THE DOUBLE-CAMSHAFT AUSTIN SEVEN ARRIVES
THE DOUBLE-CAMSHAFT AUSTIN SEVEN ARRIVES
A PERFECT LITTLE ROAD-RACING CAR WHICH ACHIEVED 121 m.p.h. ON ITS FIRST OUTING. ENGINE GIVES 116 h.p. AND DESIGNED FOR 12,000 r.p.m. COMPLETE CAR WEIGHS 91 CWT.
After a long period of waiting the new 750 c.c. Austin racing car has at last been pronounced completed, and this new aspirant for small-car honours, which was shown to members of the Press last month, certainly fulfils, both in looks and design, the high expectations which it had aroused. A 750 c.c. engine developing close on 120 h.p., itself a magnificent achievement, is fitted into a chassis with a wheelbase of only 6 ft. 10 in.—and track 3 ft. 11 in. The sidemembers and the driver’s seat are not more than 6 in. from the ground and the body and the tail, with its stabilising fin, are very little larger than those of the car which Driscoll drove so successfully last year. The latter car, of course, had offset transmission. All credit, therefore, to Mr. T. Murray Jamieson, who was responsible for the design of both cars, and to the enterprise of Sir Herbert Austin, who made possible the development of these amazing little ” seven fifties.”
Attention focuses first of all on the engine. The cylinder head and the huge camshaft casings form one unit and the cylinder block and the crankshaft the other, both being cast in R.R. 50 light alloy, while pipes and other parts not subject to stress are made in elektron. Starting from the top of the engine, the first things which impress one are the massive camshafts, running in three bearings, with cams 2 in. wide. These bear directly on the valves, and when it is considered that the triple springs required if the engine is to run up to 12,000 r.p.m. will have a combined tension of 550 lb. per valve, the need for a
robust valve-operating mechanism is readily understood. The cams and the valve-buttons are chromium-plated to resist wear. The valves are inclined at 90 degrees to one another in the hemispherical combustion chambers, and
centrally-disposed 14 mm. sparking plugs are used. The camshafts and auxiliaries are driven from the rear of the engine by a train of no less than eleven gears. A
Murray Jamieson Rootes-type supercharger, also driven by gears, is neatly built in at the back of the engine, with
an enormous single S.U. carburetter, which has a rectangular float chamber and a swinging float in order to pack neatly into the space available. The blower runs at one-and-a-half times engine speed, giving a boost of 20 lb. Wet liners of nitrided steel are used for the cylinders, the liners projecting above the block into the head, where they make metal-to-metal joints. These upper portions, which are finned, are surrounded by detachable water jackets. The crankshaft is machined from the solid and has a plain bearing in the centre and a roller bearing at each end. The connecting rods are H-section steel and have white
metal big-ends centrifugally cast. The pistons are of the slipper type, with three piston rings. The bore and stroke are respectively 60.32 by 65.09 mm. or 2g by 2 9/16 in., giving a capacity of 744 c.c.
The auxiliary drive is from the rear end of the engine. A small pinion on the crankshaft meshes with a much bigger one above it and on either side are other pinions which drive respectively the water pump and the Scintilla magneto. The starting handle connects with the near-side pinion through skew gears, as the engine is carried so low in the chassis that one would be unable to swing a handle applied directly to the crankshaft. The triple oil-pump for the dry-sump oiling system is driven by skew gears from the off-side pinion, which has high-pressure, low pressure and scavenge sections. A beautiful little oilcooler made of steel tubes six-thousandths thick is carried behind the radiator cowling, while the oil tanks, which have a capacity of three gallons, are disposed on either side of the propeller shaft. Returning to the camshaft drives, there are three pinions on the shaft marked ” B ” in the centre diagram shown on the following page, one meshing with ” A ” and a second with the trains of gears driving the camshafts at ” C ” and ” D.” By compounding the drive
in this way, the necessary gearing down and up is effected without using large pinions which would be difficult to accommodate. The blower is driven from a third pinion on shaft ” C,” and the ratio can be changed without disturbing the timing gears.
The engine has so far only been run up to 8,500 r.p.m. giving the formidable figure of 116 h.p. with which the car reached a speed of 121 m.p.h. on the Donington test-track early in March. In order to attain the intended engine speed of 12,000 r.p.m., lead-bronze big-ends will be required, but the engine has been designed to accommodate this and other alterations such as stouter connecting rods or crankshaft. Now as to the transmission. A singleplate clutch is used, but no ordinary one, as it has no less than five toggles and fifteen springs. The gear-box is in unit with the engine, and has either straight pinions or synchro-mesh gears according to the type of race for which the car is entered. The synchro-mesh mechanism, ‘ which is fitted to the top three gears, is used in hill-climbs or road-races, as it gives a distinctly quicker change than
the ” clash-type ” gears. A remote control from the gear-box brings the lever under the driver’s left hand.
An open propeller shaft with two Hardy Spicer joints takes the drive back to a cross-member under the seat. Here is situated a steady bearing, and the rear half of the transmission line passes through a torque tube to the back axle. Although the shaft is only 2 ft. long there is a danger of whipping, which might be unpleasant for the driver at 12,000 r.p.m., so in the centre of the torque tube, a second steady bearing is utilised. Mr. Jamieson considers that the heeling-over which always accompanies `• Schwingachsen ” off-sets the advantage of better wheel-adhesion, and has Right : The camshaft drive in skeleton form. The small pinion underneath ” A ” is carried on the crankshaft
therefore avoided making use of independent suspension for the rear wheels. The crux of the matter, he considers, is low unsprung weight, and this he secures with quarter-elliptic springing. The rear springs are splayed out from the sidemembers, which pass under the rear axle, and above each spring is mounted a radius rod. A double reduction is used for the final drive to avoid having a crown wheel of excessive size. The bevels give a reduction of 1.8 to 1 and a pair of straight pinions mounted above them bring the over-all ratio to about 5.5 to I.
The chassis is built on light but straightforward lines. The side-members are of channel section and almost straight from front to rear. There is a tubular cross-member in front of the engine, which is rigidly mounted and acts as an ” anti-lozenging ” tie, a banzo-forging built in at the rear spring mountings which serves to support the front end of the torque tube, a light pressing which supports the driving seat, and resists the twisting of the rear shockabsorbers, and a tubular member at the extreme rear. This and the pressing in front of it supports the fuel tank, which holds 25 gallons. The fuel consumption should work out at 7i m.p.g. for long distance events and 31 to 4 using sprint fuel. Pressure from a hand-pump forces the fuel to the carburetter. The layout of the front axle is practically the same as that used on last year’s racing cars. The front axle is tubular and straight, and made in two parts, a joint with an internal roller bearing allowing either of the parts, with its steering pivot, to rotate independently of the other. The suspension is by means of a flat transverse half-elliptic spring carried under the axle tube, and braking and springing torques are resisted by means of radius rods running to the top and bottom of each steering head. The steering wheel is mounted centrally in the chassis on a short steering column. The steering box is up in
the scuttle, and a fore-and-aft rod runs to a bell-crank lever mounted near the front of the chassis, and from here a second connecting link joins up to the steering arm. 12-in, brakes are used on the front wheels and 10-in, on the rear, the shoes being expanded by miniature crankshafts and links. The brakes are operated by enclosed cables, and are fully compensated with swinging links. The front brakes exert nearly three times as much pressure as the rear ones, and in order to prevent the front pull-off springs applying the rear brakes, one of the minor problems of the fully-compensated system, stops limiting the movement of
the rear brake arms have had to be fitted.
The cars are fitted with trim little single-seater bodies similar except for slightly higher tails to those used on Driscoll’s car last year. Complete with fuel, oil, water and tyres the weight is 94 cwt. The tyres, incidentally, are lowpressure racing Dunlops, 5.25 in. by 16, mounted on triple-spoked wheels with alloy rims. The new cars were to have made their debut at the Empire Trophy, at Donington, but the handicap allowance (seven minutes for a 250-mile race against 3-litre Alfa-Romeos) was considered too
unfavourable. Instead, one or more will appear at the Brooklands Easter meeting, and a team will be competing in the International Trophy on May 2nd and in the Isle of Man race at the end of that month.
Three cars have been made, and will be allotted to Driscoll, Charlie Dodson and Goodacre. The side-valve cars will remain at the factory and will be used to give less experienced drivers a chance of taking the wheel. In particular, an effort will be made to encourage racing mechanics from the Austin factory, a very sensible and praiseworthy scheme, and a new driver, by name, Bert Hadley, is already shaping well.