used to form adjectives from nouns or numerals.  In English, the –er suffix, when added to a verb, created an agent noun: the person or thing that doing the action indicated by the root verb.   The use in English was reinforced by the synonymous but unrelated Old French –or & -eor (the Anglo-Norman variant -our), from the Latin -ātor & -tor, from the primitive Indo-European -tōr.  When appended to a noun, it created the noun denoting an occupation or describing the person whose occupation is the noun.  The alternative spelling propellor dates from the early days of aviation in the first years of the twentieth century and is now extinct.  The standard abbreviation is “prop”, the use noted from military aviation since 1914.  Propeller is a noun; the noun plural is propellers.

Although the concept was used in antiquity and inventors and others (most famously Leonardo da Vinci (1452–1519))  had for centuries experimented, the use of the word in mechanical engineering dates from 1809 and was from nautical design describing the application of a “device for moving vessels on or under the water”.  In aircraft design the theory of the use of “propeller” appears in papers and drawings in the 1840s (in what were then described as “flying machines”) and models were built which demonstrated a “proof of concept” although it would be decades before lightweight engines of sufficient power existed to allow experiments in aerodynamics and construction to be powered.  The first known rendering of an aircraft propeller in a recognizably modern form dates from 1853.  The modern propeller uses two or (usually) more twisted, airfoil-shaped blades mounted around a shaft which are spun to provide propulsion of a vehicle through water or air, or to cause fluid flow, as in a pump.  The lift generated by the spinning blades provides the force that propels the vehicle or the fluid although this lift does not of necessity have to induce an actual upward force; its direction is simply parallel to the rotating shaft.

Lindsay Lohan getting off the propeller driven (technically a turbo-prop) NAPA Shuttle, The Parent Trap (1998).

The term “to disembark” was borrowed from nautical use and of late "to deplane" has entered English which seems unnecessary but the companion “to disemplane” seems more absurd still; real people continue to “get on” and “get off” aircraft.

The terms “impeller” & “propeller” both describe devices which use various implantations of the “rotating blade(s) design and are used in mechanical systems to take advantage of the properties of fluid dynamics to harness specific energy for some purpose.  A propeller is a type of rotating device with blades designed to propel or move a fluid (typically a gas or a liquid) by generating thrust; they are most associated with marine vessels, aircraft and some industrial applications.  In aircraft, propellers can be attached to wing-mounted engines or mounted just about anywhere on a fuselage although historically a location at the front has been most common.  In marine applications, propellers have on specialized vessels been located to the sides of the hull but they almost always emerge at or close to the stern.  An impeller is a rotating component with blades or vanes (almost always enclosed in a housing), typically used for fluid or air distribution, such as a pump or a compressor, the primary purpose being to increase flow or pressure.  The classic impellers those in centrifugal pumps where they spin, creating a flow of fluid (liquid or air) by imparting centrifugal force to the substance; in practice, impellers such accelerate liquids are more common.

So an impeller & propeller do much the same thing, using blades to propel some form of fluid.  The use of different terms is helpful because in practice they are very different devices and the distinction that one is external and the other located within a housing is handy and the origin of that seems to lie in the construct of impeller which came first, dating from circa 1680 (as an agent noun from the verb impel) in the sense of “someone or something which impels”.  What the design of an impeller does is use the energy from the rotation to increase the flow or pressure of the fluid and it that it’s the reverse of a turbine, the rotation of which extracts energy from, and reduces the pressure of the flow.  Engineers also have a number of highly technical rules about what is and is not defined as an impeller base on the whether the entry and exit of the fluids occur axially or radially but it seemed impossible to construct such definitions as absolutes so for most the simpler distinctions are more helpful.  In engineering, impellers have been recorded as a machine or component name since 1836.

News Corp website 22 January 2024.  To refer to a jet engine’s nacelle as a propeller could (almost) be defended on the basis it’s the jet engine which “propels” the aircraft but this is more likely an example of (1) the decline in the quality of journalists and (2) what happens when there are no sub-editors to correct the mistakes.  In time, artificial intelligence (AI) should improve things.    

The verb impel dates from the early fifteenth century and was from the Middle English impellen, from the Latin impellere (to push, strike against; set in motion, drive forward, urge on), the construct an assimilated form of in- (into, in, on, upon), from the primitive Indo-European root en- (in) + pellere (to push, drive), from the primitive Indo-European root pel- (to thrust, strike, drive).  The construct of the Latin impellō was in- +‎ pellō (push, drive), from the Proto-Italic pelnō or pelnaō, a nasal-infix present derived from the primitive Indo-European pelh- (to drive, strike, thrust).  The Latin prefix –in could be appended to create a negative (un-, non-, not etc) but here was used as an intensifier, another possible meaning (in, within, inside) coincidental to the mechanical devices being usually mounted within housings.

Propellers and impellers both use blades (although those of the latter are often in the form of a single piece wither cast, molded, or (occasionally) forged.  Turbines also use blade-like parts but these are called vanes and an industry which seems unable to decide on terminology is the burgeoning business of wind-power; the huge rotating assemblies on wind turbines are referred to variously as vanes, blades or rotors.  Rotor blades are familiar for the use in helicopters which is essentially an airframe where a large-scale propeller sits atop the structure, pointing upwards and rather than “propeller blades”, the accepted term is “rotor blades”, the design of which permits both lift and directional thrust although some exotic multi-engined machines have rotors in housings which, to maximize performance, can themselves be rotated to operate as conventional propellers.

Supermarine Seafang (1946) with contra-rotating propellers.  The Seafang was powered by the Rolls-Royce Griffon and was the final evolution of the Spitfire-derived Seafire and Spiteful, the trio all designed for use on Royal Navy aircraft carriers, the series enjoying success despite the basic design being hampered by the narrow undercarriage which made landings a challenge (something corrected on the Spiteful & Seafang).  Series production of the Seafang was contemplated but eventually only 18 were built because the jet-powered de Havilland Sea Vampire proved capable of carrier operations, surprising some at the Admiralty who doubted the jets could operate from anywhere but land.

The evolution of aircraft influenced propellers.  Once they had been fashioned from wood before the need for faster, more efficient shapes dictated the use of aluminium or other light metals.  By the time the first modern monoplane fighters appeared in the mid 1930s propellers were still two-bladed but as power increased over the years (something which accelerated during World War II (1939-1945)), three, four and five-bladed solutions were engineered.  The rising output however, although it permitted higher performance, created challenges for engineers, notably the “torque effect” which meant a tendency to cause the aircraft to roll in the direction of the propeller’s spin, a problem especially serious during take-offs.  In twin-engined aircraft the solution was to have the propellers rotate in opposite directions but in airframes with a single power-plant, sometimes used were contra-rotating propellers which, although introducing additional complexity and demanding additional maintenance, did offer advantages including: (1) harnessing more of an engine’s power, (2) increased thrust efficiency by a reduction in energy losses, (3) counteracting the torque effect, (4) improved low-speed manoeuvrability and ground-handling and (5) improved acceleration and climbing performance.

A flight of Republic P-47D Thunderbolts with under-wing drop-tanks.

The propeller also influenced other aspects of the aircraft.  When the prototype Republic P-47 Thunderbolt (1941-1945) first took to the air, it was the largest, heaviest single-seat piston-engined fighter ever produced (a distinction it still enjoys today).  Even the early versions used an engine rated at 2000 horsepower (later this would rise to 2800) and to harness this output demanded a large propeller.  The 12 foot (3.7 m) diameter of this four-bladed monster meant the landing-gear had to be extraordinarily long and the only way it could be accommodated was to have them retract inward, otherwise the heavy wing armament (8 x .50 inch (12.7 mm) M2 Browning machine guns (425 rounds per gun)) wouldn’t have fitted.

Chrysler XI-2220 V16.  The splined shaft is where the propeller attaches.

With things like the Thunderbolt, the Hawker Tempest and the later Supermarine Spitfires (and its derivatives), the piston-engined fighter achieved its final evolutionary form, the jet engine offering a path to performance unattainable while the physics of propellers imposed limits.  However, had the use of the A-Bombs not ended the war in 1945, development of the propeller aircraft would have continued because the early jets lacked thrust and reliability as well as suffering a rate of fuel consumption which rendered them unsuitable for long-distance operations.  With the war against Japan envisaged as lasting well into 1946, development of faster, more powerful piston engines continued although, given the parlous state of the Japanese military, it’s dubious at least there was much of a rationale for this but the military industrial complex is a creature of inertia and Chrysler’s research had perfected a new aero-engine for the Thunderbolt.  The XI-2220 was a 2,220 cubic inch (36.4 litre) V16 which was rated at a basic 2450 horsepower with some 4000 hp available when tuned for wartime use but with the end of the conflict, all such developments were cancelled and attention switched to the brave new world of jets and swept wings.  Thus ended the era of the big propeller-driven fighters, the V16 stillborn, as was the other extraordinary aero-engine on the drawing board: Britain's 32-cylinder Napier-Sabre H-32 which was a scaled-up version of their H24.

Hardtop, Hard Top & Hard-top

Hardtop & Hard Top or Hard-Top ( pronounced hahrd-top)

(1) In automotive design, as hardtop, a design in which no centre post (B-pillar) is used between the front and rear windows.

(2) As "hard top" or "hard top", a rigid, removable or retractable roof used on convertible cars (as distinct from the historically more common folding, soft-top).

(3) Mid twentieth-century US slang for an indoor cinema with a roof (as opposed to a drive-in).

1947-1949: A compound of US origin, hard + top.  Hard was from the Middle English hard, from the Old English heard, from the Proto-West Germanic hard(ī), from the Proto-Germanic harduz, from the primitive Indo-European kort-ús, from kret- (strong, powerful).  It was cognate with the German hart, the Swedish hård, the Ancient Greek κρατύς (kratús), the Sanskrit क्रतु (krátu) and the Avestan xratu.  Top was from the Middle English top & toppe, from the Old English top (top, highest part; summit; crest; tassel, tuft; (spinning) top, ball; a tuft or ball at the highest point of anything), from the Proto-Germanic tuppaz (braid, pigtail, end), from the primitive Indo-European dumb- (tail, rod, staff, penis).  It was cognate with the Scots tap (top), the North Frisian top, tap & tup (top), the Saterland Frisian Top (top), the West Frisian top (top), the Dutch top (top, summit, peak), the Low German Topp (top), the German Zopf (braid, pigtail, plait, top), the Swedish topp (top, peak, summit, tip) and the Icelandic toppur (top).

1970 Imperial LeBaron four-door hardtop.

Although the origins of the body-style can be traced to the early twentieth century, the hardtop, a two or four-door car without a central (B-pillar) post, became a recognizable model type in the late 1940s and, although never the biggest seller, was popular in the United States until the mid 1970s when down-sizing and safety legislation led to their extinction, the last being the full sized Chrysler lines of 1978.  European manufacturers too were drawn to the style and produced many coupes but only Mercedes-Benz and Facel Vega made four-door hardtops in any number, the former long maintaining several lines of hardtop coupés.

1965 Lincoln Continental four-door sedan (with centre (B) pillar).

The convention of use is that the fixed roofed vehicles without the centre (B)-pillars are called a hardtop whereas a removable or retractable roof for a convertible is either a hard top or, somewhat less commonly a hard-top.  The folding fabric roof is either a soft top or soft-top, both common forms; the word softtop probably doesn't exist although it has been used by manufacturers of this and that to describe various "tops" made of stuff not wholly solid.  In the mid-1990s, the decades-old idea of the folding metal roof was revived as an alternative to fabric.  The engineering was sound but some manufacturers have reverted to fabric, the advantages of solid materials outweighed by the drawbacks of weight, cost and complexity.  A solid, folding top is usually called a retractable roof or folding hardtop.

1957 Ford Fairlane Skyliner.

Designers had toyed with the idea of the solid retractable roof early in the twentieth century, and patents were applied for in the 1920s but the applications were allowed to lapse and it wouldn't be until 1932 one was granted in France, the first commercial release by Peugeot in 1934.  Other limited-production cars followed but it wasn't until 1957 one was sold in any volume, Ford's Fairlane Skyliner, using a system Ford developed for the Continental Mark II (but never used) was an expensive top-of-the range model for two years.  It was expensive for a reason: the complexity of the electric system which raised and lowered the roof.  A marvel of what was still substantially the pre-electronic age, it used an array of motors, relays and switches, all connected with literally hundreds of feet of electrical cables in nine different colors.  Despite that, the system was reliable and could, if need be, be fixed by any competent auto-electrician who had the wiring schematic.  In its two-year run, nearly fifty-thousand were built.  The possibilities of nomenclature are interesting too.  With the hard top in place, the Skyliner becomes also a hardtop because there's no B pillar so it's a "hardtop" with a "hard top", something only word-nerds note. 

2005 Mercedes-Benz AMG SLK55.

After 1960, the concept was neglected, re-visited only by a handful of low-volume specialists or small production runs for the Japanese domestic market.  The car which more than any other turned the retractable roof into a mainstream product was the 1996 Mercedes-Benz SLK which began as a show car, the favorable response encouraging production.  Successful, over three generations, it was in the lineup for almost twenty-five years.

Roof-mounted hardtop hoist: Mercedes-Benz 560 SL (R107).

The Fairlane Skyliner's top was notable for another reason: size and weight.  On small roadsters, even when made from steel, taking off and putting on a hard top could usually be done by someone of reasonable strength, the task made easier still if the thing instead was made from aluminum or fibreglass.  If large and heavy, it became impossible for one and difficult even for two; some of even the smaller hard tops (such as the Triumph Stag and the R107 Mercedes SL roadster) were famous heavyweights.  Many owners used trolley or ceiling-mounted hoists, some even electric but not all had the space, either for the hardware or the detached roof.

1962 Pontiac Catalina with Riveria Series 300 hard top.

No manufacturer attempted a removable hard top on the scale of the big Skyliner but at least one aftermarket supplier thought there might be demand for something large and detachable.  Riveria Inc, based in Cedar Rapids, Iowa, offered them between 1963-1964 for the big (then called full or standard-size) General Motors (GM) convertibles.  Such was GM’s production-line standardization, the entire range of models, spread over five divisions and three years could be covered by just three variations of hard top.  Made from fibreglass with an external texture which emulated leather, weight was a reasonable 80 lb (30 kg) but the sheer size rendered them unmanageable for many and not all had storage for such a bulky item, the growth of the American automobile meaning garages accommodative only a few years early were now cramped.    

1962 Chevrolet Impala Super Sport with Riveria Series 100 hard top.

Riveria offered their basic (100 series) hardtop in black or white, a more elaborately textured model (200 series) finished in gold or silver while the top of the range (300 series) used the same finishes but with simulated “landau” irons.  No modification was required to the car, the roof attaching to the standard convertible clamps, the soft top remaining retracted.  Prices started at US$295 and the company seems to have attempted to interest GM dealers in offering the hard tops as a dealer-fitter accessory but corporate interest must have been as muted as buyer response, Riveria ceasing operations in 1964.

1961 Lincoln Continental four-door hardtop (pre-production or prototype).

One of the anomalies in the history of the four-door hardtops was that Lincoln, in its classic 1960s Continental, offered a two-door hardtop, a four-door pillared sedan and, by then uniquely, a four-door convertible but, no four-door hardtop.  That seemed curious because the structural engineering required to produce a four door hardtop already existed in the convertible coachwork and both Ford & Mercury had several in their ranges, as did all other comparable manufacturers.  A four-door hard top was planned, the factory’s records indicate a handful were built (which were either prototypes or pre-productions vehicles) and photographs survive, as does the odd reference to the model in some later service bulletins but there’s no evidence any ever reached public hands.  Collectors chase rarities like this but they’ve not been seen in sixty years so it’s presumed they were scrapped once the decision was taken not to enter production.

1966 Lincoln Continental two-door hardtop.

The consensus among Lincoln gurus is the rationale for decision being wholly because of cost.  While the Edsel failure of the late 1950s is well storied, it’s often forgotten that nor were the Lincolns of that era a success and, with the Ford Motor Company suddenly being run by the MBA-type “wizz kids”, the Lincoln brand too was considered for the axe.  It did come close to that, Lincoln given one last chance at redemption, using what was actually a prototype Ford Thunderbird; that was the car which emerged as the memorable 1961 Lincoln.  But there was no certainty of success so it seems the decision was taken to restrict the range to the pillared sedan and the four-door convertible, a breakdown on the production costs of the prototype four-door hardtops proving they would be much more expensive to produce, the added inputs both of labour and materials dooming the project.  To attract attention, Lincoln anyway had something beyond the merely exclusive, they had the unique four-door convertible.

1967 Lincoln Continental four-door convertible.

It did work, sales volumes after a slow start in 1961 growing to a level Lincoln had not enjoyed in years, comfortably out-selling Imperial even if never a challenge to Cadillac.  Never a big seller, achieving not even four-thousand units in its best year, the four-door convertible was discontinued after 1967, the two-door hardtop introduced the year before out-selling it by five to one.  The market had spoken; it would be the last convertible Lincoln ever produced.

Deconstructing the oxymoronic  "pillared hardtop"

1970 Ford LTD four-door hardtop (left) and Ford's press release announcing the 1974 "pillared hardtop", September 1973. 

So it would seem settled a hard-top is a convertible’s removable roof made with rigid materials like metal or fibreglass while a hardtop is a car with no central pillar between the forward and rear side glass.  That would be fine except that in the 1970s, Ford decided there were also “pillared hardtops”, introducing the description on a four-door range built on their full-sized (a breed now extinct) corporate platform shared between 1968-1978 by Ford and Mercury.  The rationale for the name was that to differentiate between the conventional sedan which used frames around the side windows and the pillared hardtops which used the frameless assemblies familiar from their use in the traditional hardtops.  When the pillared hardtops were released, as part of the effort to comply with pending rollover standards, the two door hardtop switched to being a coupé with thick B-pillars, behind which sat a tall “opera window”, another of those motifs the US manufacturers for years found irresistible.

1976 Cadillac Eldorado convertible, at the time, “the last American convertible”.  The aluminium wheels were a rarely ordered factory option.  On paper, combining a 500 cubic inch (8.2 litre) V8 with front wheel drive (FWD) sounds daft but even in the early, more powerful, versions GM managed remarkably well to tame the characteristics inherent in such a configuration.  The styling of the original FWD Eldorado (1967) was one of the US industry's finest (as long as buyers resisted ordering the disfiguring vinyl roof) which no subsequent version matched, descending first to the baroque before in the 1980s becoming an absurd caricature.

When ceasing production of the true four-door hardtops, Ford also dropped the convertible from the full-sized line, the industry orthodoxy at the time that a regulation outlawing the style was imminent, and such was the importance of the US market that expectation that accounted also for Mercedes-Benz not including a cabriolet when the S-Class (W116) was released in 1972, leaving the SL (R107; 1971-1989) roadster as the company’s only open car and it wasn’t until 1990 a four-seat cabriolet returned with the debut of the A124.  Any suggestion of outlawing convertibles ended with the election in 1980 of Ronald Reagan (1911-2004, US president 1981-1989).  A former governor of California with fond memories of drop-top motoring and a world-view that government should intervene in markets as little as possible, under his administration, convertibles returned (including Cadillacs) to US showrooms.  In the even then litigious US, that prompted a class action from disgruntled collectors who had stored 1976 Cadillac Eldorados with the expectation of them increasing sharply in value, the suit filed alleging a “breach of promise” on the basis of Cadillac advertising the things as “the last American convertible”.  Historically, breach of promise actions were most associated with women seeking redress against cads who promised marriage and then refused to fulfil the pledge (an action still technically available in some US states) but the courts quickly dismissed the claims of the Cadillac hoarders as “groundless”.  Legal opinion at the time was that the suit might have had a chance had the words been "the last Cadillac convertible" and even then it would have had to withstand (1) the precedents which underpinned the notion of what in contract law was called "mere puffery" and (2) the then still prevalent sentiment that "what was good for General Motors was good for America", something which critics noted was still a detectable feeling among US judges.

1966 Lincoln Continental Sedan (left) and 1974 Buick Century Luxus Colonnade Hardtop Sedan (right).  Luxus was from the Latin luxus (extravagance) and appeared in several Germanic languages where it conveyed the idea of "luxury".   

It was actually only the ostensibly oxymoronic nomenclature which was novel, Ford’s Lincoln Continentals combining side windows with frames which lowered into the doors and a B pillar; Lincoln called these a sedan, then the familiar appellation in the US for all four-door models with a centre pillar.  Curiously, in the 1960s another descriptive layer appeared (though usually not used by the manufacturers): “post”.  Thus where a range included two-door hardtops with no pillar a coupés with one, there was among some to adopt “coupé” and “post coupé” as a means of differentiation and this spread, the term “post sedan” also still seen today in the collector markets.  Other manufacturers in the 1970s also used the combination of frameless side glass and a B-pillar but Ford’s adoption of “pillar hardtop was unique; All such models in General Motors’ (GM) “Colonnade” lines were originally described variously as “colonnade hardtop sedans” (Buick) or “colonnade hardtops” (Chevrolet, Oldsmobile & Pontiac) and the nickname was borrowed from architecture where colonnade refers to “a series of regularly spaced columns supporting an entablature and often one side of a roof”.  For whatever reasons, the advertising copy changed over the years, Buick shifting to “hardtop sedan”, Chevrolet & Oldsmobile to “sedan” and Pontiac “colonnade hardtop sedan”.  Pontiac was the last to cling to the use of “colonnade”; by the late 1970s the novelty has passed and the consumer is usually attracted by something “new”.  Because the GM range of sedans had for uprights (A, B & C-pillars plus a divided rear glass), the allusion was to these as “columns”.  Ford though, was a little tricky.  Their B-pillars were designed in such as way that the thick portion was recessed and dark, the silver centrepiece thin and more obvious, so with the windows raised, the cars could be mistaken for a classic hardtop.  It was a cheap trick but it was also clever, in etymological terms a “fake hardtop” but before long, there was a bit of a vogue for “fake soft-tops” which seems indisputably worse.

1975 Imperial LeBaron (left) and 1978 Chrysler New Yorker.  The big Chryslers were the last of the four-door hardtops to be produced in the US.

The Americans didn’t actually invent the pillarless hardtop style but in the post-war years they adopted it with gusto.  The other geo-centre of hardtops was the JDM (Japanese Domestic Market) which refers to vehicles produced (almost) exclusively for sale within Japan and rarely seen beyond except in diplomatic use, as private imports, or as part of the odd batch exported to special markets.  As an ecosystem, it exerts a special fascination for those who study the Japanese industry.  The range of high performance versions and variations in coachwork available in the JDM was wide and for those with a fondness for Japanese cars, the subject of much envy.  By the late 1970s, the handful of US four-door hardtops still on sale were hangovers from designs which dated from the late 1960s, behemoths anyway doomed by rising gas prices and tightening emission controls; with the coming of 1979 (coincidently the year of the “second oil shock”) all were gone.  In the JDM however, the interest remained and endured into the 1990s.

1965 Chevrolet Corvair Corsa (two-door hardtop, left) and 1969 Mazda Luce Coupé (right).

The first Japanese cars to use the hardtop configuration were two-door coupés, the Toyota Corona the first in 1965 and Nissan and Mitsubishi soon followed.  One interesting thing during the era was the elegant Izuzu 117 Coupé (1968-1980), styled by the Italian Giorgetto Giugiaro (b 1938) which, with its slender B-pillar, anticipated Ford’s stylistic trick although there’s nothing to suggest this was ever part of the design brief.  Another of Giugiaro’s creations was the rare Mazda Luce Coupé (1968), a true hardtop which has the quirk of being Mazda’s only rotary-powered car to be configured with FWD.  Giugiaro’s lines were hardly original because essentially they duplicated (though few suggest "improved") those of the lovely second generation Chevrolet Corvair (1965-1969) and does illustrate what an outstanding compact the Corvair could have been if fitted with a conventional (front-engine / rear wheel drive (RWD)) drive-train.

1973 Nissan Cedric four-door Hardtop 2000 Custom Deluxe (KF230, left) and 1974 Toyota Crown Royal Saloon four-door Pillared Hardtop (2600 Series, right).

By 1972, Nissan released a version of the Laurel which was their first four-door although it was only the volume manufacturers for which the economics of scale of such things were attractive, the smaller players such as Honda and Subaru dabbling only with two-door models.  Toyota was the most smitten and by the late 1970s, there were hardtops in all the passenger car lines except the smallest and the exclusive Century, the company finding that for a relatively small investment, an increase in profit margins of over 10% was possible.  Toyota in 1974 also followed Ford’s example in using a “pillared hardtop” style for the up-market Crown, the exclusivity enhanced by a roofline lowered by 25 mm (1 inch); these days it’s be called a “four door coupé” (and etymologically that is correct despite the objections of many) and Rover had actually planned their 3.5 (P5B; 1967-1973) to include a four-door coupé featuring both pillarless construction and the lowered roof; as it was the former proved too difficult within the budget so only the chop-top survived.  In the JDM, the last true four-door hardtops were built in the early 1990s but Subaru continued to offer the “pillared hardtop” style until 2010 and the extinction of the breed was most attributable to the shifting market preference for sports utility vehicles (SUV) and such.  In Australia, Mitsubishi between between 1996-2005 used frameless side-windows and a slim B-pillar on their Magna so it fitted the definition of a “pillar hardtop” although the term was never used in marketing, the term “hardtop” something Australians associated only with two-door coupés (Ford and Chrysler had actually the term as a model name in the 1960s & 1970s).  When the Magna was replaced by the doomed and dreary 380 (2005-2008), Mitsubishi reverted to window frames and chunky pillars.

Standard and Spezial coachwork on the Mercedes-Benz 300d (W189, 1957-1962).  The "standard" four-door hardtop was available throughout the run while the four-door Cabriolet D was offered (off and on) between 1958-1962 and the Spezials (landaulets, high-roofs et al), most of which were for state or diplomatic use, were made on a separate assembly line in 1960-1961.  The standard "greenhouse" (or glasshouse) cars are to the left, those with the high roof-line to the right.

Few European manufacturers attempted four-door hardtops and one of the handful was the 300d (W189, 1957-1962), a revised version of the W186 (300, 300b & 300c; 1951-1957) which came to be referred to as the "Adenauer" because several were used as state cars by Konrad Adenauer (1876–1967; chancellor of the FRG (West Germany) 1949-1963).  Although the coachwork never exactly embraced the lines of mid-century modernism, the integration of the lines of the 1950s with the pre-war motifs appealed to the target market (commerce, diplomacy and the old & rich) and on the platform the factory built various Spezials including long wheelbase "pullmans", landaulets, high-roof limousines and four-door cabriolets (Cabriolet D in the Daimler-Benz system).  The high roofline appeared sometimes on both the closed & open cars and even then, years before the assassination of John Kennedy (JFK, 1917–1963; US president 1961-1963), the greenhouse sometimes featured “bullet-proof” glass.  As well as Chancellor Adenauer, the 300d is remembered also as the Popemobile (although not then labelled as such) of John XXIII (1881-1963; pope 1958-1963).

Few European manufacturers attempted four-door hardtops.  One of the few was the Mercedes-Benz 300d (W189, 1957-1962), a revised version of the W186 (1951-1957) which came to be referred to as the "Adenauer" because several were used as state cars by Konrad Adenauer (1876–1967; chancellor of the FRG (Federal Republic of Germany (the old West Germany) 1949-1963).  Although the coachwork never embraced 1950s modernism, the W186 & W189 obviously an evolution of pre-war practices, much of the engineering was advanced and the factory used the chassis to produce spezials including long wheelbase "pullmans", landaulets, high-roof limousines and four-door cabriolets (Cabriolet D in the Daimler-Benz system).  The W189 is remembered too as the state car of the Holy See, used by popes in the days before fears of assassination.  Most however were the "standard", four-door hardtop.

Pillars, stunted pillars & "pillarless"

1959 Lancia Appia Series III

Actually, although an accepted part of engineering jargon, to speak of the classic four-door hardtops as “pillarless” is, in the narrow technical sense, misleading because almost all used a truncated B-pillar, ending at the belt-line where the greenhouse begins.  The stunted device was required to provide a secure anchor point for the rear door's hinges (or latches for both if suicide doors were used) and in the case of the latter, being of frameless construction, without the upright, the doors would be able to be locked in place only at the sill, the physics of which presents a challenge because even in vehicles with high torsional rigidity, there will be movement.  The true pillarless design was successfully executed by some but those manufacturers used doors with sturdy window frames, permitting latch points at both sill and roof, Lancia offering the configuration on a number of sedans including the Ardea (1939-1953), Aurelia (1950-1958) & Appia (1953-1963).  The approach demanded a more intricate locking mechanism but the engineering was simple and on the Lancias it worked and was reliable, buyers enjoying the ease of ingress & egress.  It's sad the company's later attachment to front wheel drive (FWD) ultimately doomed Lancia because in every other aspect of engineering, few others were as adept at producing such fine small-displacement vehicles.

1961 Facel Vega II (a two door hardtop with the unusual "feature" of the rear side-glass being hinged from the C-pillar).

Less successful with doors was the Facel Vega Excellence, built in two series between 1958-1964.  Facel Vega was a French company which was a pioneer in what proved for almost two decades the interesting and lucrative business of the trans-Atlantic hybrid, the combination of stylish European coachwork with cheap, refined, powerful and reliable American engine-transmission combinations.  Like most in the genre, the bulk of Facel Vega’s production was big (by European standards) coupés (and the odd cabriolet) and they enjoyed much success, the company doomed only when it augmented the range with the Facellia, a smaller car.  Conceptually, adding the smaller coupés & cabriolets was a good idea because it was obvious the gap in the market existed but the mistake was to pander to the feelings of politicians and use a French designed & built engine which proved not only fragile but so fundamentally flawed rectification was impossible.  By the time the car had been re-engineered to use the famously durable Volvo B18 engine, the combination of the cost of the warranty claims and reputational damage meant bankruptcy was impending and in 1964 the company ceased operation.  The surviving “big” Facel Vegas, powered by a variety of big-block Chrysler V8s, are now highly collectable and priced accordingly.

1960 Facel Vega Excellence EX1

Compared with that debacle, the problem besetting the Excellence was less serious but was embarrassing and, like the Facellia's unreliable engine, couldn’t be fixed.  The Excellence was a four-door sedan, a configuration also offered by a handful of other trans-Atlantic players (including Iso, DeTomaso & Monteverdi) and although volumes were low, because the platforms were elongations of those used on their coupés, production & development costs were modest so with high prices, profits were good.  Facel Vega however attempted what no others dared: combine eye-catching suicide doors, frameless side glass and coachwork which was truly pillarless, necessitating latching & locking mechanisms in the sills.  With the doors open, it was a dramatic scene of lush leather and highly polished burl walnut (which was actually painted metal) and the intricate lock mechanism was precisely machined and worked well… on a test bench.  Unfortunately, on the road, the pillarless centre section was inclined slightly to flex when subject to lateral forces (such as those imposed when turning corners) and this could release the locks, springing the doors open.  Owners reported this happening while turning corners and it should be remembered (1) lateral force increases as speed rises and (2) this was the pre-seatbelt era.  There appear to be no confirmed reports of unfortunate souls being ejected by centrifugal force through an suddenly open door (the author Albert Camus (1913–1960) was killed when the Facel Vega HK500 two-door coupé in which he was a passenger hit a tree, an accident unrelated to doors) but clearly the risk was there.  Revisions to the mechanism improved the security but the problem was never completely solved; despite that the factory did offer a revised second series Excellence in 1961, abandoning the dog-leg style windscreen and toning down the fins, both of which had become passé but in three years only a handful were sold.  By the time the factory was shuttered in 1964, total Excellence production stood at 148 EX1s (Series One; 1958-1961) & 8 EX2s (Series Two; 1961-1964).

The Mercedes-Benz R230 SL: Lindsay Lohan going topless (in an automotive sense) in 2005 SL 65 AMG with top lowered (left), 2006 SL 65 AMG with top erected (centre) & 2009 SL 65 AMG Black Series with fixed-roof.

At the time uniquely in the SL line, the R230 (2001-2011) was available with both a retractable hard top and with a fixed roof but no soft top was ever offered (the configuration continued in the R231 (2012-2020) while the R232 (since 2021) reverted to fabric).  Having no B pillar, most of the R230s were thus a hardtop with a hard top but the SL 65 AMG Black Series (2008–2011) used a fixed roof fabricated using a carbon fibre composite, something which contributed to the Black Series is weighing some 250 kilograms (550 lb) less than the standard SL 65 AMG.  Of the road-going SLs built since 1954, the Black Series R230 was one of only three models sold without a retractable roof of some kind, the others being the original 300 SL Gullwing (W198, 1954-1957) and the “California coupé” option offered between 1967-1971 for the W113 (1963-1971) roadster (and thus available only for the 250 SL (1966-1968) & 280 SL (1967-1971)).  The California coupé was simply an SL supplied with only the removable hard top and no soft top, a folding bench seat included which was really suitable only for small children.  The name California was chosen presumably because of the association of the place with sunshine and hence a place where one could be confident it was safe to go for a drive without the fear of unexpected rain.  Despite the name, the California coupé was available outside the US (a few even built in right-hand drive form) although the North American market absorbed most of the production.

1969 Chevrolet Corvette L88 convertible with soft top (or soft-top) erected.

1969 Chevrolet Corvette L88 convertible with hard top (or hard-top) attached.

1969 Chevrolet Corvette L88 convertible, with soft-top lowered (ie topless).