It is a circular metal part with teeth that mesh with the drive chains. The 170mm dimension refers to the length of the crank arm, which is the part of the driveline that connects the chainwheel to the pedals.
In bicycles, the chainwheel assembly is responsible for transferring power from the rider’s pedals to the rear wheel. The number of teeth on the chainwheel determines the gear ratio, which affects the speed and torque of the bicycle. In fire trucks, the chainwheel assembly is used to drive the rear axle.