This expression has been part of the English language for well over 400 years. You may wonder how putting a spoke in a wheel will impede anyone's progress. After all, a bicycle wheel is full of spokes, putting another one in is unlikely to slow things down. What you must remember is that the wheel referred to here is the ancient wheel — the solid wooden wheel, one which didn't have any spokes. These wheels, however contained a hole, and whenever someone wanted to stop the wheel from moving, he inserted a spoke into the hole. The spoke or pin acted as a brake and thus impeded movement.