Our initial design for this project was very simple. Two small, fixed-gear bicycles welded parallel with a frame in the middle (to look like a carriage/cart type thing). Then, we'd mount an old horizontal-shaft lawn mower engine on it. We'd connect the output of the motor via slip clutch and V-belts to the bike's drive sprocket and tie the steering together. Then, we'll install more brakes if needed.
That was the idea, but didn't exactly work out like that.
(And sorry there are no pictures of the build. I decided to do this instructable after I had already finished it, so the documentation is mostly text. Hence why the metal is so rusty in all of the pictures...)
Step 1: Get a Motor
The next step in getting the motor ready was to clean the carburetor. The idle was really rough and it was evident that the fuel mixture wasn't being delivered properly. I pulled off the carb and cleaned the entire thing with Gumout carb cleaner. Those chemicals are nasty and will eat through anything, including latex gloves. I soaked the carb in the Gumout and using a toothbrush to knock off old deposits. I replaced the gaskets to the fuel tank and the head, and the motor ran much better. I could use the fuel adjustment screw to adjust the amount of fuel getting to the motor, and tuned it to a nice low idle speed.
Now, the motor was ready to be put into action.
Step 2: Change of Plans
We bought an overpriced used Huffy from the pawnshop. After a lot of adjusting, the bike performed the basic functions we were looking for: steering, stopping, rolling well, and shifting gears. We didn't need the front dereilleur, so we pulled that off.
After that, we found a free Schwinn. This bike was totally classic, straight out of the 40's or 50's. It was pulled out of a barn and was completely non-rideable. It had some sweet fenders and a pair of baskets flanking the rear wheel. It was the perfect bike to cannibalize. Luckily enough, the back tire still held air. We cut off the back half of the frame and welded it parallel to the rear of the Huffy with some scrap angle iron. We didn't think that it would be very stable, but it turned out to be rock solid. We had some fun pushing each other around in it and learning to balance with the sidecar.
Step 3: Failed Drive System
The basic idea of the drive system was to use the motor to drive the front sprocket of the Huffy, and utilize the rear gears from the bike to give us some options for speed. We decided that a clutch was important. Without one starting and stopping would be a challenge, as we later found out.
To accommodate our parts acquisition we ordered a centrifugal clutch online. It is designed for go-karts, and was supposed to engage at 2,200 RPM. After receiving it and hooking it up to the motor, we quickly found out how inadequate it was. It didn't engage until the throttle was almost entirely open, and once engaged, it didn't grab the clutch shoe at all. We ended up with a $40 clutch that couldn't even drive our unloaded wheel. After trying to contact the company, we discovered that the spring wasn't adjustable so we couldn't change the engagement speed. Time for plan B.
Step 4: More Failed Drive Systems
Step 5: Belt Drive
This system worked until the pulley on the motor melted. The ratio of the system is still really off. It takes a lot of slipping to get the thing moving. The 4hp motor gets bogged down trying to move the 26" wheel. This puts a tremendous amount of slipping on the motor pulley. So when I was driving up a slight incline, the whole pulley melted and tore apart. It was a cheap die cast part, so we went ahead and ordered a cast iron pulley from McMaster.
Step 6: Improvements
Some ideas to fix the gear ratio are to implement a friction drive at the edge of the wheel, rather than the middle. Another option is to change out the bicycle sprockets and fix the ratio that way.
This project would be super cool with an electric system rather than a gasoline. With a big 5hp electric motor like those used in big air compressors, this go-kart would be pretty sweet. The problem with that is merely the cost. A new motor cost up to $200, and I currently don't have any sources of a used one. A big battery array would also have to installed. The advantage of an electric system is that it has maximum torque at the stall speed, which is what we really need. Of course, it is also emission-free and much quieter.