In early 1936 a ‘new engine’, funded by a ten thousand pound injection of capital by Sir Arthur Sutherland, was being developed by Claude Hill. Two litres in capacity, it was producing about twenty five percent more power than the 1½ litre engine, which had given such good service since 1928. The works had experimented previously with a hugely over-bored engine as far back as 1931, but more recent work had indicated that simply increasing the bore size was not likely to produce a sufficiently reliable gain to be worthwhile. The 1½ litre blocks could be stretched to over 1700 cc but the stroke also needed to be adjusted and this was not so easy to accommodate in the 1½ litre cylinder block.
So, the new 2 litre engine was a completely new unit, with redesigned patterns and tooling and with only a very few parts carried forward from the Mark II engine. The potential of the new engine was so encouraging that a decision was made to design and build a new chassis into which it could be fitted. This was to become the ‘Speed Model’. As always, with an eye to racing success driving sales, Gordon Sutherland sanctioned the production of two new works team cars to be designated LM22 and LM23, for yet another entry at the Le Mans 24 Hours Race.
Along side the new ‘Speed Model’ a production version was being developed, known as the ‘15/98’. Initially as a long chassis saloon, the new 2 litre Aston Martin was something of a disappointment. However, the shorter chassis 2/4 seater tourer was quickly developed into a very nice touring car. A drop head coupe version was also offered on the short chassis and open four seater tourer coachwork was an option on the longer chassis.
When news of a new model reached the press, a lot of interest was provoked, causing some alarm to Lance Prideaux- Brune who was about to buy all the remaining stock of ‘Mark IIs’ and could immediately see them as being difficult to sell if there was indeed an imminent new model. However, the works were keen to emphasize in their publicity that, ‘the new chassis will not in any way supersede the famous 1½ litre which has been the sole model manufactured for so many years’. It was initially intended to sell the ‘Speed Model’ as a racing car for the road, to appeal to those who wanted particularly sporting performance.
The new chassis was heavier and stronger even than the ‘Mark II’. Right from the outset it was decided that the new car would have hydraulic brakes, so the chassis was relatively uncluttered by brake cross shafts. The deep channel section tapered from the front, but was parallel to the rear from the cross member beneath the flywheel bell housing. It was braced by four pressed steel and one fabricated channel section cross members and was under slung at the rear, as per the earlier models. Tubular cross members at each end again provided the location for the spring pins. The wheelbase was 1" shorter that the 1½ litre cars but the track 2" wider.
The front axle, bought in from Alford and Alder was unusual in that it pivoted in four silentblock bushes, mounted in trunnions, clamped to the front springs and free to pivot both backwards and forwards. Two heavy cables attached to the chassis side braced the trunnions and prevented any torque movement of the axle under braking, and was so designed to minimize changes in the steering geometry not only under braking but also under cornering forces. The cable could be adjusted easily to obtain the correct front axle caster.
This patent design was innovative, but it soon became apparent that while it worked well going forward and in a straight line, there was not sufficient stability in the axle through corners, and none at all when the car was braked in reverse, leading to alarming movements in the whole front axle assembly. The answer was to fit a solid rod instead of the cable, what we would describe today as a radius arm.
The brake system by Lockheed used a single 1¼" tandem master cylinder so that the front and rear brakes were in effect separate circuits, and the failure of one would not render the entire system useless. It was largely developed by Aston Martin itself, the company designing and building the entire back plate assembly including brake shoes, adjuster mechanism, handbrake assembly and brake drums, only buying in the master cylinder and the slave cylinders. The cost was enormous, allegedly almost 15% of the cost of the whole car. However it did work superbly well, and few cars with drum brakes, right up to the advent of discs, had better braking than a ‘Speed Model’ Aston Martin.
The dry sump engine was mounted on two large rubber blocks mounted in the main channel chassis cross member and with a single silentblock at the front. The main difference between the 1½ and 2 litre engines (apart from the capacity) was that the cylinder head breathed the other way around compared to the earlier models, with the exhaust side now the inlet side, and visa versa. This marginally improved the efficiency of the over head cam and rocker assembly. With larger 1⅝" carburettors and higher lift cams the works engines (with development help from Jock Horsfall) eventually produced 125bhp.
In addition to the new engine, an entirely new gearbox was also developed. Still crash type, it was based on the earlier 1½ litre box but was stronger and more robust, with wider straight cut gears and a redesigned selector mechanism. Easily capable of handling 200bhp, with close ratios and constant mesh gears on the input and main shafts, (the ratios of which could easily be changed to lengthen or shorten the intermediate gear ratios), it was one of the finest pieces of engineering to come out of the pre-war Aston Martin factory. Whilst other manufacturers were developing synchromesh gearboxes, and they were relatively common, Bert Bertelli and Claude Hill understood that all racers needed to do was get into the next gear as quickly as possible, and by far the best method of doing this was to double de-clutch going down the box and simply ‘smash it in’ going up. Strong straight cut gears with a simple selector mechanism were ideal for the job. In publicity material the ‘racing type gearbox, designed for lightning changes up or down’ was particularly featured. One of the joys of driving a good ‘Speed Model’ is still, to this day, using its gearbox.
With relatively short and stiff springs and friction dampers all round, the ‘Speed Model’ chassis was in fact quite old fashioned, but it provided a very stable base and the cornering behaviour was predictable and progressive. One of the main characteristics of all the Bertelli 1½ litre cars was that they had very good handling. Although relatively underpowered, they could maintain a very high average speed, just what you needed for long distance racing, and the new ‘Speed Model’ was very much in the same vein.
Sadly, the 1936 Le Mans 24 Hours race did not take place due to industrial action by French workers, so the two works team cars were quickly sold on to defray the costs of development. However, work did progress on more than half the twenty three chassis required to homologate the car for Le Mans, and these had a mix of coachwork styles, and for the first time there was not a single readily recognizable body work for a production Aston Martin. E. Bertelli Ltd. produced a number of coachbuilt 2/4 seater versions (the ‘Type A’) and a one off four seater, and Abbey Coachworks made two, slightly ungainly 2 seaters (the ‘Type B’).The remainder of the early cars had different two seater bodies, at least a couple very similar to the works cars and one with coachwork very similar to the 1½ litre ‘Ulster’. The final eight cars to be assembled, late in 1939 and 1940, had very unusual steel framed bodies designed by Claude Hill, (the ‘Type C’) with a real emphasis on aerodynamic efficiency. With rather a bulbous front with headlamps behind the radiator grill, and long tapering tails, they were something of an acquired taste. They were however nearly 20 mph faster than the open wheeled and 2/4 seater bodied cars almost directly as a result of their shape, a fact which did not escape the designer, Claude Hill.
Lastly, mention should be made of the efforts of Jock Horsfall in his ‘Speed Models’. He owned two cars, a 2/4 seater ‘Type A ‘which had been rebodied as a two seater, and was eventually developed as an out and out racing car; and another Speed Model which was fitted from new by the works with a 1½ litre ‘Ulster’ style body . With works assistance, Jock significantly developed his own cars and did in a two year period, what without doubt the works would have wanted to do, had the racing programme been continued. Jock’s road sports car was spectacularly successful both at home at Brooklands, and in Ireland, and later, post war, at the Belgian grand prix which it won, remaining (and likely to remain), the only Aston Martin to ever win a Grand Prix. The racing car was also very successful. Producing in excess of 125 BHP in its most powerful form on methanol fuel, it was, and probably still is, the fastest un-supercharged pre-war Aston Martin. Rebuilt by Jock Horsall after the war it came second in class and fourth overall at Spa in 1949, some thirteen years after manufacture, with Jock driving the entire twenty-four hours himself.
Sadly, Jock Horsfall was to be killed in a racing accident, late in 1949, while driving an ERA. He was a very good self taught development engineer, a superb driver and had a fascinating wartime history as a spy in MI5. His efforts were to show just what a remarkably good engine the overhead cam 2 litre is, and what could have been, if the ‘Speed Model’ had been further developed.
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