With FBS Spacers you can ride single bearing wheels such as Oak Wheels or Blackriver Wheels on your Ytrucks, which are designed for wheels with 2 ball bearings (dual bearing wheels). Ytrucks have a longer axle than for example Blackriver Trucks or Dynamic Trucks.
Your single bearing wheels slip back and forth on the axle and therefore don't roll perfectly.
FBS Spacers. They replace the 2nd bearing and you can ride single bearing wheels on your Ytrucks without any problems.