Conception to Production

Some Data on the Sunbeam Rapier, Now Coming Off the Rootes’ Assembly Lines Coventry 

The new Sunbeam Rapier caused considerable interest at the last Earls Court Motor Show, and as this general-purpose, high-performance car went into production at the beginning of last month (H.R.H. the Duke of Kent being amongst the first to receive delivery) we decided the time had come to visit Coventry and glean some facts about this new product of the Rootes Group.

Accordingly, in the middle of March, we paid a visit to the Humber factory, contriving to average over 40 m.p.h. (and over 40 m.p.g.) on this journey, in our low-powered German car, although impeded by crawling ‘buses along Oxford’s tedious ring-road and by several single-line sections on the main Oxford-Banbury Road.

The approach to the Humber factory is along Humber Road, a fact which should delight members of the Humber Register.

It is a far cry from vintage Humbers to the Sunbeam Rapier, several of which we had encountered on test as we neared Coventry. The car has an attractively-compact two-door hard-top body in the Rootes “Gay-Look,” nicely offset by the chromium-ringed, hooded in-built headlamps.

The conception which brought the Rapier into being was that of an all-round high-performance car, possessing a more general appeal than its famous predecessor, the Mk. III Sunbeam, which is, perhaps, a car primarily for the enthusiast. It was felt that a small, compact five-seater saloon, outstanding in respect of speed and acceleration allied to good fuel consumption, easy on the eye and of modern appearance, would admirably meet the needs of world markets. A stylist of international repute planned the body lines round a modified o.h.v. Hillman Minx engine, an entirely new chassis/body structure, possibly slightly heavier but certainly stiffer than that of the Minx, being designed for the Rapier. This has only two transverse members, one located at the front of the back seat, the other under the front seats, embryo chassis/body sills extending forwards and backwards from these cross-members, deep foot-wells being used. This structure spaces the stresses equally, the propeller-shaft tunnel sharing this duty, and A-post weakness, inherent in many integral body/chassis structures, is avoided.

The employment of a modified Hillman Minx engine is an instance of skilful planning on the part of the Rootes Group engineers. This engine, in its latest o.h.v. form, was introduced only about eighteen months ago and it was designed to be fully capable of considerable development. For example, crankpins and bearing areas of this 1,390-c.c. engine are of practically the same dimensions as those of the 2.3-litre Sunbeam Mk. III engine. Thus as used in the Minx there is a very high safety margin ensuring long life and dependability, while the engine is suitable for development to high-performance form. The Rapier has much the same power/weight ratio as the Sunbeam Mk. III but, because the total weight is lower, an output of less than 60 b.h.p. suffices instead of 80 b.h.p. to provide equivalent performance. Consequently, the Minx engine, which has “square” dimensions of 76.2 mm. bore and stroke, and the aforesaid sturdy crankshaft and bearings, is working well within itself when developed for the Rapier, to give 57½ b.h.p. at 5,000 r.p.m.

Very few modifications were necessary to produce this power output. The compression-ratio is increased from 7 to 1 to 8 to 1, a Stomberg 1¹/_32-in. choke DIF36 carburetter is fitted in place of a Zenith, using modified rnanifolding, and a larger clutch is necessary to cope with the increased torque. The Minx valves and the Minx camshaft are retained, giving a valve timing of inlet opens 11 deg. b.t.d.e., inlet closes 55 deg. a.b.d.c., exhaust opens 53 deg. b.b.d.c. and exhaust. closes 13 deg. a.t.d.c. The Minx exhaust system is also retained, although this had been designed from the inception with high performance in mind. A Humber Hawk gearbox is used in unit with this engine, but this is turned over so as to bring the selector-mechanism on the top instead of at the side of the box, thus providing a deeper, stiffer casing and eliminating oil starvation at the front of the box when ascending hills.

A back-axle ratio of 5.22 to 1 was decided upon as offering a lively performance, together with an effortless cruising speed, in conjunction with a manually-operated small Laycock de Normanville overdrive applicable to third and top gears. Rootes may be said to have pioneered enthusiastically this new D-type epicyclic overdrive unit.

Suspension is by coil-springs and wishbones at the front, with a single-piece wheel carrier, and ½-elliptic leaf-springs at the back. The roll centre is, of course, closer to the centre of gravity than in the Minx. Experimentation resulted in spring-rates of 94.3 lb./in. at the front and 114 lb./in. at the back, the latter being changed to 143 lb./in. when the auxiliary damper blades of the ½-elliptic springs come into action over severe bumps. Rootes are firm believers in telescopic-type shock-absorbers, the lever-type having suffered breakages over rough tracks overseas, and Girling telescopic dampers are fitted all round on the Rapier. An anti-roll bar at the front was experimented with but was found to be unnecessary.

That, then, was the conception of the new Sunbeam Rapier. By simple modifications the Rootes engineers increased the output of the Hillman Minx 1.4-litre engine from 43 b.h.p. at 4,400 r.p.m. to 57½ b.h.p. at 5,000 r.p.m.. the b.m.e.p. being 130 lb./sq. in. at 3,000 r.p.m. and the torque 878 lb./in. at 3,000 r.p.m. These figures relate to the Sunbeam Rapier engine with ignition adjusted to optimum torque, on premium fuel, with air-cleaner, fan, dynamo and test-bed exhaust system in place but without the gearbox. At 1,000 r.p.m. the power developed is 10½ b.h.p., at 2,000 r.p.m. it is 27 b.h.p., 3,000 r.p.m. equals 41½ b.h.p. and 4,000 r.p.m. equals 53 b.h.p.

Initial testing, we were told, dates back some two years, and three prototype Rapiers were taken last year by a test team through Spain, Switzerland, Italy, etc. Buying local fuel, not always suited to the engine, the overall fuel consumption worked out at 30 m.p.g., which suggests that a feature of the production version will be extremely good fuel economy allied to performance of Mk. III standard.

Mr. B. B. Winter, Rootes’ Engineering Director, is a great advocate of the M.I.R.A. Testing Ground and prototype Sunbeam Rapiers were tested there exhaustively, as are subsequent production versions. Although the maximum speed claimed is 90 m.p.h., we believe this has been exceeded appreciably over the M.I.R.A. speed-course. Much rig-testing is done of chassis components, much of it to destruction, and on the day of our visit Norman Garrad, Competition Manager and Sales Manager of Sunbeam-Talbot Ltd., had called at the works, and left for London on his return from testing a production Rapier in Sweden. Just as soon as Mr. Garrad is convinced that the new Sunbeam is ready for competition work a Rapier team will be entered for the major rallies, probably this season, because the Rootes Group has great faith in the sales-value of competition successes.

Great attention has been paid to quiet running and, apart from normal sound damping, rubber insulation of scuttle, gearbox, etc., has been incorporated to obviate engine noise tunnelling back into the car.

Each production engine is bench-tested and required to give within 3 b.h.p. of the designed maximum power output, and each complete car receives a local road test. Production of the Rapier is planned to reach 250-300 cars a week and will almost certainly be double, possibly treble, the output of the Mk. Ill. as the assembly lines get into their stride. The engines are assembled at the Humber factory and the body pressings come from the Pressed Steel Company.

The foregoing information is that supplied in the course of an interview with members of the Rootes Group engineering staff. It is no longer possible to identify this latest Sunbeam with one man, as the later Wolverhampton Sunbeams were associated with Louis Coatalen and the later London Talbots with Georges Roesch. Mr. Winter controls a design team, of which Mr. G. W. M. Lush is Chief Experimental Engineer and Mr. T. Jump the Chief Designer of the Group’s small cars, while Mr. H. G. Payne is the Chief Body Engineer. Mr. Cutler, as Chief Designer of the Group’s medium-size cars, does not come into the Rapier scheme.

While we were at Coventry we were able to drive a Sunbeam Rapier for a few miles. In overdrive an indicated 70 m.p.h. is an effortless cruising speed at 3,500 r.p.m., and a speedometer 85 m.p.h., equal to roughly 4,500 r.p.m. in overdrive, came up along brief straights. Overdrive is manually controlled by a little lever protruding from the centre of the dash. Quite useful acceleration is available by merely selecting normal top from overdrive top. The steering-column gear-lever worked stiffly on the car we tried, which is unusual for a Rootes product. The car cornered well, tending to understeer. but seems softly sprung and there was appreciable tyre howl, while the steering is heavy. Some vibration was transmitted, it seemed, from the front wheels to the region of the scuttle. The driver’s seat is adjustable for height and holds the occupant securely. The instruments are very nicely arranged, speedometer and rev.counter being immediately in front of the driver, with pleasantly thin needles which move in the same plane, the speedometer being calibrated in both m.p.h. and k.p.h. A row of four small dials set above the main dash comprise oil gauge, ammeter, fuel gauge and water thermometer. Before the passenger is an under-dash lidded cubbyhole. The hand-brake lever is well located, horizontally on the right. and a stalk protruding from the off side of the steering column operates the “winkers.” The horn is actuated by a steering-wheel ring. The rather small pedals are biased rather over to the right of the seating position. The boot lid lifts to reveal very generous and accessible luggage space, shared by a vertically-mounted spare wheel. Dunlop “Cushion” tyres are standardised. Access to the rear seat of the two-door body is rendered easy by wide doors and front-seat backs which fold absolutely flat. There is a knee-height crash-pad below the dash. The Sunbeam Rapier feels a lightweight (we estimate it to weigh about 19½ cwt.) and, with its output of nearly 60 b.h.p. from the flexible short-stroke engine, should be able to stand on the merit of its performance/economy figures. At present the only body style will be the two-door hardtop, which, incidentally, lends itself better than any to the “Gay-Look” colour-scheme; nor is the present sturdy push-rod o.h.v. engine likely to be supplemented or replaced, we gather, by the twin o.h.c. Singer Hunter 75 power unit! The price of the new Sunbeam Rapier is £1,043 17s., inclusive of p.t. We look forward to endorsing this data and these brief impressions with an early road test of this interesting high-performance vehicle. — W.B.

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Individuality Essential in Brtish Designs

Below we publish a letter from a reader experienced in design in the U.S.A. as well as in this country who believes, as we do, that to secure sales in dollar-markets Britain should cease to imitate American styling and methods and revert to traditional individuality.

Sir,

I was recently in charge of the engineering laboratory of an American car manufacturer, and now have a similar position in this country, and count myself among the more fortunate of mortals.

It is usually unfair to generalise, but since the destiny of the American engineer is largely governed by the all-powerful female section of the buying public, he is the first to admit that his cars are designed for women, whereas it appears likely that British cars are created for the masculine taste. The power equipment is aimed at making the lot of the American lady less arduous, and the overhang and vast grille and bumpers at giving her the illusion of security. Suspensions, until very recently, have been solely developed towards the “million-dollar ride,” and bodies for sidewalk-distance eye appeal. Fortunately, the Industry is now beginning to realise that this design policy may have something to do with the appalling road casualty incidence and is seriously considering the matter of roadholding.

If one evaluates a car in terms of a machine intended for the safe, speedy conveyance of people over all types of road surface with the least nervous strain, then the normal Arnerican car will often fall far short of the normal British car. However, if the aim is a sitting-room on wheels with a gaudy, massive exterior, that is intended only for straight, wide, smooth roads, then the British car is a ghastly contraption.

If one is an engineer with a love of functional beauty and craftsmanship in both design and execution, and a hatred of sham and pretentiousness, then the American car can be the object of revulsion, though in fairness it must be admitted that some of the current British products evoke the same emotion.

The power unit and its output is a favourite subject for comparison, and for the benefit of those who feel outgunned by American advertised h.p., the truth of the matter is that actual  b.h.p. at the clutch on an average production engine is between 30 and 40 per cent. less than the catalogue figures. For example, a popular V8 takes 4½ litres to produce 110 b.h.p. and is stated to give 162 b.h.p. With “hop-up” equipment claimed to give 185 b.h.p. it will reach 122 b.h.p. On the other hand, a well-known British sports-car engine of 1¼ litres claimed to give 54 b.h.p., not only comes within 4 per cent. either side of this figure but is always mounted on the dynamometer with the gearbox, so that the claimed figure includes the transmission-loss.

The matter can be summarised by observing that if you have a lot of money and a lot of metal you can afford to design inefficiently, whereas if your circumstances are the reverse you have to do the most with the least, and relatively we do, which academically means that we make better machines.

.A very large number of Americans are acutely conscious of this situation and heartily deplore it. They largely constitute our dollar-market, and they part with their dollars not because of any magical association with the word “British,” but because they appreciate quality. If our Industry persists in its present policy of imitating American styling and methods we shall lose that market. Given the choice of junk they will buy their own and rightly, because nobody can make better junk than the Americans. Let us live up to our traditional boast of good design, good quality, good craftsmanship. We shall then continue to eat.

Iam, Yours, etc., E. C. Martin. Besselsleigh.