I have had a lot of feedback on my channel from people asking me to make a gas powered bike.
I found a good running chainsaw with a bent and broken bar at a garage sale for 20 dollars. I also was able to pick up a cheap mountain bike from a thrift store.
This bike is for a friend to use to commute to college. Since parking at the school is expensive ($6 a day) and a bus ride on public transit (also $6 a day) would take longer than walking we decided to hack this together.
Step 1: Tools and Parts Required.
Drill and bits
1inx1in box steel (about 3 ft required)
4inx4inx1/8in metal plate
Bicycle brake lines and handle
Random bolts, nuts, and hose clamps
Bike axle peg
long spring to tension the engine to the bike tire. A bungee cord may work in a pinch.
Step 2: The Drive "peg"
I used a axle peg from a BMX bike. The axle pegs are the things that stick off the side of the bike's axle to stand on or do tricks with.
Taking apart the clutch to the engine i extracted the nut that holds the clutch to the engine crankshaft. This is the only part i cannot figure out how to do without a welder. You may be able to go to a muffler shop or school metal shop and ask them to weld the nut on as centered and straight as possible to the peg.
If you do not get the nut solidly welded and centered you will get a bad amount of vibration which will cause the engine's crankshaft bearings to wear quickly. precision is key on this part.
After you finish this step screw the peg onto the engine shaft.
Step 3: Making the Bracket to Hold the Engine.
Take your stick of 1inx1in box steel and weld or bolt the metal plate to the end. Now drill 2 holes into the plate to slide the bolts that held the chainsaw bar onto the saw.
Drill your holes for the bike frame mount and for the vertical bolt.
After all holes are drilled you can start bolting on the chainsaw motor and get it hooked to the bike frame.
Step 4: Throttle Linkage.
The throttle linkage is a little tricky.
Re-using an old bike brake cable and brake handle is an excellent idea. You must look at how far the brake cable travels when you squeeze the handle. I can almost guarantee that it will travel way further than the throttle cable on the engine.
This poses a problem because if you pull on the saw's throttle cable too far you can break the cable off or worse.
The workaround to this is to leave some slack in the cable. Make sure the carb is at wide open throttle when the bike brake handle is fully depressed.
This is explained in the video.
Step 5: Tension and Friction Drive
After the engine is bolted to the bike i need to find a way to put some force down on the engine to make it ride along the top of the tire. If you don't have some sort of pressure pushing the engine onto the tire the bike will not go fast and the drivetrain will slip a lot.
I recommend a spring of some sort to do this job. For now i will be using a bungee cord.
This is shown in the video.
You want light tension but not too much. Too much tension will cause the engine's bearings unneeded stress and wear them out quicker. Just enough tension to let you accelerate.
Step 6: Test and Tune Run.
The chainsaw engine ran perfectly before i installed it onto the bike so it should run fine after it has been started. A couple pumps on the fuel primer and choke set at 1/3 gives it just enough draw to get the bubbles out of the fuel lines and gets it going.
The engine will not start until i get to about 6 to 10mph. This is because the engine needs a certain RPM to start generating electricity to fire the spark plug.
The bike is FAST! Almost scary fast. Since the speed limit for gas powered bikes in my state is 25mph I have governed the engine to run no faster than that. If i am on private property i can increase the tune and run it up to maximum speed.
Please check all requirements in your state/county if you wish to ride on public roads. ALWAYS WEAR A HELMET!!! broken helmets are cheaper to replace than shattered skulls.
Step 7: Run Time, Fuel, Maintenance.
After you build your machine you should take it for increasing range of rides. Start small and check over the bike every ride until you are certain it is solid to ride.
I did a 1mi, 3mi, and a 5 mile run to check for fuel range, heat, and durability. The furthest i have ridden the bike in one trip is 16 miles (8 there and 8 back) and i am happy with how it preformed.
Make sure you are not overworking the engine. If it gets too hot you can seize it. If your engine is getting too hot you should look into gearing it down with a smaller peg. The smaller the peg on the engine the more times it has to turn to make 1 bike wheel rotation. A smaller peg will increase torque giving better acceleration at the loss of top speed.
You want your engine to run about half its maximum speed to cruise. This insures that you have good gas milage and good cooling.
Check your spark plug at least once a week. This will give you a good idea of how the engine is internally preforming.