These are two brackets that can mount to the bottom of the Trekker to hold photo reflectors.� The disks stick to either the 3.5" wheels or the 4.5" wheels sold with the Trekker Chassis.� Black and white contrast on the wheels will be read by the sensors which will tell the OOPic or other microcontroller how far the robot has traveled. Hardware is included to mount the brackets to the Trekker Chassis.� If you have the Trekker Line Following package, two of the sensors can be mounted to this kit and you'll be ready to go.