Originally Posted by Richard Kennedy
Close, but not quite the full story.
Just because 17 is prime doesn't mean you'll get 17 skid spots. Consider the gear 51x17. 51/17=3 , so assuming you skid with your pedals in the same place each time you'd only get 3 skid spots.
No, 51x17 gives you 1 skid patch assuming the pedals are always in the same location, or 2 for two pedal locations. 51/17 = 3 means the wheel spins three times for each crank rotation, so every full rotation of the crank results in the tire being in the same exact position as at the start of the revolution. However cause half a rotation of the cranks moves the wheel 1.5 times, you get two skid patches for each half revolution.