NOTE: This is a placeholder instructable. The real instructable is gigantic and is still in progress.
This instructable is part of a series I am making which details the process of making a motorized bicycle from scratch. The parent instructable is coming soon, and will include everything in this instructable, however since that is still over a month out, I figured I'd let you guys get started with part 1. I will add a link to the parent instructable once it is complete.
The first step (and one of the most important) in motorizing a bicycle is to choose a good bike to motorize. You can't just go slap a motor on any old bike and expect it to work, especially one as large and powerful as the motor we'll be using. A good bike to motorize has several key attributes, all of which must be fulfilled if the build is to be successful. The bike in the above image meets all of my requirements except having a steel frame, but I will use it to illustrate the other things to look for because I already have pictures of it and it's convenient.
Step 1: The Frame
First, the bike must have a large enough frame gap to fit the engine we will be using, which is the 79cc harbor freight predator. This is a fairly large engine to be cramming into a bicycle, so as far as frame gaps go, the bigger the better. Remember that the pedals must still be able to turn after the engine is placed into the frame.
The frame must not be made of carbon fiber, as carbon fiber will crush and splinter if we try to mount the motor to it. Aluminum is not great, but it will do if you don't intend to ride it much. Be warned that if you do use an aluminum frame, the frame will eventually develop cracks and break from metal fatigue. Steel is great, by far the best. The bike in the picture has an aluminum frame.
Some specific frames that work well are the Trek 820 (one of the bikes I will be motorizing for this series will be a trek 820) and the older steel frame models of the Specialized Stumpjumper and Hardrock. Look for a 21" frame (or larger if they exist).
Step 2: No Road Bike Handlebars
You will want to avoid using a bicycle with ram's horns or other handlebars with curved ends, as it will force you to use a thumb throttle instead of a twist throttle, the former of which makes your thumb get tired fairly quickly on long rides. If you have a road bike and don't want to spend the money on a new frame, however, there are workarounds. For example, I have a motorized road bike which has a thumb throttle and also has the rear shifter converted into a second throttle control that locks in place.
Step 3: Good Brakes
One of the most important features of a motorized bicycle is good brakes. Because you will be going faster, you need more stopping power than an average bike. I'd recommend getting a disc brake for the front, however you can not have any sort of disc brake in the back, as the sprocket that drives the bike will be mounted to the disc brake mount. In order to compensate for having normal (although still good) bicycle brakes on your bike, we will be setting up the engine in such a way that letting off on the throttle makes the engine act as a secondary brake.
Step 4: Wide Rear Hub Clearance
The rear sprocket will be mounted to a disc brake mount on the rear wheel, so we need enough clearance that the chain does not rub against the frame (this will kill a bike frame very quickly).
Step 5: Crank Clearance
The cranks must be able to clear the engine and jackshaft once it is mounted in the bicycle. If you have a 21" frame, this probably won't be an issue. Don't worry about not being able to put your feet on the pedals with the engine in place, we can extend the pedals with pieces of metal if necessary.
Step 6: Nothing on Top Bar
It is helpful, although not absolutely necessary, to not have cables running along the top of the top bar, as they get in the way of mounting the gas tank. If your bike does have cables on the top bar (like the Trek 820), don't worry, we can always reroute the cables later.
Step 7: Sturdy Wheels
Because weight is not really an issue now that you won't have to propel the bike yourself, you should make sure to beef up the bike's wheels. They will be hitting bumps and rocks at much higher speeds than they normally would, and normal bike wheels won't hold up too well to prolonged riding. I recommend getting wheels with 12 gauge spokes (this will probably not happen unless you get really lucky or are willing to spend $400 on custom-built wheels), quad laced if you can find them, with as many spokes as you can get. Double-walled rims also help, but are not really necessary unless you plan to ride a lot.
Step 8: Rear Disc Brake Hub
You will need a rear hub with a disc brake mount.
Step 9: Good Tires
Get some puncture-resistant tires. Also, try to choose thick tires with a lot of tread, as this will both reduce your risk of crashing and provide you with a more comfortable ride.
Step 10: No Rear Suspension
Rear suspension makes motorizing a bicycle much more complicated. Stay away from this unless you have built these bikes already and want a challenge.
Step 11: Buy a Bike
Now start looking for a bike! I recommend craigslist as a good starting point, as you can usually find suitable steel frame mountain bikes at good prices without too much searching.