Oblongata writes -
“This will show you one way to build yourself a Ski Bike (aka Snow Bike, Skibob, Skike, or Skicycle). It is a downhill only machine and if you like bikes a lot, then it will probably keep a stupid grin plastered on your face all the way down whatever crazy hills you choose to take it on. It is easier to ride than it looks and is quite stable at high speeds (not sure about the speeds you could achieve at a ski hill, but its worked great for average hills in Wisconsin). There are many ways you could build one of these, this is just one way to get you started.” – Link.