If you are riding at 20MPH, the axis of the wheel is traveling forward 20mph, but the other parts of moving forward at different speeds, ranging from 0-40 MPH, but always with an average speed of 20 mph. The speed of any given wheel part is depends on how far it is from from the axis and the point in the rotation. The bottom of the wheel is always moving forward 0 MPH, otherwise the tire would be sliding across the pavement. The top of the wheel is therefore always moving twice as fast as the moving speed (40mph), because it is spinning forward at 20mph as well as moving forward with you at 20mph. The spokes are between the axis and the tire and at the top of the rotation will be moving forward at a speed that is between 20-40. The distal end of the spoke will be moving closer to 40 and the proximal end will be moving closer to 20. Likewise the spoke at the bottom of the rotation is moving at a speed somewhere between 0-20, depending on the distance from the axis.

Heres another way to explain it...if you are going along at 20mph, and you lock your brake into a skid, how fast is the top of your tire moving at the moment it locks?....its moving 20mph. In order for it to spin, it must move twice as fast at the top (20mph) and zero MPH at the bottom.