Tuesday, 8 October 2013

Saab 92 aerodynamics

Talking, as I have been, about manufacturers’ publicity pictures, I always liked cutaways. It’s the engineer in me. Nothing ever conveyed a car’s structure like a good graphic. The Motor road tests used to do it well, showing how the mechanical bits of a car fitted in with rather stiff-looking occupants. The Saab 92 (left), produced from 1950-1955, makes the point. You can see where the tiny engine leans forward ahead of the front wheels, which it drives, and where the radiator is mounted to catch the air-flow. The upholstery looks a bit thin and rear people have to tuck their feet below the front seat.

In 1950 they made 1246 Saab 92s, every one the same shade of green. It was more important to get production started, and eliminate bottlenecks in the paintshop, than offer buyers a range of colours. It’s said the aircraft factory had over-stocked on green paint for its aeroplanes. Modest power propelled the little car at barely 100kph (60mph). There were only three gears and reaching 50mph occupied the best part of half-a-minute. It scarcely mattered; there was no shortage of customers in 1950.

Although slow, the Saab had clear-cut qualities. Encouraged by the success 2-stroke DKWs had in Sweden before the war it went for an engine cheap to make and simple. A parallel twin-cylinder 3-port 2-stroke of 764cc (80 x 76mm) with Schnürle scavenging, producing 24bhp at 3,800rpm, it was narrow enough to be set across the front with an aluminium head and flat-topped pistons. It had three main roller bearings, a built-up crankshaft with a pair of 180-degree-spaced crankpins, and three main bearing journals pressed into disc crankwebs. Only the small-end bearings were plain, not ball or roller. An extension from the crankshaft carried the cam for the dual-coil ignition, and lubrication was by 4 per cent oil added to the petrol.
A single-plate clutch and 3-speed gearbox, with synchromesh on third and top and a steering column gearshift, formed a unit with the engine. Engine mountings were unusual, a single transverse leaf spring supporting the forward part on rubber torque-resisting buffers, and a rubber cushion at the rear. The result was virtually vibrationless, especially at low revs.

Laurence Pomeroy, who had conducted experiments on aerodynamic cars at Brooklands in 1939, tested a 92 in 1950: “… a most interesting example of the type of car which emanates from an aircraft factory, and shows the benefits of clean lines by giving nearly 65mph (105kph) on less than 25bhp. Excellent roadholding and direct steering were also characteristic of this model, but, as is often the case with 2-strokes, the fuel consumption was not the best feature, failing to reach 40mpg (7.06l/100kms).”

Pomeroy’s advocacy of the slippery shape was only partly justified, for far from being worthy of an aircraft manufacturer, the Saab fell short of ideals laid down by German aerodynamics pioneer, Paul Jaray. Despite the classic teardrop shape, it had a thoroughly average air drag coefficient of 0.35. The bulbous front wings gave an unnecessarily large frontal area so the puny power had a lot of air to displace at 60mph (97kph). Had it been slimmer below the waistline, fuel consumption might have been better.

On the steering, however, Pomeroy was characteristically correct. The Saab’s rack and pinion took only 1.75 turns from lock to lock so it was high-geared, light, accurate and by comparison with nearly all its contemporaries (with worm and nut, cam and roller, recirculating balls, and other nightmares) sheer delight. Tactile, direct, strongly self-centring, drivers could feel road shocks but they could also feel what the wheels were doing, adding amply to the control that compensated for the car’s relative sloth.
In 1950 The Motor commented: “The Saab's layout is ingenious both in conception and in detail. Its unorthodoxy sets a reviewer a task which is difficult yet exceptionally interesting: difficult because of lack of standards for comparison, and interesting for revealing the gains and losses resulting from new layouts and construction methods.” The Motor wanted to be convinced. Its authors liked the principles Saab employed, but they were not finding the results entirely bore out their expectations.

It says something for the speed expected of a 1940s small car that they observed: “The surprise comes in experiencing the power. The car is fast, but what distinguishes it is acceleration in top gear in the vital 15-45mph speed range, which would not disgrace a car of twice the engine size.” There were doubts about refinement: “The Saab lacks the smoothness and silence which the average baby car has acquired between 1940 and 1950.”

The problem of stiffness around a boot lid aperture was solved by not having one on the 92. Access to luggage was through the rear seat. The smooth underside had stiffening ribs and box-section sills, its flat profile a great boon on loose, gritty Swedish roads while the designers concentrated the strength of the body in the middle. The burden of suspension loads were fed into the strong central structure by mounting the front torsion bars in the forward scuttle with tubular bolsters. Torsion bars for the trailing arm independent rear suspension were well forward of the rear hubs, so that the length of the frame subject to suspension-induced loads was less than 85 per cent of the wheelbase.

It was a strategy adopted more than a decade later by cars as diverse as the D-Type Jaguar and the Rover 2000, both of which had stiff centre sections carrying the strain of the suspension, so that the outermost extremities of the car could be thin-skinned and light weight. Above: prototype Saab 92s. Below, later Saab 96 with in-line engine.

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