This is a set of optional parts for Mini 4WD car models.
From the manufacturer:
These are lightweight versions of the popular 19mm aluminum rollers, with 6 holes in the body for a weight savings of roughly 30%! Rollers are given a black finish, and have a 520 ball bearing installed for super smooth rotation. This product includes 2 rollers, plus spacers and screws for attachment.
Compatible Cars: Mini 4WD and Mini 4WD PRO cars (Some bodies may require modification)