Tamiya Mini 4wd 95563 HG Lightweight 17mm Aluminum Ball-Race Rollers
From the manufacturer:
These are lightweight versions of the popular 17mm aluminum ball-race rollers, with 520 ball bearings built in for superior smoothness of rotation. The product includes two rollers, plus screws and spacers for use in attachment. These components are around 25% lighter than the base design.
Mini 4WD and Mini 4WD PRO cars