Clutch 49cc Chainsaw W Centrifugal Clutch


New Member
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12:06 PM
Jun 3, 2008
I took a rear sprocket off of a BMX bicycle and welded it to the centrifugal clutch drum of a Craftsman 49CC 16inch bar chainsaw. Can anyone recommend if I should mount it above the front middle or rear? What size sprocket do I need to mount within the spokes? What ratio would I end up if I used the sprocket from the crank of the same BMX bike?

Is this going to buck me off the bike when the clutch disengages? How fast would I have to be going for it to not strain things?

Some photos to come soon.
great idea, chainsaws rock. you can pick them up on craigs list for 100-150 all day long. if you get it figured out it would be great. i considering one on a pusher.
Gear ratios


I used 13000RPM as Engine RPM for my 32cc chainsaw engine.
16 teeth for front primary and 44 teeth for rear primary.
I used 26in as the tire diameter.
Thats 2.75:1
The rest of the inputs I filled in as 1.
It reported my max speed at 365.65MPH
For 4:1 it reported 251.38MPH

I am frustrated because this does not take into account my weight or a hill. How do I include these variables into an equation to figure out if I need a different gear ratio or an engine with more torque?
your engine has plenty of torque.
to reach 251 mph, you'll need to pedal up to around 220mph, for the engine to take over.:p:LOL:
try your rpm's at 20:1 and see your results.
(last year, we put a 36cc poulin chainsaw on a bike....our final gear reduction numbers were around 25:1. this bike pulls good, from a standing start....and tops out at 35 mph.

Warning!! this is NOT pretty! :LOL:

take a minute & introduce your self here--->
so we can give you a proper welcome.
You want something more like a 18-25:1 ratio, which is not practical with a 16T sprocket because your rear sprocket will need to be so large. A jack shaft setup will help you out, but increases complexity.
I'm gonna use my Titan just so you have an idea of what you need here. The gearbox allows the engine to spin 5 times for every one turn of the output sprocket. Then the final drive at the end of the jackshaft which is identical to the output rotation due to the matching number sprockets is 10 tooth. On mine it's 10 tooth to 44 tooth rear sprocket.
To get the same ratio on yours I don't think they make a 10 tooth BMX sprocket. If I remember right the smallest common sprocket is 14. If my sprocket is 14 tooth my rear sprocket would have to be roughly 4 teeth more for every one tooth may be 5 teeth but let's just say it's 4 teeth. So add 16 teeth (4 teeth times the 4 extra teeth on the front sprocket) so my rear sprocket would have to be a 60 tooth sprocket.
Now because you don't have a 5 to 1 gear reduction rather I believe it's a direct drive you'll need 60 times 5 or a 300 tooth rear sprocket to get the same ratio as mine which is pretty decent if you use a 14 tooth front sprocket.
But here's another way. Weld a 26 inch rim without the spokes right along sides your 26 inch wheel. Weld a small pulley maybe 1 1/2 inch diameter on the inside. Then wrap a serpentine or whatever automotive belt would work on that bad boy.
That should be fairly close to the 16/300 gear you would need.
..Or just make it friction drive.
Maybe Andy,the sponsor on your left there can make you a 300 tooth sprocket?
I have decided to extend the existing chainsaw drive shaft and keep the clutch at the end of the extended shaft. Then working back towards the chainsaw, connect a hollow sleve - bike peg sort of thing to the clutch housing. It will rotate freely around the extended shaft using a bearing.

The first problem is that none of the machine - fabrication shops around here have the left hand tap. Nobody can even tell me what thread it is.

Can anyone tell me?
If a gearbox or jackshaft are not in the cards- friction drive is the way to go- easy reduction.