The Evolution of the Chevrolet Corvette
Last summer we were promised a Chevrolet Corvette Sting Ray GT coupé for road-test, through Vauxhall Motors Ltd. and early this year it materialised, and, after some rather alarming vicissitudes, it was possible to put it through its paces last month. The report appears on page 253.
A project of the Chevrolet Motor Division of General Motors, the Corvette dates back to the winter of 1953, when it was introduced as a moderately-powered car with conventional chassis features, and automatic transmission. This emerged in 1957 as the Corvette SS, which had a de Dion back axle and a power/weight ratio of 5.8 lb./h.p. Demand had been exceeding output and a new version was initiated for 1959. In 1958 William L. Mitchel, G.M.'s Vice-President in charge of styling, had built the experimental XP87 Corvette on a Corvette SS chassis and the G.M. engineers looked at this body styling and at the Cerv 1 of 1960 which had i.r.s. and a power/weight ratio of 4.2 lb./h.p., later improved to 3.9 lb./h.p., when they began to visualise the current Sting Ray.
The target they had set themselves was expressed as "A vehicle that would provide luxurious transportation for two and give an exceptionally high level of performance, stability and safety, with a high emotional appeal." If that isn't planning a car to GT requirements I don't know what is, and how closely General Motors has succeeded you can judge for yourselves after reading the road-test report by a colleague, in this issue of Motor Sport. I will say that after driving the rather clapped-out 1963 Sting Ray submitted to us, I was suitably awed by the performance, liked the layout of instruments and minor controls, found the driving position excellent and the seats comfortable, the steering very light and the gear-change excellent, and, except for the vulgar embellishments and dummy body items, I was certainly not disappointed with this American GT coupé.
The conventional front location for the 327 cu. in. Corvette engine was retained because G.M. wished to combine the shortest wheelbase (8 ft. 2 in.) with no increase in cost. An engine of this weight, put at the rear, would entail an increased wheelbase length and a costly power train, while a front engine with rear trans-axle would have raised the cost appreciably with no perceptible improvement. However, G.M. engineers agreed that i.r.s. with frame-mounted differential was essential for eliminating wheelspin due to torque transference, and to improve sprung to unsprung weight.
Care was taken to achieve a good weight distribution. Not only was the passenger compartment set as far back as possible but the engine centreline was off-set an inch to the of o/s., which brought crankshaft and off-set back-axle pinion on the same line, thus reducing the width of the prop.-shaft tunnel. Luggage was carried behind the seats in true GT manner and the spare wheel was carried below the fuel tank. The result was a front/rear weight distribution of 47/53, compared to 51/49 of the 1962 Corvette, and a reduction in c.g. of 3.3 in. The track was reduced by an inch in front, two inches at the back (56 in. and 57 in.), compared to the 1962 car. The lower c.g. was attained by placing the occupants within the frame, whereas previously they sat above it, although this involved re-routing the exhaust system through the frame members.
The improved 1963 Corvette could be sold for the same price as the 1962 version largely because normal production front suspension and brakes were adapted for the car. Special i.f.s. was needed for the earlier model.
The decision to use a fibreglass body for the Corvette was made in the early '50s. G.M., like other far-smaller concerns, realised that for small output cars plastics are more economical than steel, while giving excellent durability and high re-sale value. Engineering and Styling combined to some extent in evolving the Sting Ray body with its separate frame design. In spite of many heavier components (more durable exhaust system, retractable headlamps, larger fuel tank, bigger brakes, body reinforcements, etc.), the 1963 body is somewhat lighter and 90% torsionally stiffer than the 1962 convertible; the 1963 convertible is 10% stiffer than the 1962 convertible.
New wishbone and coil-spring i.f.s. was devised, with the roll centre raised horn near ground level to 3¼ in. by lowering the inner pivots of the upper wishbones. This gave higher roll resistance for a given spring rate and better cornering power, with understeer characteristics. These improvements were deemed worth the greater sensitivity to wheel alignment and a tendency to steering kick-back.
The roll centre of the new i.r.s, is 7.56 in. In conjunction with the ball-joint i.f.s., unsprung weight was reduced by 11 lb. at the front, 89 lb. at the rear, for the same overall weight, and weight distribution is claimed to be such that the shock-absorbers are valved for damping only, without compromise for ride balance. Rubber insulation is employed through the suspension, including at the eight rear links, for the differential carrier and even at both ends of the shock-absorbers. The c.g. is 16½ in., the roll couple was reduced by 18.5%, and the static ride rates/wheel are 93 lb./in. front, 125 lb./in. rear. Roll rates are 420 lb./ft.-degree front, 325 lb./ft.-degree rear. Rear suspension is by a transverse multi-leaf spring, torque and braking forces being taken by radius arms, and the wheels are located by upper and lower links, the drive shafts acting as the upper links.
To special order, a heavy-duty rear spring and 3/8 in. larger diameter, re-calibrated shock-absorbers are available, which increase the spring rate from 162 lb./in. to 305 lb./in. The new spherical ball-joint front suspension replaced the reversed Elliott king-pins of the earlier Corvettes. Lowered cost due to the use of many normal Chevrolet suspension components has already been mentioned; this also reduced the lubrication points from 16 to 4 and raised the lubrication interval from 1,000 to 6,000 miles. Even the front anti-roll bar is attached by means of standard links, brackets and grommets. New, faster (16 to 1) steering was introduced for the 1963 Sting Ray and power steering put in hand. Heavy-duty i.f.s. is available, using a stabiliser bar of 0.19 in. greater diameter (0.94 in.), the spring rate going up from 260 to 550 lb./in.
The brake drum area was kept at it 11 in. but the area was increased from 84 to 116 sq. in. front, 73 to 84 sq. in. rear, comparing the 1962 with the 1963 Corvette. Lining area went up by approx. 18% to nearly 186 sq. in. Heavy-duty brakes, with larger, finned drums and metallic linings, etc., are available as part of the Sting Ray Special Performance package option. The X-braced chassis of 1962 was replaced by five cross-members for the ladder-type welded steel chassis frame, torsional rigidity being improved by nearly 50% and the maximum deflection in beam stiffness improving by 0.002 in. Fuel capacity was increased from 16.4 to 20 (U.S.) gallons and the filler size enlarged from 2 1/8 in. to 3 in. For the Aerocoupé only, a 36-gallon tank is available, made of fibreglass.
New elliptical silencers were designed, 6¼ in. shorter but of 9% greater volume, and 2-in, exhaust pipes standardised with the 250-b.h.p. engine and the 300-b.h.p. engine when used with Powerglide automatic transmission. Otherwise, 2½-in, pipes are specified for the 300-b.h.p. engine, and for both special-camshaft high-performance engines. Silencers on the n/s. are aluminized inside and out; those on the (cold) o/s. are a combination of stainless steel and aluminization.
Down the years, Corvettes have retained 6.70 x 15 tyres, but the rim width was increased by ½ in. for 1963 (15 x 5.5K welded steel wheels). A 15 x 6 aluminium wheel is available, with malleable iron adaptor to which the wheel is secured by a locking nut, a simple knock-off design. With these wheels the tyres are also 6.70 x 15, but 6.50/6.70 x 15 racing tyres can be fitted on the front wheels and 8.00/8.20 racing tyres on the rear wheels of the Aerocoupé only, when it has the re-positioned big fuel tank.
Four versions of the basic 327 cu. in. (5,363 c.c.) V8 engine power the Chevrolet Corvette, with three transmission options. These engines stem from the 283 cu. in. fuel injection engine of 1957, which gave 1 h.p. per cu. in. Bored and stroked to its present size, it gives 1.1 h.p. per cu. in. The fully synchronised 4-speed gearbox was introduced in 1957. These power unit developments, up to a maximum of 360 b.h.p., have benefited General Motors' saloon-car range.
The Corvette body began as a 2-seater 2-door convertible built in Flint late in 1953. Side windows were removable. In 1956 a new top and windscreen and a removable hard-top were introduced. The front-end and facia were changed radically in 1958, the rearend revised in 1961. The 1963 body was very different from that of 1953, yet some of the original tools were used for making the underparts. G.M. had always thought the Corvette "a little big," so for 1963 dimensions were cut down and entry and exit made easier. The new GT coupé body was styled to look European, yet not lose Corvette identity. Luggage space was almost double that of the convertible and 60% greater than in the trunk of the 1962 coupé. The steel framing for the convertible's fibreglass shell weighed 82 lb., compared to 48 lb. for the 1962 body.
The original 1953 plastics weighed 357 lb., those of the 1963 convertible 300 lb. Total weights: 1953 = 411 lb., 1962 = 405 lb., 1963 = 397 lb. The big plastic doors, cutting up into the roof, presented problems due to being large doors on very small hinge spans. Their glasses are solid tempered and slightly curved in two directions, with C.V. window, frameless on the convertible, and with normal door locks.
Colin Chapman (who uses a different and quicker system on the Lotus Elan) may be interested, and B.M.C. (who gave up in despair on the Sprite) take heart from the knowledge that G.M. designed five different mechanisms to operate their concealed (rotating) headlamp clusters, from simple rods-cum-cables to a complicated Rube-Goldberg single-switch system, before arriving at the present two-motor electrical layout that requires two switches "but has no little dogs running on treadmills"! The motors are adaptations of C.V. window-lifting motors, one for each headlamp-cluster door.
The first Corvette presented door scaling problems, since the subject of much attention. That original Chevrolet Corvette had body parts made by the Molded Fiberglas Body Co. of Ashtabula. Ohio, which, in 1952, employed about 85 operatives. The first 300 Corvettes of 1953 were produced practically by hand. Today the same concern supplies the body parts from the St. Louis plant opened in December 1953; it now has some 1,100 employees, and last year Corvette assembly space was doubled.
General Motors claim that Corvette demand now exceeds supply and it is perhaps significant that for three-quarters of those purchased in recent years the 4-speed manual gearbox has been specified.—W. B.