I recently came across plans for building a go-kart without an engine. It is a sport called “Soap Box Karting” in which people take their cart to the top of a hill and coast down, usually racing their friends in their friends’ own soap box go karts.
Caution: this is a very dangerous sport and it is advised that the roads be blocked off to the public whenever using the public roads for competitions. Also be sure your cart is equipped with an excellent braking/steering system and follow the instructions in the plans!
Plans can be found here (scroll down, you’ll see them): http://makezine.com/projects/Adult-Soapbox-Kart/
A lot of pictures of the Weekend Warrior Soap Box Kart can be found here: https://www.flickr.com/photos/11436606@N02/sets/72157623575987498/
And if you’re still curious and want more information from the builder, check out the forum thread here: http://forum1.aimoo.com/MISFITS/General-Discussion/The-Weekend-Warrior-Chonicles-1-1763287.html