Tamiya 95683 HG Lightweight 13mm Tapered Aluminum Ball-Race RollersTapered rollers are a great addition to your Mini 4WD, giving it a better chance of coming down and back onto the track should it ride up onto the course fence. These rollers are 13mm in diameter and have ten holes around the outer section that save about 20% in weight. Built in 520 ball bearings ensure smooth rotation, and the set includes two rollers, plus two screws and four aluminum spacers for use in their attachment.