Pololu Wheel for Micro Servo Splines (20T, 4.8mm) - 40×7mm, Black, 2-Pack
These black plastic wheels have silicone tires and measure 40 mm (1.57″) in diameter. They mount onto micro servo splines with 20 teeth and a 4.8 mm diameter, such as the splines on FEETECH’s FS90R micro continuous rotation servos and FT90R digital micro continuous rotation servos. This product is a pair of wheels.
These wheels are great for robotics, custom RC vehicles, or any other project you need to get rolling. The combination can serve as a small, inexpensive, and easy-to-control drive system for a miniature robot. The wheel presses onto the servo spline just like a typical servo horn, and the small screw included with the servo accessory pack for mounting servo horns should be used to hold the wheel in place.