A mountain bike (MTB) cassette is the cluster of gears on your rear wheel that determines your pedaling resistance and climbing ability. Upgrading or replacing your cassette is common for improving gear range, shifting smoothness, and reducing overall bike weight.Before upgrading, you must verify your rear hub’s driver standard (e.g., Shimano HG, SRAM XD, or Shimano Micro Spline) and the maximum gear size your derailleur can clear.