This was one of those projects I couldn't get out of my head. I had seen, on the internet and in person, the small 2-stroke engine powered scooters which were becoming increasingly popular. Naturally I wanted one, but I'm not the type that would just buy a commercially available scooter that will work well and last for years and be happy about it: I would have to build my own.
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
First, a quick disclaimer about the design: Since there is a lot of variation between different bikes and small engines, this design will likely need to be adapted to fit your equipment. It's important to figure out what you have before you commit to an engine or bike since some types won't work very well.
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
The 1.25 inch spindle with a threaded hole that I used on my bike is a peg which came with my all-terrain scooter. The shaft was already threaded so the spindle was easily tightened onto the shaft and secured with two more nuts. In a spindle driven design, the diameter of the spindle is the only factor that can change the effective gearing of the bike (the tire diameter doesn't matter.) A 1.25 inch spindle works well on this bike as a nice middle ground between top speed and acceleration. It would still be interesting to try out some other sizes.
Step 3: Strapping on the Engine
My engine came completely encased in plastic shrouds. Once the engine was free from its casings it was possible to lay out a pattern for the engine mount. I rough cut the holes for the flywheel and coil so I could layout the pattern of the screws. This engine has a cast bracket perpendicular to the shaft which made it easy to screw to the 1/4 inch plywood plate. The thickness of that plate was my first mistake. Thicker plywood would have been much better for this purpose for reasons I will explain further down the page.
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
I was able to ride the bike at this point, but I didn't get very far: after a few miles of high speed bumps and vibrations, the motor brackets loosened up. After a bit of time thinking up solutions, I ended up buying a spring from a local hard ware store. I rigged up a wire hook to a hole on the bikes fork and a brass hook on the far end of the plywood plate between which the spring can be tensioned. The spring allows for changes and movement of the engine while keeping plenty of traction between wheel and motor.
Step 5: Throttle
All of the parts for the handlebars were scavenged. The gas lever was originally a brake pedal on a free bike I found during one of my yard-sale searches. The irony of using a brake for the throttle should not go unnoticed. In fact, it's a borderline bad idea considering this bike has coaster brakes and I'm used to quickly squeezing brake levers for panic stopping. That could be a nasty surprise... The brake levers originally slipped over the ends of the handle bars. Since I couldn't slide the lever into its new position, I instead cut a section of the levers clamping attachment. It still allows for a tight fit without having to remove the handle bar grips. I was able to use the original cable form the brakes of the donor bike, which was plenty long. The end of the cable was secured to the engine in the same way as the original throttle cable. To attach to the throttle lever, I crimped on an electrical terminal with an eyelet and used a small 4-40 screw to bolt the terminal to the lever.
Step 6: Killswitch
The kill switch is very handy. For example, you can let the engine cool down when it starts overheating and pinging. It can also let the engine assist with braking which is a helpful safety feature. I used a simple momentary, normally closed switch with a flush mount housing and basic copper wire for this task. The wiring just connects to the two terminals on the coil to allow the switch to interrupt the spark when pressed. The switch is mounted into a piece of plywood and fixed to the handle bars with an aluminum bracket and a single bolt.
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.