Introduction: Bicycle Go-Kart

About: Developing projects for HowToons @ Squid Labs.

This is a physics project that I did. The only specifications for the project were "Do something cool with technology." It also meant "Build something cool." That set my mind racing. I grouped up with a friend, TehFunk, and we decided to build something amazingly cool out of some old stuff. What we came up with is a design for a go-kart.

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

Luckily we found a free motor. It's an old 4 HP Briggs and Stratton horizontal shaft 4-stroke engine. It hadn't been running. Apparently it was running at one point, but when we got it it wasn't throwing a spark. After opening it up and filing and gapping the points, it still wouldn't throw a spark. I found out that the wire running to the ignition coil had broken, so i soldered it back together and threw some heat shrink on it. Then we cleaned up the magnetos and did a general cleanup of the engine. We tried again, and luckily we got a spark. After some ether in the carburetor, the thing fired right up. It was pretty rough. The idle wasn't even at all and it kind of sputtered and shuttered. We got a new spark plug and muffler for it.

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

The initial design of a "quadricycle" type vehicle proved to have some problems. The kart would be relatively big and bulky, and it would be too difficult to find two similar bikes on our budget. We opted to find a main bike, and weld a motorcycle type sidecar.

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

A lot of this project is based around what parts we can get. We aren't in the position to spend a lot of money, so we have to work around what kind of things we can mooch off of people. We acquired some ANSI 40 roller chain and a 23 tooth sprocket. Working around these parts, we decided on trying a chain 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

Now that we had chain and a sprocket, we attempted a direct drive, just to see what would happen. After hooking it all up, the engine started, and the wheel spun at a ridiculous speed. Although it should be known that the wheel was in the air and had no load on it during this test. The whole system moved so fast that the bike chain wouldn't stay on the sprocket. Our problem was that we had a horrible gear ratio. Trying to drive the 26" back wheel at the hub wasn't putting the mechanical advantage on our side. We were going from a 9-tooth sprocket at the motor to a 23-tooth at the bike crank. This system obviously was not working, and we needed to reduce the speed from the motor more.

Step 5: Belt Drive

The next thing to try was a belt drive. The ability of the belt to slip would work to our advantage. An 8" pulley was attached to the bike crank and a 1.5" pulley to the motor drive shaft. We rigged up a belt tensioner to act as a clutch. It is a lever arm with a idler pulley on the bottom and a spring to hold it against the belt. The whole thing is hinged off of the frame of the kart. When the lever is pulled up, there is no tension on the belt and it can slip. When the lever is let down, the belt engages.
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

The go-kart doesn't go really fast. The motor is also really out of tune, and sounds completely sick when it recieves a load. Some simple tune-up can help the motor, but fixing the gear-ratio would be the ultimate fix.
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.