The inner side of the crank is hollowed out to reduce the weight to reduce the burden of riding. It is not easy to be deformed when pedaling hard, and it has better rigidity and force transmission effect. Material: aluminum alloy body + chromium phase steel shaft.Bottom shaft: 24mm screw-in bottom shaft Length of shaft center: 115mm. Crank + bottom bracket + sprockets. Also, the each additional item means. The the same color and size. Transit time may be a little delayed because of the. If you have any questions.