Mixing metals

With few exceptions, con rods for automotive engines are split at the big end, and the joint is held together by a pair of threaded fasteners. The pattern is much the same for racing con rods, the only common exceptions to this being rods for two-stroke engines or single cylinder four-stroke engines. In these cases the rod has no split at the big end and the crankshaft is made up of two or more pieces.

Where the rod is of the usual split type, it is generally the case that the rod and its cap will be made from the same piece of material, quite often in the form of a forging. A wide range of metallic materials are used, from aluminium through titanium to steel and even metal matrix composites. Where billet con rods are made, it is still conventional to produce the rod and cap from the same billet. However, there might be occasions where the designer has to consider a cap made not only from a different piece of material but from a different material altogether.

The reasons for doing this would have to be compelling as there could be problems with taking this approach. Where a different material is specified for rod and cap, it is will generally in a search for increased cap stiffness. If the cap is felt to lack stiffness, it might be the case that other components limit the scope to increase the stiffness of the cap through geometry changes.

For instance, in an inline four-cylinder engine, the distance between the crankpin and the closest components in the sump/lower crankcase might not allow any appreciable increase in cap stiffness. To increase the stiffness through geometry changes, we will generally look to increase the height of the ribs on the cap. Increasing the height of the ribs is the most efficient way to improve stiffness in terms of adding the least weight per unit stiffness increase.

However, where we have reached the limit of increased rib height, a material substitution for the cap might be our only option. This is really only sensible for rods made from materials that are less stiff than steel. For a steel rod, there are only a few materials that offer an appreciable increase in stiffness, and these are generally much more dense.

Compared to titanium, steel offers an increase in stiffness of 75% or more, depending on the precise grades of titanium and steel in question, but we can see that large changes in stiffness are made possible by material substitution. If the change required is less than this 75% then the mass penalty of the steel cap can be lessened by reducing the rib height.

The problems associated with using a steel cap on a titanium rod will come from the physical properties of the material. As the rod reaches working temperature, the thermal expansion of the rod relative to the cap will tend to superimpose a distorted shape on the big-end bore, which is additional to whatever dynamic distortion is present. The problem would be worse if we were to use a steel or titanium cap on an aluminium rod, owing to the greater difference in the coefficient of thermal expansion.

Written by Wayne Ward