I decided to do a little write-up on my motorized bicycle, as a request from many viewers of my previous instructable, "How I Spent $6.79 on Gas in One Semester."
In no way am I the very first to build such a machine. I am just a relatively recent addition to a long history of building these motorized bicycles. In fact, these bikes were used in the turn of the century. The very first Harley-Davidson motorcycle was a motorized bicycle, and modern motorcycles have evolved since. In the 1950's motorized bicycle kits called "Whizzers" were made in the United States, and then once again in about 2008 with high gas prices these bikes have made a comeback. Nowadays, most motorized bicycles are made with chinese 2-stroke engines. There are many people much more experienced than I on forums such as http://www.motoredbikes.com/ and http://motorbicycling.com/
The legality of these bikes varies a lot. It is unfortunate that in modern times these bike are often associated with the DUI crowd or tweakers. I believe they are banned altogether in Australia, and heavily regulated in some US states. In most states, the law limits the engine displacement to 50cc. Here are the states that I do know the laws on:
In California, the motorized bicycle cannot have more than 2 horsepower, and can't go faster than 30 mph. You also need to register it, have a M2 license, and wear a DOT motorcycle helmet when you ride. Unfortunately, these laws are not widely known in California. Many people mistakenly believe the law mentions a 50cc limit, which it doesn't. And nearly every Craigslist ad for these bikes says they are "perfectly street legal no registration just ride BUY NOW PL33ZE CASH ONLY." Another problem is that there is an old law that is still listed under the California Vehicle Code that says that "Motorized bicycles are exempt from registration." This law, passed in the 1970's, rather than being removed, has just had additional laws piled on top of it. So if you talk to different police officers, you will get multiple answers.
In Utah, the only requirements is that the bike cannot be above 50cc, no faster than 30 mph, and that you have a drivers license. Pretty easy, no police have seemed to care about me, and the only one who did check it out just said, "Thats weird, man. Looks like something from the 1920s" and left.
Ok, now for the cool part.
The bicycle itself is a Schwinn Delmar, purchased from Walmart. It is a single speed, coaster brake, and cost me about 100 dollars. Walmart bikes are not recommended for motorized use, because they are poorly assembled, and the welds can't hold up to high vibrations. I chose a single-speed model because I did not want to deal with Walmart derailleur components or cluttered handlebars (you need room for the throttle and clutch lever). I am a bike mechanic, so when I bought the bike I gave it a complete overhaul. I repacked both hubs with high-temp red automotive grease, and made sure everything was tight. You do not want a wheel to seize up on you at 30 MPH. I also fabricated a new mount for the front fender out of thick steel, because there are horror stories of people being thrown over the bars by a front fender mount that snapped from high vibration.
Also, just a coaster brake won't be enough to stop a motorized bicycle. If the chain were to be thrown, or the hub get too hot, you would lose all braking power. So I took some caliper brakes off of an old BMX bike and threw them on. The caliper brakes do not have a lot of stopping power, but they were easily outfitted to my bike and are good just to supplement the coaster brake.
The engine was purchased as a kit online, for about $180.00. There are many sources to buy them from, and aesthetically all the engines look identical to me. Problem is, most people cannot ship the engines to California (where I was building it) because of CARB regulations. Eventually I found a site based in california who could ship to me, http://www.californiamotorbikes.com/ The kit comes with all you need to get the bike up and running.
The engine is 50cc, single cylinder, air-cooled, single speed (even though it has a manual clutch), and 2-stoke. I feed the beast with 32:1 gas to oil mixture. I had a bit of trouble mounting the engine to the bike, but the kit came with a handy U-bolt to help attach the engine to the oversized downtube on my bike. The finished bike can go about 30mph, but I'm sure at a lower altitude and with performance parts you could get more speed out of it.
It uses very little gas. Many sources say it gets 150 MPG, but realistically it probably gets 100 MPG. The tank holds half a gallon, so wasting gas on joyrides and engine revving doesn't worry me.
These chinese engines aren't known for their reliability, but I make sure to take care of it. Every once in a while I make sure to oil the clutch gears (its a dry clutch) and in the one year I've had it, only had to replace two parts, the magneto and CDI (which were $10.00 apiece). The engine is simple enough to fix, and parts are cheap.
The only other parts I added onto the bike and kit were:
Caliper brakes. Taken from a donor BMX bike.
Analog speedometer. From Amazon. I had to use an analog one because for some reason my digital speedometer has a fear of engines. Whenever I started up the bike, the screen would go black, go bat-crap crazy, and reset. My guess is from spark-plug interference. So analog I went.
Double brake lever. From Amazon. This allows me to apply both brakes with one lever, and leave enough room on the handlebars for the clutch lever.
Headlight. Homemade, basically just a flashlight strapped to the handlebars.
Dirtbike seat. At one point, I had an old dirtbike seat on my bike that was attached with zipties. Currently I do not have it on anymore, so that I look more like a bicycle (can't beat bicycle parking!).
Really cool bike.
30 MPH, maybe more
Single cylinder 50cc
Chinese kit purchased online
Bike from Walmart
Other stuff added for cool factor
PLEASE obey the law if you end up building a motorized bicycle. People and legislators already think these bikes are the spawn of Satan and Rock n' Roll and Homosexuals and Methamphetamine.
As I've said in a previous 'ible, all it takes is one of us to whiz by a congressman's cousin on the sidewalk with one of these bikes to ruin it for everyone.
These bikes are fun and economical, please use them responsibly!
Wear a helmet, obey traffic laws, and have fun!