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Chainsaw bike

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Chainsaw bike

Unread postAuthor: stuffbuilder » Mon Sep 12, 2011 4:08 pm

I've been planning on putting a chainsaw engine on my bike so I can get around town easier. I've seen it done on youtube plenty of times, but the main difference in my plans is that mine will be chain drive, not friction drive. The rear wheel on my bike is a 24" with a 24 tooth sprocket. I know chainsaws generally put out about 12000 rpm, but I'm not sure what kind of ratio is most efficient for the different sizes of engines (most common sizes seem to be 33ish cc and 42ish cc). I figured a 24:1 reduction would be good but I have no idea if that would let me use the full potential of the engine (can I go faster or will the engine struggle?).

P.S. 500 rpm on the wheel will make the bike go about 35 mph
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Unread postAuthor: jhalek90 » Mon Sep 12, 2011 4:35 pm

A friend of mine hooked a chainsaw up to a mountain bike, a 7 speed.... and was able to shift gears.... i think that is the only way to go if your going for speed and torque.
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Unread postAuthor: Technician1002 » Mon Sep 12, 2011 4:44 pm

Checked your math. looks good so far.

3141.6 ft_per_min =>
16 m_per_s
57.5 km_per_h
3142 ft_per_min
52.4 ft_per_s
1047 yards_per_min
35.7 mph
31 knots


The next part is to find the power requirement to push a bike to 35 MPH with normal air resistance and then compare that to the power of the engine. A relatively easy power unit to use instead of HP is Watts. I recommend using Watts as many electric bike conversions advertise the speed and watts. Converting the chainsaw engine size to watts is not difficult to finish the math.

Alternatively, you could convert the electric bike conversion stuff to HP instead and compare to the gas engine. We can double check the math for you if you wish.

Start by finding the proper size e-wheel or other motor conversion power requirement to drive the bike at your target speed. You will want the power supplied to exceed the power required to leave some hill climbing ability
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Unread postAuthor: stuffbuilder » Mon Sep 12, 2011 6:46 pm

The problem is that I don't know the HP of any of the chainsaw engines. The 35 mph top speed was just a guess too. I'm not sure what each engine is capabable of, but I want to make sure that it is not geared too low.
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Unread postAuthor: Fnord » Mon Sep 12, 2011 7:52 pm

Do you have at least a CC capacity printed on the chainsaw engine to plan to use? That would be a nice ball-park figure.
Or do you still need to buy one?

If we're assuming you don't have any data on the chainsaws you're looking at, the only way to really compare is to try them out individually, if only as a "saw". Wood varies greatly in toughness with species and moister content so it isn't an ideal test media.
From experience I'd say any saw that can cut through a 130-pound cheerleader in less than a second is upwards of 2.5 HP and will work well as a bike engine.
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Unread postAuthor: jor2daje » Mon Sep 12, 2011 7:58 pm

One thing to look out for is you can not hook up bike chain to a chain saw as easily as you might think, the sprockets and chains are opposite, on a bike the sproket fits into holes in the chain while on a chainsaw a the teeth on the chain fit into holes in the sprocket.

Also if you want to use the bike without the chainsaw you will need one with a clutch. I would just go friction drive because its easier and you can just lift the engine off the wheel instead of needing a clutch. Unless ofcourse you are capable of machining a new gear/clutch then go for it.
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Unread postAuthor: ramses » Mon Sep 12, 2011 8:50 pm

All chainsaws have clutches; the engine can idle without the chain spinning. Many weed eaters have this feature too.

It is worth noting that when moving "at speed", if you kill the gas engine (or lift off the throttle), the bike wheels will backdrive the engine to whatever speed, but the engine won't be receiving oil in its fuel supply through the carb.

engine + spinning - oil = seized (but it can likely take some abuse, but wear will increase)


Early 2 stroke cars (they stopped because they sucked) use overrun clutches to prevent this engine braking.
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Unread postAuthor: stuffbuilder » Tue Sep 13, 2011 11:46 am

I'm hoping to get a 42cc engine if I can find one for a good price. As for the sprocket, I would just get the chainsaw chain sprocket machined down and have a normal sprocket welded in place of it. Most likely I would have someone with the proper tools do this for me so it can all be perfectly centered.
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Unread postAuthor: Alster370 » Tue Sep 13, 2011 12:06 pm

Don't you still have to have car/bike licence to ride it , even though its still a bike, just because its powered by a petrol engine?
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Unread postAuthor: Technician1002 » Tue Sep 13, 2011 12:11 pm

The requirements vary depending on your location as to the license requirements. In most places a power assisted bicycle is considered a moped as long as the displacement is below a set value, the top speed is below a limit, and it can still be pedaled like a bike. Removal of the pedals makes it a motorbike, not a moped.

Regulations in your area may vary. Traveling at 35mph on level ground in my state would not be considered a moped.

In my state the definition of a moped is as follows; from
http://moped2.org/laws/Washington.htm
"Moped" means a motorized device designed to travel with not more than three sixteen-inch or larger diameter wheels in contact with the ground, having fully operative pedals for propulsion by human power, and an electric or a liquid fuel motor with a cylinder displacement not exceeding fifty cubic centimeters which produces no more than two gross brake horsepower (developed by a prime mover, as measured by a brake applied to the driving shaft) that is capable of propelling the device at not more than thirty miles per hour on level ground.
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Unread postAuthor: Zeus » Tue Sep 13, 2011 5:50 pm

42cc is a lot for a saw. I use a 55cc for firewood (shock horror!!!) and it'll push you faster than you'd ever want to go. I've had kickback from a 36cc and that's fairly powerful for your purposes.

My 55cc, if the kickback brake didn't work I'd be talking to you now with a few bits missing. And I'm of a large stature.
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Unread postAuthor: stuffbuilder » Tue Sep 13, 2011 6:45 pm

I've seen saws as big as 100cc, but I would never even think about using anything bigger than 42cc. I would think that a huge engine would simply snap the chain when trying to accelerate. And +50cc engines cost a LOT.
And the main reason I'm not using a friction drive is that I've heard they tend to wear out tires pretty quick.
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Unread postAuthor: POLAND_SPUD » Tue Sep 13, 2011 7:11 pm

I know it's totally useless but....
when I first saw this thread title I though that someone wanted to attach chainsaws to the handlebars
:D
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Unread postAuthor: Zeus » Tue Sep 13, 2011 9:37 pm

stuffbuilder wrote:I've seen saws as big as 100cc, but I would never even think about using anything bigger than 42cc. I would think that a huge engine would simply snap the chain when trying to accelerate. And +50cc engines cost a LOT.


They aren't too bad, looking at Stihls, a 54cc is only $950 out here. It shouldn't snap the chain, it takes quite a bit to snap chainsaw chains, and they're a lot thinner. If you're worried about it, replace the sprockets on the bike with motorcycle sprockets and chain.

If it snaps beside your leg though, you'll be in for a lot of hurt. Chainsaw chains pull flesh off when they snap, but the bruising from a bike chain would be nasty. I've recieved a whack with a bike chain, but putting a couple of kW behind it wouldn't be fun.

stuffbuilder wrote:And the main reason I'm not using a friction drive is that I've heard they tend to wear out tires pretty quick.


I concur wholeheartedly, friction drive is no good for the application.

And regarding CCs and power, a 30cc Stihl puts out 1.3kW, that's (I think) around 2.4 HP, a 40.8cc is 2kW, 3.6 HP. Hope that helps a bit.

Poland, do you think I haven't already tried that?
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Unread postAuthor: ramses » Wed Sep 14, 2011 4:48 pm

Don't ignore the need for an overrun clutch. Otherwise, if you close the throttle and coast, you may have lubrication issues.
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