The best replacement for quads is dirt bikes.

The best alternatives to quads are rated as following:
★★★★★  dirt bikes
★★★  hamstrings
★★★  triangles
★★★  quads
★★★  royal flush
★★★  straight flush
★★★  inlines
★★  thrusters
★  atv
★  hamstrings first
★  tris
★  full house
★  snare
★  inline
★  hamstrings bigger
★  bikes
★  4 wheelers
★  quads poker
★  four wheelers
★  glutes
  long arc quads
  atvs
  triangles opengl
  dirtbikes
  straight flush odds
  atv's
  hams

Your alternative for quads is not listed? Suggest!
Speaking about quads, one probably shouldn't forget about two moms.

Other search terms related to quads and their ratings are:
★★★★★ two moms
★★★ 2 moms
You have in mind a companion for quads and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of quads in all years. Click on graph for more.
quads
dirt bikes
hamstrings