In these threads, I've seen reference to "spacers", in reference to tires and wheels. What are these? What is the up-side and the down-side in using them?

Thanks,
Richard

 

Upside- no rubbing with big brake kits (with the wheels) and/or wheels (with the wheel well), in other words, it helps you clear things.

Downside- hopefully someone will help me with this one, cause i really have no clue.