WHAT COMMEI :IAL OPERATORS
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CAN LEARN from tie MOTOR SHOW MANY chassis features of the private cars displayed at Olympia this week are of interest to engineers connected with road transport of all kinds. Technically, it is not an outstanding exhibition, but the very fact that there is no new component of absorbing interest has given designers a facility for studying detail which they have turned to good advantage during the past year.
Consequently, the outstanding trend of the 1936-1937 period is one of general refinement with especial attention to silencing, riding comfort and improved performance, All engines, for example, are now fitted with air silencers connected to the carburetter intake—often of very large size—and with one or two exhaust silencers of the " straight-through" type. Engines are mounted on rubber in various ways and as a rule the whole exhaust system, from the manifold to the tail pipe, is flexibly suspended from the chassis. The Rover company has adopted the interesting plan of fitting the exhaust system between the frame side member and the overhanging sill of the body.
Mechanically, engines have been quietened by paying special attention to valve-gear design and to its lubrication. In all but the least expensive models the crankshaft is fully counter-balanced. There is a tendency to shorten the stroke, this being particularly noticeable in the new V-type engines, of whith there are several. The 12-cylinder Lagonda, for example, has cylinder dimensions of 75 mm. by 84.5 mm. and consequently is of quite reasonable overall width, despite the use of overhead camshafts which extend the engine laterally.
Another new V-type engine is the eight-cylinder Standard, but this has side-by-side valves. As in many other power units at the Show, the cylinder heads are of aluminium alloy. Inlet and exhaust manifolds are both cleverly fitted between the banks of cylinders, the former being fed by a dual downdraught carburetter.
Also interesting as an example of clean and compact design is the new Sunbeam straight-eight engine ; the capacity is 41 litres, but the overall length is slightly
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less than that of the former six-cylindered 3i-litre engine, Nevertheless, each bore is separated from its neighbours by water spaces and room has been found for 10 main bearings. A novel point is the way in which the water pump is driven from the forward end of the camshaft, the timing gear being at the rear of the engine; the pump provides a useful damping effect. Another novel feature is the location of the two carburetters, these being placed at the ends of a long inlet manifold.
Tappets requiring no adjustment are becoming popular with American engine makers following the lead set by Buick a year ago, when this concern brought out an oil-cushioned tappet mechanism. This is alternative to the Ford plan of providing bearing surfaces so _large that wear is negligible. Whereas the Ford is a side-valve engine, the Buick has push-rod-operated overhead valves and, owing to temperature effects, it is essential to make allowance for the varying distance between the camshaft in the crankcase and the overhead valve rockers.
In order to improve distribution many makers use two carburetters for six-cylindered and eight-cylindered engines, and others (notably the Alvis concern) use three carburetters to feed six cylinders. Again, mainly
as a means for improving distribution, the American Auburn and Graham models are fitted with high-speed centrifugal blowers ; the term " supercharger " is scarcely appropriate, because rup serious attempt is made to boost the engine beyond a normal output. The blower does, however, help to prevent the power from falling off at high revs.
On a few makes separate cylinder liners are employed, for example, the Siddeley Special, A.C. Ace, Citroen and Lancia. Separate exhaust-valve seatings are used in a number of American engines and in the six-cylinder Hurnbers. A novel form of combustion chamber is a feature of the new 25 h.p. six-cylinder Vauxhall.
There is not much to be noted in the design of transmission systems, synchromesh being generally popular with the exception of a number of well-known British cars on which the Wilson self-changing epityclic gearbox is used. A detail of interest is the removal of the gear lever of an orthodox transmission from its usual central location to a position on the facia where it does not obstruct the front compartment.
This is done in various front-drive cars, such as the B.S.A., Citroen, Adler and D.K.W., and is also a feature of the bigger Renault models.
The Cotal electro-magnetic gearbox is a point to notice in the Delage exhibits ; as in the Wilson system, the gear lever is on the steering column.
More makers than formerly are using the Newton centrifugal clutch. Hypaid bevels are popular for final drives on many cars this year, mainly because the offset of the pinion enables the propeller shaft to be lowered to a position where it does not need a tunnel in the floor.
In American cars the outstanding transmission feature is the overdrive, which is usually arranged to give an overall (indirect) gear ratio of about 3 to 1 for highspeed cruising, as compared with a direct top-gear ratio of about 41 to 1. The main advantages are reduction in wear, improved petrol consumption and an engine which seems less " fussy " owing to its lower revs.
Few chassis manufacturers now make their own brake gear, one or other of the three leading proprietary systems being generally employed. The Clayton Dewandre servo motor is once again becoming popular for fast and heavy cars. The leading development in brakes this year is, however, the wide adoption of rigid iron alloy brake drums in place of the pressed-steel type. Although heavier, the iron drums are relatively free from distortion, are not so liable to scoring and give a smoother braking action.
Suspension systems continue to show a sharp division of opinion amongst designers as between the merits of an independent action for the front wheels and a conventional front axle. No prominent firm has gone over to the opposite camp during the year. The number of cars using torsion bar " springs " is increased by three new models : the 25 h.p. Vauxhall, the 12-cylinder Lagonda, and the Lancia Aprilia. In each case the torsional scheme is employed only at one end.
In the form of a stabilizer a torsion bar is now used by• many makers. This is largely due to the general adoption of softer springing which makes it necessary to provide some method of controlling the roll on corners.
Larger hydraulic shock absorbers, in many cases automatically controlled in relation to road surface and speed, are used by a number of British manufacturers.
Steering gears are mainly of the various well-known proprietary types, the only real novelty in this department being the new Citroen system in which a springloaded pinion engages with a rack.
In the new Austin Ten the steering gearbox has been moved to the front dumb-iron with the drag link trailing to the steering arm as in a forward-control commercial vehicle. A somewhat similar plan was adopted for the Hillman Minx for 1936. The Bluemel telescopically adjustable steering column is widely used.
Chassis frames become more and more rigid as the springing is made more supple ; a" logical corollary. Designers are finding some difficulty in obtaining this rigidity without running into the disadvantage of excessive weight. Welded box sections of great inherent stiffness are providing a solution, as they enable lightgauge material to be employed ; the side members of the new Alvis Seventeen are well worth inspection in this connection. Another point is the increasing popularity of the tubular cross-member.
Summed up, it is an exhibition which reflects the serious attention that the motor industry has been giving to the manifold details upon which the satisfactory performance of a car depends.