De Dion Suspension:
Refers to French auto designer de Dion and his independent
rear suspension that utilizes a connecting tube. Rear suspension system which allows the wheels to retain their correct camber no matter how much the suspension is deflected. In a typical application, two tubes run across the car, one being free to telescope slightly inside the other, and the wheel hubs are fitted to these tubes by rigid arms. The track is controlled by fixed-length halfshafts connecting the hubs to the differential, which is attached to the underside of the car. In this way, torque reaction is taken by the car frame, and the car's unsprung weight is reduced. De Dion suspension is a long-established system used by, among others, Ferrari, Alfa Romeo and Rover.
The History of the De Dion Suspension System
Designers of motor vehicles have always been confronted with serious problems when they set about the lay-out of power transmission. The conventional type of vehicle with rear drive has several disadvantages�the mass of the differential considerably increases unsprung weight and thus riding comfort and controllability, the necessary allowance for rise and fall of tail-shaft causes either a hump in the floor or a high level floor, and under conditions of driving where road surfaces are slippery the full power of the motor cannot be used because of the torque effect on the back axle which tends to lift one wheel. When this occurs with the conventional type of differential unit the wheel just spins and no power is transmitted to the other wheel.
In 1921 a car was exhibited at the Berlin Motor Show by a Professor Rumpler. This car featured swinging half axles at the rear, i.e., the differential unit was mounted rigidly to the chassis and on each side of it the axle was universally jointed so that it could rise and fall; located in position by radius rods. The springing was provided by coils. There have been many derivations of this system since, but there are certain drawbacks, the main one of which is that the layout gives inherent over-steer, (i.e., the car tends to go round a sharper corner than that steered for). When the Nazi State in Germany subsidised the Mercedes and Auto-Union factories to build world beating Grand Prix racing cars during the period 1934-1939, the designs turned out by both groups at first made use of the Rumpler system of swing axle. However the Horch section of the Auto-Union group developed a form of rear suspension for their large touring car, known as the De Dion system. To trace the evolution of the De Dion design we must now step back in time almost forty years.
An engineer called Georges Bouton and his brother-in-law, Trepardoux, went into partnership with the Count Albert De Dion for the purpose of building steam cars. Bouton and De Dion, however, began to experiment with petrol engine developments while Trepardoux continued to develop his steam car design. His eventual design for the engine and transmission system consisted of a unit-built twin-cylinder engine and differential mounted at the rear of the chassis, on either side of the differential were universal joints from which exposed live shafts went to another universal joint mounted on each hub. The hubs were tied together by a dead axle tube which acted as a load carrier also. The springs used were leaf springs. This device was covered under a British patent issued in 1894. A feature of the design was that each half-shaft was extended from the outer universal through a hollow hub and drove the rim of the wheel by means of special steel spokes as the wooden load-carrying spokes were not considered strong enough for driving loads. This design was successfully incorporated into a steam tractor which, towing a car, put up the fastest average speed at 11.2 m.p.h. in the first motoring competition, the 78.75 miles long Paris-Rouen trials of 1895.
With the increasing interest of the firm in petrol engine cars Trepardoux resigned and so his system became known as the De Dion axle. It was employed on the partner's petrol engine cars with practically no change at all in the design as they used a rear-mounted single cylinder engine. However, in due course their cars began to adopt the normal front mounting for the engine with the propeller shaft driving the back axle. Hence some of the merit of the layout was lost as the unit construction system was no longer used, also the use of an overhead worm drive cancelled out the possibility of having a low floor.
In 1910 the original De Dion patent expired and at the London Motor j Show of that year in addition to six , of the eight De Dion models shown, the Pilain car was also making use of this design. However, the added , complications of engineering, and the additional cost outweighed die advantages, and when production of cars was revived in 1919 after the First World War, the De Dion rear axle was not being used. Until 1931 the only optional design to that of a normal rear axle was some form of the Rumpler design of 1921. However, in 1931 the famous American constructor of Indianapolis track cars, Harry Miller, used a modified layout with which he had experimented on a front wheel drive car the previous year. In this system the wheels were connected by an axle tube. Thus Miller was the first man to apply the De Dion system to the front end of a motor car, and when he used it on the back end of the car he became * the first user of the De Dion rear axle in racing for thirty-four years. His adoption of the design created little technical interest in Europe, and in 1934, as we have already recorded, Auto-Union and Mercedes-Benz used the Rumpler swing axle design on their Grand Prix cars.
The De Dion layout used by Horch on their eight cylinder cars between 1936 and 1939 closely followed the original De Dion layout, except in the use of conventional universal joints instead of the De Dion type, which allowed telescoping action as well as angular action. Therefore a telescopic joint was incorporated in each half shaft. Mercedes-Benz, who were then, and are now, using swing axles on their touring and sports cars, apparently took notice of the Horch design, and in 1937 their Grand Prix car appeared with the De Dion type of rear axle. To save weight torsion bar springs were used in place of the multiple leaf type which had always been used with this design hitherto. To positively locate the axle tube radius rods had to be used and to allow one wheel to rise while the other one stayed at road level the De Dion tube had to be split to allow one half to rotate slightly in relation to the other. This system proved to be very successful. It gave the best compromise of eliminating torque reaction from the back wheels, and gave a chassis that was completely stable at high speeds.
In 1938 Auto-Union abandoned the swing axles on their racing cars and used a De Dion system very similar to that used by Mercedes. In the post-war era we find the De Dion layout being used by Delahaye in conjunction with leaf spring on the original De Dion principle and by Dodge on a delivery van, the layout in this case also being close to the original scheme, except in the use of an H section forging for the load-carrying axle instead of the De Dion tube. Both designs use conventional universal joints and splined shafts for telescopic movement.
Also see: How it Works - Suspension 
