Selecting Materials for Component Construction

Story and Photography by Doug Gore

All successful racers are interested in saving weight and they should be. As long as sufficient traction is available, the acceleration of a car equals its engine torque (force) at the rear wheels divided by the car’s weight (mass). Less weight means greater acceleration, including cornering (radial) accelerations.

Racers also know that aluminum is much lighter than steel, and that magnesium is lighter still, so common sense would seem to indicate that aluminum and magnesium are better choices of materials for race car parts than is steel. Indeed, most racers automatically assume that aluminum parts are always lighter than steel counterparts and therefore choose aluminum whenever possible. That is not always the case however. To understand why, we need to know more about the metals commonly used in race car construction.

For the following discussion we will focus on materials used for suspension and steering links, but the material characteristics are generic and apply to all components.

Steels have the widest variety of alloys and different mechanical properties to choose from. Low carbon, or “mild”, steel is a mixture of iron and between 0.10-0.30 percent carbon. Mild steels have yield strengths in the range of 40-60 thousand PSI and tensile strengths in the range of 50-70 thousand PSI. Yield strength defines the minimum force necessary to pertinently stretch or bend a piece of metal and tensile strength is defines the amount of force necessary to break the metal apart.

The greater the difference between a metal’s yield strength and its tensile strength, the more it will stretch or bend before it breaks. The amount that a metal stretches before breaking is called its elongation. Metals that stretch appreciably before breaking are called ductile or malleable. Lead and copper are both very ductile. Materials whose yield strength is close to their tensile strength are said to be brittle. Glass and very high strength tool steels are both brittle. All mild steels are between these two extremes and are generally considered to be ductile, meaning that they will bend and stretch a fair amount before they break. High elongations are desired for race car chassis parts so that they will bend a lot before breaking apart.

Another important characteristic of mild steels is that they are not heat treatable. That means that heating those steels to their melting temperatures and rapidly cooling them will not appreciably change their mechanical properties including their strength. That is one reason that mild steel can be welded without regard for heating and cooling rates. Welding will not change mild steel’s base properties.

If the amount of carbon in the steel is increased beyond 0.3 percent, it becomes known as “high carbon” steel. The higher carbon increases the steel’s yield and tensile strengths, while also decreasing its elongation. It also makes the steel responsive to heat treating and thus more difficult to properly weld. Race car chassis parts are rarely made from high carbon steel because they tend to snap on impact.

The addition of small percentages of other elements to mild steel can turn it into alloy steel. Alloy steels, and there are hundreds of them, offer many advantages over carbon steels, including greater strengths. While the increased strength often comes at the price of increased brittleness, that is not always the case.

For example, 4130 chrome-moly steel is a popular low alloy steel used for race car parts. When formed into aircraft quality tubing, this steel is supplied with a heat treatment that results in a yield strength of 75 thousand PSI, a tensile strength of 95 thousand PSI and an elongation comparable to the mild steels. . Annealing the tubing reduces its strength to 60 and 90 PSI respectively, and additional heat treatments can increase the tensile strength to over 180 thousand PSI. At that high strength the metal becomes quite brittle. That is why care should be taken when welding chrome-moly tubing to avoid rapid cooling and thus hardening of the steel.

One of the interesting things about the many different types of steel is that they all weigh virtually the same, regardless of the alloy content. Another is the fact that they all have the same springiness, or “modulus of elasticity”. That means that a coil spring wound from ordinary mild steel wire will have exactly the same rate as one wound from the best “super alloy” steel if the dimensions are the same. The difference will be that the “super alloy” spring will compress more before taking a set. See Table 1 for selected steel properties.

Aluminum is different. First, aluminum and its alloys only weigh a third as much as steel. Second, the strongest aluminum alloys have less than a third the strength of the strongest steels but are stronger than the mild steels. Third, the higher strength aluminum alloys tend to have low elongations so they often to break rather than bend in a crash. And finally, all of the high strength aluminum alloys require specific heat treatments to gain their strengths. They also loose most of their strength if welded.

Pure aluminum is the weakest alloy, with a tensile strength of only 13 thousand PSI, or one third that of the weakest mild steel. Never the less, the ratio of strength to weight of pure aluminum is close to that of the weakest mild steels. That can be a plus for aluminum. See Table 1 for selected properties of different steel and aluminum alloys.

As with the steels, the addition of small quantities of other elements to pure aluminum results in alloys that have greatly increased strengths and reduced elongations, especially when followed by heat treatments. Several of the popular aluminum alloys like 2024-T4 and 7075-T6 can have strengths equivalent to that of the stronger mild steels. The ubiquitous 6061-T6 has a heat treated strength in the mid range of the mild steels, and, at only one third the weight. That makes it very attractive to racers.

An important difference between the high strength aluminum alloys and the mild steels is the aluminums require a heat treatment for strength and the mild steels do not. The heat treatment limits what racers can do with the higher strength aluminums. When 6061-T6 is welded, it looses its strength in the area of the weld, and reverts back to a strength of only 18 thousand PSI. All aluminum alloys with a –T3, –T4, or –T6 at the end of their alloy number have been heat treated and will loose most of their strength in the area of a weld if they are not re-heat treated after welding.

While aluminum is only one third the weight of steel, its stiffness is also only one third that of steel. That fact tends to cancel out aluminum’s weight savings potential on parts like suspension links where stiffness is required since three times the amount of aluminum is required for the equivalent stiffness. Further, in the cases where high strength is required, selecting a metal whose ratio of strength to density is high is generally a better choice as long as its elongation is high enough to prevent fracturing. That is an area where chrome-moly shines.

That said, suspension links fabricated from fairly large diameter chrome-moly tubing with welded in place spuds are generally the stiffest links for a given weight and almost always the lightest links for a given strength. The steel links generally have the lowest material costs but also have the highest fabrication costs. Dispite the popular belief, aluminum suspension links are not always the lightest choice for equivalent strength or stiffness.

Engine blocks and heads are a different case. There, aluminum parts offer a tremendous weight reduction but come at a higher cost over cast iron.