Development of Fracture-Split Connecting Rods Made of Titanium Alloy for Use on Supersport Motorcycles

A connecting rod made of titanium alloy is effective for lower fuel consumption and higher power output comparing to a steel one because the titanium connecting rod enables to reduce the weight of both of reciprocating and rotating parts in an entire engine substantially. But up to now, it has been adopted only to expensive and small-lot production models because a material cost is high, a processing is difficult and a wear on a sliding area should be prevented. In order to adopt the titanium connecting rods into a more types of motorcycles, appropriate materials, processing methods and surface treatment were considered. Hot forging process was applied not only to reduce a machining volume but also to enhance a material strength and stiffness. And the fracture-splitting (FS) method for the big-end of the titanium connecting rod was put into a practical use. The brittle fractured surface of the big-end by FS method is useful for not only cost reduction but also ensuring the stiffness of the connecting rod compensating the low elastic modulus of titanium alloy comparing to that of ferrous alloys. In this study, the evaluation results of materials characteristics related to these processes and functional property of surface treatment is described. From these evaluations, the capabilities of FS titanium connecting rods were quantitatively shown. FS titanium connecting rods have been produced for the supersport motorcycle engines from November 2014.