3 speed Shimano internal gear hub w. Coaster brake build in 16 Rhyno Lite wheel

  
My build took too long, though, and my son was outgrowing it by the time it was ready, and he only rode it a few times before moving to a bigger bike. This is a great upgrade for a single-speed bike in a hilly area! It came with a 19-tooth sprocket, which I switched to a 24-tooth sprocket for better performance in the hills (both are included - see pics).

Has coaster brakes for kids who haven't transitioned yet - great for putting on a bike with hand brakes to facilitate learning both before transitioning out.