From the first team car, LM1, the specification changed from car to car as the team gained competition experience. The first four team cars were built in this slightly haphazard way and it was not until 1931 that a team of three almost identical cars were built for that season’s racing which included the Brooklands Double Twelve and the Le Mans 24 Hours Race. These were LM5, 6 and 7. All three cars survive in a more or less original mechanical specification, though LM5 was rebodied by the works in 1932 with a 2/4 seater body.
Specification of LM7
Chassis. The short 11 6" chassis as per Four-Seater "International" Sports. Wheelbase: 8’ 7". Track: 4’ 4"
Engine. The Renwick and Bertelli designed overhead camshaft 4 cylinder engine, with dry sump.
The sump capacity was increased to 5 gallons and was protected by a wire mesh stone shield. The timing gears were narrower than standard, straight cut and drilled. The cylinder head was to ‘Ulster’ specification with lowered combustion chamber roof and significant gas flowing. The ports were separated by welded in pieces which effectively turned it into a four port cylinder head. The inlet manifold was re-designed to fit this configuration and it had a much longer throat. Electron was used for all engine ancillary castings including rocker box, timing cover, bell /housing and sump. Various camshafts were used and the rockers were shimmed out with spacers rather than the normal ‘Thackeray’ spring washers. Longer oil pumps were used to increase the volume of oil supplied.
Bore 63.9 mm, stroke 99 mm, 1495 cc.
Compression ratio: 9:1 using 75% ethyl and 25% benzol racing fuel.
Power: approximately 70 bhp at 5000 rpm.
Torque: approximately 70 lbft.
Twin 1⅜" SU side draught carburetors (TT specification).
Magneto ignition.
Two ‘Autopulse’ fuel pumps.
Transmission. Aston Martin designed 4 speed crash gearbox with drilled straight cut gears, constant mesh main and layshaft, dog clutch 4th speed and reverse. The gear ratios were closer by altering the ratio of the constant mesh 3rd speed pinion. The gearbox casing was cast in Electron. Two stays were fitted to reduce torque twist under load. Ratios: 12.18 7.57 5.56 4.75.
The rear axle was reinforced with a steel tube from the steel bearing shells of the differential, outboard to the hub.
Steering. ‘Marles’ worm and peg.
Wheels and tyres Rudge Whitworth 52 mm well base wheels with sixty spokes and 2½" rims. Fitted with 5.00 x 19" tyres.
Suspension. Semi-elliptic leaf springs front and rear. The springs have extra leaves and each leaf is polished. ‘Hartford’ dampers front and rear, but with larger 106 mm discs at the front.
Brakes. 14" Electron drums with shrunk in steel liners and 1¼" wide shoes and linings. Each shoe has an individual pivot. ‘Perrot’ operated at the front.
Dimensions
Wheelbase: 8’ 6".
Track: 4’4".
Length: 12’.
Width: 5’ 3".
Height: 4’ 4".
Weight: 17 cwt.
Fuel tank capacity: 20 gallons.
Performance: approximately 90 mph at 5000 rpm.
Coachwork. The works cars at this period had the lowered radiator (by about 4½") covered by a stone guard which included the top header tank. The helmet type wings were initially fitted to a strong cross tube at the front, and to brackets to the body frame at the rear. Later (for the Ulster TT) mounting reverted to the tried and tested production method of fixing them to the brake back plates by three wing stays.
The heavily humped scuttle was to a degree the fore runner of the 1932 "Le Mans", via the "Le Mans" Two-Seater Sports Model. All three of the cars had rounded, pointed tails in which the spare wheel was stowed above a large capacity fuel tank. All three cars were painted green.