2 Questions about my chain drive back mounted engine

[tylerscool45]

1st ya i know i already got a 14 tooth sprocket which i am welding on tonight
2nd will a 90 tooth work
3rd i dont know if a can get a jackshaft or gearbox if i can't will it still work
Im not using friction drive btw

 

[nathanl]

26 inch wheel with a 9000 rpm motor driving a 14 tooth cog to a 90 tooth cog on the wheel gives you a topspeed of over 100mph. It wouldn't really work. Check out http://www.motoredbikes.com/showpost.php?p=87814&postcount=1 It has a gear calculator that gives you an idea of what you need to get in the 30mph redline gear area.

 

[tylerscool45]

So what would happen at 2 to 1 gear ratio? Thats what i want to know

 

[nathanl]

2:1? The numbers you were asking about before were 6:1... neither would work. You'd just damage your chainsaw engine/clutch.

 

[tylerscool45]

You mean it would be broken, Chainsaws are meant to be at that high rpms.

 

[HoughMade]

Yes they are, but what everyone is telling you is that they run way, way, way, way, way too fast without reducing the crankshaft speed of the engine by about 18 to 20 times for use on a bicycle. You want the engine running at its natural sweet spot, which is at a high rpm over 7,000 in this case, maybe 10,000, while reducing the speed with sprockets or gears enough get adequate torque to the wheel.

Think about this- take any manual transmission car, put it in the highest gear, 5th or 6th, and try to leave from a stop. Doesn't work well and most times will kill the engine. Now imagine a car with even more gears, 10 or 11, and trying to start from a stop in 11th gear- that is what you are asking that chainsaw engine to do at a ratio of 6 to 1, let alone 2 to 1- that would be a theoretical 16th gear. Won't work.

 

[tylerscool45]

so 18 to 1 would be the sweet spot cause i can get up to 21 to 1 or how bout 17-1 its a 22 inch wheel btw

 

[loquin]

18:1 with a 26 inch wheel results in apx. speed of 43 MPH at 10000 rpm

the ratio of 22 to 26 is 0.846

0.846 times 43 mph is 36 MPH

Here's the calculation that you need to use with your calculator:

Speed = RPM / reduction ratio * wheel diameter / 336

10000 / 18 * 22 / 336 = 36.3 MPH for 18:1, 10K RPM, 22 inch wheel

and

10000 / 17 * 22 / 336 = 38.5 MPH for 17:1, 10K RPM, 22 inch wheel

 

[Luka]

Well yes the one that the other guy has is ugly, but it works well if its not for the fact that he has no throttle. Weedeaters and engine kits all go one way which where you can put the engine in the frame. But most if not all chainsaws you cannot do that unless you put it upsidown. So adding to the back is the only option. I am doing it differently because instead of making another sprocket on the other side im basicly moving the chain from the front to the chainsaw. Ill show you what i mean

ill post another one friday when it should be done

Tyler,

If you put what I would call a 'half'-jack' on the seatpost downpipe...

A half jack being a simple post with two gears on it. One larger than the other, spinning freely, but welded/bolted to each other...

Then you could run a chain from a smaller gear at the chainsaw, to the larger gear on the half-jack. Then run another chain from the smaller gear at the half-jack to the largest gear at your crank.

This way, you could reduce the gearing twice. Plus you would retain full shifting on the rear.

If I did this, (and I may. Using an old husqvarna chainsaw), I would put a freewheel crank on, so that i wouldn't have to keep pedaling when the engine is running.

I have drawn on your own illustration, what I mean.

You would eliminate the chain going down to the back wheel.