Bikes are pretty easy and very rewarding to modify on the street. Here's two very invention street modifications from Australia. They were found by Wade Hatler. He writes on his travelog: "I took off for a six-month bicycle tour two years ago and never quite managed to go back home." Along his travels on the backroads of Australia he spied these two remarkable bikes.
"Why would someone build a homemade trike you ask? Well, because trikes have some compelling advantages, but commercially built trikes are pretty expensive. Someone with some skill in the machine shop can take a couple of old bicycles and make a nice stable, comfortable commuting vehicle with practically zero expense. The one [below] here has an actual office chair as the seat. You can see he hauls his daughter around on the trike, and life is good. Front wheels are simply the front steering assembly and part of the frame from a diamond frame bicycle. A crossbar hooks them together, and to the frame that holds the foot cranks and pedals. A small tie-rod connects the front two tires together, and steering is just a chunk of the original handlebars. You don't get a blisteringly fast performing trike with Ackerman steering in this fashion, but you can get a good, solid, dependable, environmentally friendly vehicle for just about free."
"This invention [below] is called the BiQuadly. It's a four wheel, fully independent suspension, two wheel drive, multi-speed rig that's powered by both the arms and the feet. I've never seen anything quite like it. It's all a handmade prototype: Basically, you power the bike by cranking the pedals, which go through a conventional bike's derailer to a jackshaft, which then goes back to each of the rear wheels. The two hand levers you see move in both the back to front and the left to right planes. In the back to front direction, they hook to a crankshaft. Once you get going, you can crank back and forth on the hand levers, and it applies power from your arms onto the pedals to assist your legs. In the left to right direction, the handles are hooked to the steering, and they make the front tires turn to steer."