These are new, tapered versions of the smooth-spinning ball bearing rollers from Item 15464 JR HG Alum Ball-Race Rollers 19mm (Ringless). Tapering them encourages the car to stay on the course when it mounts the walls, with the aim of raising your race completion rate!
Tapered Aluminum Ball-Race Rollers (Ringless) x2
Built-in 520 ball bearings give smooth rotation.
Screws and spacers for use in attachment are included.