What do those two little wheel on your rear derailleur do? Bicycle pulley wheels or jockey wheels feed the chain evenly into the rear derailleur maintaining even tension on the chain for smooth shifting. They are generally inexpensive to replace and are also an opportunity to upgrade your bicycle drivetrain. Oversized pulley wheel systems developed by Ceramicspeed reduce friction through less chain contact and extremely precise bearings. Of course keeping your pulley wheels clean helps tremendously as well as this is an area that builds up dirt and excess lube creating chain drag.