What is the name of the part, often seen on BMX bicycles, that allows an additional rider on the back the bike? What is the name of the part, often seen on BMX bicycles, that allows an additional rider on the back the bike? It appears to be a set of relatively short but thick poles that go on either side of the axle of the rear wheel so a passenger can stand on it and—holding onto the shoulders of the driver—can be transported with minimal effort.

What is that part called?
They are called Pegs.
Pegs are mainly used by BMX riders to help perform various tricks.
Flatland trick (standing with all of his weight on one peg, not damaging anything):

Grinding a wall on the rear peg (imagine the forces when the rider jumps on the wall, still: no damage to the hub but of course to the peg and the wall):

Images from Wikimedia Commons