Automotive Suspensions

Automotive Suspensions

The reason cars have suspension systems are so that the passengers do not feel the uneven road being driven on, effectively for a smooth ride. The primary elements that make up an automotive suspension system are springs, shock absorbers and anti sway bars attached to the axles and frame. The materials used to compose these parts have developed dramatically from when the first suspension system was created. The materials used are crucial to create a successful system, new materials have been developed such as alloys, which allow suspensions to be lighter, stronger and more efficient.

Springs are a vital part the automotive suspension, the initial shock impact is felt by the tyres, shortly followed by the springs. A spring can be described as:

"An elastic body that has the primary function to deflect or distort under load, and return to its original shape when the load is removed."[3]

The torsion bar spring is a bar or tube which has been twisted lengthwise to give a spring action. The torsion bar, unlike a conventional spring, twists longitudinally rather than compressing down and then releasing a load. A lot of strain is put on the torsion bar spring; therefore it is made from an alloy to deliver strength and toughness. Some vehicles use a steel alloy but there is an example of Toyota utilising an alloy called nickel chro-moly, which has also been used in aircrafts [4]. The alloy nickel chro-moly is a combination of heat treated nickel, chromium and molybdenum. Together these produce a tough metal with great fatigue resistance.

Older leaf springs are manufactured from thin steel plates layered on top of each other. More recent suspension systems will have fiberglass coated leaf springs. Fiberglass has a larger tensile strength than steel so for this reason it is used for leaf springs [2]. Tensile strength is measured as how much strain can be put on a material before it breaks, for this reason a high tensile strength is a sought after property in suspensions.

The most common type of spring used in cars is a coil spring. These are generally made from alloy steels. An alloy steel is a mixture of Iron (Fe), Carbon (C) and any other elements that better the physical properties. An element called Molybdenum is often added to steel as an alloying element [1]. It improves the material properties because it increases toughness, strength and corrosion resistance. This is increased further when molybdenum is used alongside silicon, manganese or chromium.

Choosing the raw materials for the manufacture of the coil spring plays an important role. The steel has to be checked thoroughly for defects which could affect the end product. Some of these defects include inclusions, an inappropriate microstructure or even decarburization [3].

There are several different methods of manufacture of coil springs which have been developed. The method I am going to discuss is one adopted by a company called Tanabe USA Inc.; who have been in the automotive industry for over 20 years. In production they wind high-tensile steel at a cold temperature, rather than heating it first like many other manufacturers do.

The reason the steel alloy is usually heated first is because this makes the metal soft and therefore easier to bend into a coil however heating the metal changes the "internal molecular structure" [5], this change in structure then actually deteriorates the strength of the steel. For this reason Tanabe USA Inc. is cold winding instead using a machine called "WAFIOS" [5], where the machine uses a large amount of strength to bend the steel alloy.

The function of automotive suspension is to absorb shock; this shock does not directly hit the springs though. First the tyres take the jolt, this shock moves from the tyres and into the suspension system.

As can be seen in [Figure 1], after the shock has been to the tyres it moves to the springs which are attached to the axles of the wheels. The coils then take the impact load. From this process it is easy to tell that the coil springs go under a vast amount of strain so the physical properties the springs deliver are very important.

The wear and tear of automotive suspension coils can sometimes fatigue, therefore the test of this is crucial because if a car suspension was to fail it could cause an accident. Ideally the strength of the metals need to be tested and this can be done by tensile testing machines. This can help justify the metal to be used. To test fatigue, car manufacturers have simulation machines which can perform operations to see what the material would be like after for example 5 years of use.

The automotive industry is constantly looking for newer, stronger, more reliable materials to use in car suspensions. As new alloys are being developed, car suspensions are at a technical high. In the years to come I think that suspensions will keep the same concepts but the materials will change vastly.

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