This was also a project that kept changing. The initial design worked, but not very well (or for very long.) I kept redesigning as I went along, tweaking the bike for more reliable use. In its current condition it is quite effective at moving someone around and makes for a quick and easy to build project. My initial build lasted about 8 hours over one day. When I hopped on the thing and went flying down the road at speed, I was very thrilled and surprised to have gotten that much of a result out of one day of bodging. While you view this project, please keep in mind that some major improvements could be made to my design to fix various safety and performance issues. If you plan on building a similar design, make sure check out the lessons learned step before you build.
For more info about this project and a bunch of others, check out my website: thewidgetforge.com
Step 1: Design
The design was constrained most by the motors available to me at the time and my lack of welding capabilities. I wanted to use my all-terrain style 12 inch tire scooter along with a clutch but I couldn't come up with the necessary equipment. My searches of local (and on trip that was not so local) yard sales and eBay returned few useful motors. Mounting the motor without welding also posed a challenge since that was the efficient and obvious way to fix everything together.
When I couldn't get a motor with a clutch, I got frustrated enough to haul off and improvise a spindle driven design for my bike from 5th grade. The bike is quite small for me, but still allows pedal starts and has coaster brakes which frees the handle bars for the gas and kill switch. I decided to make it front wheel drive for ease of construction: there's a lot more free room up front.
For the engine, I used a small 26cc McCullough engine which came from a hand-held leaf-blower. It is a half shaft motor (only one side of the crankshaft is supported) without a clutch, but it had a threaded shaft which allowed for easy attachment of the spindle so I was happy.
I initially went very simple with the design: just make some brackets and bolt it on. That worked for a few miles when the engine mounts loosened up and the spindle stopped transferring power. I eventually modified the bike to include a spring tensioning system to keep the motor firmly on the tire.
Step 2: Attaching the Spindle
Step 3: Strapping on the Engine
With the engine screwed securely to the plywood, I continued by making brackets to connect the plywood to the bikes front fork. These brackets bolt around the front fork, through the plywood and to a set of flat brackets with 5/16 inch bolts. The Brackets are able to tighten down on the fork so there is enough tension to hold the spindle to the wheel.
First Test Run
At this point, I took the bike on its first test run, which didn't last very long at all. Actually, more accurately, it never started. The 1/4 inch plate tended to bend rather than engage the tire. I couldn't get enough traction to kick over the engine. Hence why thicker 1/2 inch plywood, or even better 3/4 inch, would be much more effective. To stop the plywood from bending without starting over, I used more aluminum strapping bolted to the face of the plywood. Amazingly, this worked! With some furious pedaling I kicked the bike over and the motor started, rocketing me to the end of my street. This was very surprising and quite exciting to me since most of my poorly thought out, frustration ridden second attempts tend not to work out.
Step 4: Adding the Spring
Step 5: Throttle
Step 6: Killswitch
Step 7: Results + Video
Here's a video of a "high-speed" run (about 25 mph) down my street. I hadn't added the hand throttle yet so I had to pull the engine's original throttle cable to accelerate, which is why I'm awkwardly leaning over the front of the bike.
To start the bike, I use the standard priming and choking procedure. This usual includes a priming until the bulb fills with fuel followed by a quick pedal start with the engine fully choked until it kicks over. Once their is fuel in the lines, I open up the choke to the middle setting. The motor starts when the bike is pedaled to around 8 mph, which can take some furious pedaling, but isn't too bad. The bike is even easier to start when it's hot. At about 12 mph the bike has enough power to start accelerating up to speed.
Acceleration is very sluggish at low speeds, but over 15 mph the bike zooms along with adequate acceleration. According to the bike odometer I have attached, it has hit about 29 mph on a slight down hill. It can go up significant hills and still hold 15 mph. Average speed riding around the neighborhood is about 18 mph. If you think of the machine as a bike with a power-assist rather than a mini-bike, it is very reasonable. With a little gas it is effortless to cruise at 20 mph, a speed barely attainable on a downhill when pedaling this bike. Since your rear is near the ground, it feels even faster than it is.
Is it worth it?
For all of the frustration that comes with keeping the machine running, the question you have to ask yourself is: Is it worth it? Well, as long as you enjoy hot exhaust blowing on leg and the wind in your hair while flying down the road at obscene speeds with a small two-stroke between your legs screaming at 7000 rpm, it is definitely worth it.
Step 8: Lessons Learned
- Use at least 1/2 inch thick plywood for the plate (1/4 inch thick aluminum or steel would be even better it you can cut it)
- Add a tensioning mechanism from the start.
- There is definitely a bracket geometry that would work better. A stiffer set up that connects on the other side of the wheel would make for a much stronger and more stable platform.
- The rims that came with the bike are not nearly strong enough to take the 25 plus miles an hour the bike can achieve. Mine have come out of alignment and been seriously bent a few times already.
- Steel brackets would be better to attach the motor plate for added strength and safety. One of the aluminum brackets I made broke at one of the bends. A set of store-bought u-bolts would probably be the best solution and they would be even easier to use as long as you can find the right size.