Theoretical way to make a cheap 79cc friction drive.

Jan 21, 2015
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Portland, Oregon
So I haven't actually tried this, but the idea popped into my head and it's too simple to not throw out there. You could take a 79cc predator, or any similar or larger 4 stroke, and a centrifugal clutch like a max torque, and use the outside of the centrifugal clutch as the roller to drive the wheel. You might have to grind some grooves in the outside of the clutch to help it grip the wheel. I would think that since the clutch is like 3 or 4 inches in diameter, the gear ratio would be pretty much perfect for a 79cc if you used something larger than a 26" tire. 26" would probably be pushing it though. You could probably build this friction drive easily for under $200 (not including a bike). Thoughts?
 

Slogger

Member
Sep 8, 2014
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nohio
A 3 1/2" clutch housing would get you about 42 mph at 4000 rpm if the bearings can take the stress and you can get traction on the tire. You'd probably want something bigger than the 79cc to go fast.
A small clutch that could handle 3 to 5 hp and it might be doable.
It definitely obeys the KISS principle, keeping it simple.
 
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Aug 17, 2011
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Springfield IL.
Ive never thought about it but it seems feasible!
And like Slogger said....Simple enough!
But then again, with the addition of a chain and sprocket, you would eliminate all the drawbacks of a friction drive.
 

racie35

Active Member
Nov 17, 2012
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usa
I think the bushing or bearing in the clutch...without support from the outside..might say bye bye. Then again, it might work!
 

curtisfox

Well-Known Member
Dec 29, 2008
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minesota
I think the bushing or bearing in the clutch...without support from the outside..might say bye bye. Then again, it might work!
I think you are right ,as I have a old KS bike machine motor from the 50's. It has a clutch, but has a shaft hooked to the end with bearings on both ends off the shaft..............Curt
 

Slogger

Member
Sep 8, 2014
545
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nohio
The stress on the output shaft and the clutch bearings would be increased, that's why I mentioned it up above there. The bearing is designed for the torque load from the chain, then you add the force to get good traction on the tire and you have a load on it from two directions.
If that traction could be gained with a minimum of force on the tire, it might hold up ok, who knows? Some sort of gnurled surface or a rubber sheath with knobby bumps maybe.

What about a toothed belt turned inside out and glued on to the clutch bell...
(you'd need some darned good glue.)
If it works even temporarily some clutch company would make a good one sooner or later.
 
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DRATS

New Member
Nov 8, 2016
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Cincinnati
I will let you know if it works out. I had the same idea. Right now i am using a v-belt pulley filled with jb-weld. The pulley is a 3 inch diameter. It is about 10 inches when measured around the perimeter.

With the 3 inch wheel it cruises at 31.3 mph. It doesn't go any faster than that.
I have been using the brakes to stop and I feel comfortable coming to a stop.
it goes up hills ok.

When i track down a large enough clutch I will throw it on and report.
 

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a_dam

New Member
Feb 21, 2009
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Momence, IL
Sorry for bringing up old thread. Haven't been around much - just noticed it.

Slogger is right on his math. 4000 rpm with 3.5 in roller would give about 42mph. Sounds doable if the motor has the torque.

I just wanted to throw in these couple of pennies. Wheel size doesn't matter when dealing with friction drive. Your "gear ratio" is determined by roller diameter.

I wouldn't build a friction drive without a bearing on both sides of the shaft. But I've found that you really don't need a lot of pressure on the tire, even with my skinny 10-speed tires and little weed-wacker motors. A bigger motor/roller like you're suggesting would take even less pressure because of the bigger contact area.

Since TheNecromancer13 is talking about a Predator and centrifical clutch, this will be a homemade build. You won't be wasting chain-drive parts from a china girl kit.