I've got the one with the dangly bit at the back, the open bulldog clip thing that the metal belt runs between, 2 spikey plates at the front, a collection of spiked saucers nested together on the backwards wheel, and extra wobbly mini-brake handles on the hand holders which adjust the hardness of the pedalling. Seems to work quite well on my town racer.
I've got the one with the dangly bit at the back, the open bulldog clip thing that the metal belt runs between, 2 spikey plates at the front, a collection of spiked saucers nested together on the backwards wheel, and extra wobbly mini-brake handles on the hand holders which adjust the hardness of the pedalling. Seems to work quite well on my town racer.