Development of High Strength and Low Cost Motorcycle Transmission Shafts

Evolution continues for small, light weight and high output engines for motorcycles. As a result, high strength steel is a necessity for internal engine parts, including mission shafts. The transmission shafts must have sufficient torsional and bending strengths to withstand the higher torque, along with intermittent sharp torque inputs, and higher bending force. From a production point of view, using a coil fed cold forge process results in superior productivity. This productivity cannot decrease by changing to a high strength material. Also, there needs to be a balance of high strength versus post carburizing straightening. A new class of steel was developed by adding boron and adjusting Cr content of a conventional case hardening steel to produce a flat quench curve (Jominy curve). A flat Jominy curve is desired to increase core hardness and reduce variability. An increase in core hardness results in higher torsion strength and the reduced variability results in a more consistent final product. In order to maintain the ability to straighten the transmission shafts, a maximum internal hardness value was set. This value is controlled by carbon, boron and Cr contents. Additionally cost reduction was realized through the elimination of a post cold forge normalizing process by the addition of Ti and Nb grain refiners. With the implementation of the newly developed steel, a higher strength transmission shaft was achieved at a lower cost.