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TECHNICAL TALK 5: Small racing cars in 1939

The cutaway drawings in the motoring magazines during the 1930s were always delightful – but they seldom extended to explanations of engineering details.  The drawings in this Technical Talk are an exception: they are beautifully clear and come with some fascinating information.  They were published to help racing-car engineers interested in the 1½-litre formula that was the main focus of racing in Britain at the time.

 

Maserati 6CM – front suspension

Here is the front suspension of the six-cylinder Maserati 6CM produced in 1937 and 1938.  Its torsion-bar suspension gave much improved roadholding and cornering speed.  The torsion bars were 24 inches long; the left-hand bar can be seen extending back from the pivot of the top wishbone.  The friction shock-absorber is linked to its front end.  The hydraulic drum brake is wide and well-ventilated.

 

Maserati 4CL – rear suspension

By 1939, fierce competition in the international voiturette class, and the introduction of the Alfa Romeo 158 and ERA B- and C-types, forced the Maserati brothers into designing a new, four-cylinder engine with square dimensions of 78 by 78 mm.  This developed 30–50 bhp more than the previous six, mostly through an increase to four valves per cylinder and a more powerful supercharger.  The car was the 4CL.  Here are the conventional leaf springs and box-section chassis spars.  But back in 1937 someone had already modified the rear suspension of the previous generation of Maserati…

 

 

Maserati 4CM – Tecnauto rear suspension

For 1937, the basic design of the 6CM was followed, but a 4-cylinder 1½-litre engine was used; it was thought to be livelier than the six for twisty circuits.  Thus was born the 4CM.  A private group of racing enthusiasts got together to form the Scuderia Ambrosiana, including Count Giovanni “Johnny” Lurani Cernuschi.

In 1938 the privateer Maserati, No. 1128, was rebuilt for the European season and Lurani had the rear end modified by the Turin-based firm Tecnauto.  Their suspension units, with a coil spring within a tube and springs actuated by a trailing arm, were mainly intended for the front end of racing cars, to give a “bolt-on” independent front suspension. Lurani had these units fitted to the rear of the Maserati, to replace the semi-elliptic springs, but kept the one-piece rigid rear axle.  He felt that the Tecnauto coil-spring units would give a better ride to the rear end and stop it hopping about – and he was proved right.

 

Austin – Roots supercharger

In 1935 T. Murray Jamieson designed the twin-cam Austin for voiturette racing.  He intended to develop it to reach 12,000 rpm, using extremely heavy valve springs and wide cams to achieve this, but the cars were never tuned to give more than 116 bhp at 8,590 rpm, and only 90 bhp at 7,000 rpm for long distance events.  Critical to the success of the engine was also the Roots supercharger shown here, that too designed by Jamieson.  These blowers were volumetrically highly efficient at low engine speeds, but had low adiabatic efficiency (in other words, tended to waste too much heat) above 16 psi boost – and in the Austin ran at a remarkable 20 psi boost.

 

M.G. – Centric supercharger

For high pressures, a vane-type supercharger was more effective.  The Zoller vane blower on ERAs could reach boost pressures as high as 30 psi.  Here is a similar but simpler Centric vane-type supercharger from ‘Goldie’ Gardner’s 1939 200-mph M.G. record breaker.  The small drawing top left shows that the vanes run on a main shaft that is concentric with the outer casing; their tips stay almost in contact with the casing.  A rotor is driven by an internal gear from that shaft.  Its centre is eccentric to the casing, always higher than the main shaft centre.  Four trunnions (effectively reinforced slots) in the rotor allow it to rotate around this higher centre and move axially in relation to the vanes .  The vanes therefore sweep air from a large space at the inlet to a small space at the outlet, thus compressing the air.

Drawings courtesy of the Richard Roberts Archive


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