Side Cap Bearings, Stacks, Side Stacks, or Hubstacks are a yo-yo design element or auxiliary modification consisting of bearings that are added to the hub and covered with caps, exhibited on many yo-yos including the MiRoC, eight8eight and 4xl. The purpose of Hub Stacks are that they allow the player to grab the bearing caps while the yo-yo is spinning, allowing for a variety of tricks that were previously not possible. The most popular setup for Hub Stacks is when the inner race of the bearing is resting on the yo-yo "post" and the outer bearing race is resting on the cap, making it easy to grab with much less friction, making the yo-yo spin freely in between your fingers like a gyroscope. Side Cap Bearings are also commonly known as side bearings, bearing caps (AnY) and hubstacks (YoYoFactory).
The Side Cap Bearings first showed up as Modders adding ball bearings to the sides of their yo-yos. Yo-yo maker and modder Frank Difeo of Dif-e-Yo eventually added bearings to the side tips of the Spike-Fly to make the Gladiator, which he described as a “bearingized” version of the Spike Fly. Next, the AnY MiRoC was the first mass produced yo-yo with Side Cap Bearings, naming them Bearing Caps (However, this design had problems with the caps staying on the yo-yo). Soon afterwards, YoYoFactory came out with the G5, installed with side cap bearings referred to as Hubstacks. After this many other companies have followed suit, creating a variety of side cap bearing yo-yos that have helped change the way yo-yos are used and perceived.