Atomic Zombie has a great step by step on making your own chopper bike, KoolKat writes – “Kids’ bikes with 20-inch wheels are abundant at the dump and yard sales, especially the cheap steel frame units. These bikes not only take a beating, but they are outgrown in a year or two, so you will probably find a lot of them at your favorite scrap yard or city dump. These frames are easy to chop, and 20-inch bikes make great choppers, but unless you are only 4 1/2 feet tall, there won’t be much leg room on one of these bikes.
The problem of size is compounded even more if the head tube angle is taken back to add more rake, as this pushes the handlebars even closer to the seat. At this point, your only option is to move the seat higher or farther back, creating either a goofy looking chop, or a flying death trap that pulls uncontrollable wheelies on so much as a sneeze.
To get a little more leg room on a chopper made from kid’s 20-inch frame, two frames will be joined together in order to move the bottom bracket further up. The head tube will also be pushed forward, allowing for a nice long set of forks to be installed without creating a super tall wheelie machine.” – Link.