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Aston Martin Second Series

The 1 ½ Litre.

The Bertelli first series chassis was designed in the mid 1920’s and with rod and Perrot shaft brakes, worm drive axle with torque tube and chassis mounted gearbox, it was, even in its time somewhat out of date. Indeed, it was not unlike the Bamford and Martin side-valve chassis which had been first laid down in 1912. However, the second series chassis was designed in late 1931 and was therefore a much more contemporary technical design.

Still under slung at the rear and with similar wheel base and track dimensions as the ‘International’ it was designed for the gearbox to be fitted in unit with the engine, and for cable brakes to be fitted directly on to the brake back plates. The engine was mounted in the chassis on four tubular engine mounts each fitted with a ‘Silentbloc’ rubber bush, with a single ½" diameter bolt running through a ‘U’ shaped bracket, bolted to aluminium blocks. These were securely bolted into the chassis side frame by two vertical and three horizontal high tensile 5/16" bolts. They were fitted with bronze chamfered washers on the top, bottom and outside to take up the different rake of the chassis on each face. Thus the engine became a semi-stressed member, considerably stiffening up the front of the chassis. Without the engine fitted into the chassis, the steering wheel can easily be moved six inches each side of its resting position as a result of the bending of the side frame at the mounting bracket.

The foot brake, mounted on a bracket bolted to the chassis operated two tubular cross shafts linked by a short pushrod, onto which the brake cable ends were secured with yokes and pins on short levers. The cross shafts were mounted in spherical bearings at each end, on the near side secured by a tapered pin and on the off side fully floating, so that the shafts did not seize when the chassis flexed . The cables could be adjusted at each wheel by the simple expedient of a thumbscrew on the end of the cable which could take up slack either caused by cable stretch or brake wear. Adjustment was therefore very simple, for both the front to back balance and side ‘pull’ without the need for any compensating mechanism. However, front to back balance was set by the factory by the simple expedient of drilling the cross shaft lever on the rear cross shaft slightly closer to the pivot point, thereby reducing the leverage of the rear brakes. The shoes now had the wider 1½" brake linings first fitted to the 1931 team cars, with each shoe mounted on a separate pivot and being opened by a simple cam in an aluminium housing, actuated by the pull of the cable on a short lever. The large surface area of the brake linings on the 14" diameter drums made for very efficient brakes, more than capable of locking up all four wheels in an emergency stop. Almost every road test carried out by the many motoring magazines of the day were to comment on the marvelous and most effective brakes. The brake system was indeed a typically Bertelli design, simple yet elegant and efficient.

With the gearbox now in unit with the engine, transmission to the (bought in) ENV differential mounted in a simple ‘banjo’ type axle casing, was by common propeller shaft, which had expensive to make ‘doughnut’ type couplings with a cruciform spider and phosphor bronze bushes. Rather heavy and difficult to lubricate they were always prone to wear.

With the ENV differential came a variety of final drive ratios, the most common ratio being 4.75:1 or 5.1:1 for the longer and heavy tourer and 4.66:1 for the lighter short chassis cars and the competition cars. This was particularly useful in the racing cars, as the different crown wheel and pinion ratios and wheel and tyre sizes gave a very useful variety of final drive ratios to suit almost any road or track circuit. With 18" wheels the final drive ratio of 4.66:1 gave very close to 20 mph per 1000 rpm.

The engine upgrades for the second series cars were also an important improvement on the ‘International’. The valve gear was improved with threaded valve spring collars (now triple springs instead of open coil double springs) secured with hardened steel threaded caps. The cylinder block was almost identical to the ‘International’ but without the side pedestal for the magneto. The dynamo was now driven directly off the front of the crankshaft and featured in advertising publicity as ‘acting as a torsional vibration damper’. As a result the timing gear, which was mounted on the spindle of the dynamo in the ‘International’ engine, was now supported on its own shaft with bearings in housings back and front. The rear bearing housing also doubled as the mounting bracket for the flange mounted magneto which was coupled directly to the auxiliary shaft via a Simms vernier coupling. The same shaft now drove an up-rated water pump mounted on the front of the timing cover. The timing chain was now of the ‘Duplex’ roller type and was tensioned by a more sophisticated (but still rather noisy) ‘Weller’ blade spring correctly tensioned by a coil spring and plunger in the rocker cover.

However, the most significant change was a redesigned cylinder head with much improved porting which gave the possibility of designing a proper balanced inlet manifold, no longer a simple cover plate with two carburetors attached. This alone improved power by nearly ten percent and was to remain almost unchanged right up to 1936. With the improved breathing came new camshafts and carburettor combinations. The ‘New International’, still fitted with the old ‘International’ cylinder head, had a rather inefficient and heavy cast iron exhaust manifold, now with the exit at the rear of the manifold, so not to overheat the magneto from a central downpipe. However, the ‘Competition 2 Seater’ and ‘Le Mans’ with the new cylinder head, now had a pair of neat cast iron manifolds which fed into twin downpipes, clad in chromed copper flexible covers, feeding into a ‘Brooklands’ style exhaust box, at first with a single pipe entering at the front of the box, but later with the iconic twin stubs on the top of the box, very much a style feature of the ‘Le Mans’ and the later ‘Mark II’.

At the rear of the cylinder block, there was a new flywheel bell housing with the starter now mounted on the left side of the engine. On to the flywheel bell housing, was bolted the clutch bell housing, which provided a mounting for both the clutch and throttle pedals on the off side, and which supported the unit gearbox. For the ‘New International’ and the ‘12/50’ this was a bought in Laycock gearbox with ‘silent second and third speed gears’, being replaced on the ‘ Competition 2 seater’, and long and short ‘Le Mans’ by an Aston Martin designed gearbox, a development of the ‘International’ box. Although the Aston Martin designed gearbox was more difficult to use than the ‘Laycock’ gear box, being built with simple straight cut gears, it was much easier to fit with different ratios depending on the weight of the vehicle and also the requirements of serious competition, where carefully chosen gear ratios could make a significant difference to lap times.

Two long chassis variations were made, the ‘12/50’ or ‘Standard’, in tourer and saloon form, was a lengthened version of the ‘New International’ complete with early first series ‘International’ cylinder head. The second, the ‘Special 4 seater’ , the lengthened version of the ‘Le Mans’, was identical in mechanical specification to the short chassis car including the better second series cylinder head.

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