Chainsaw bikes

[inspectorcritic]

I got a guy who wants a motor on his bike and I mentioned motorized bike kits. But then he found videos of bikes with chainsaw motors on their bikes. Is a 2-stroke kit better than a chainsaw bike? I understand that a chainsaw would take more work, but in the end, which out preforms the other? These motors don't really seem too different from a chainsaw motor to me.

Ask Jason he's real good with chainsaws he's down in Texas.

 

[Matt Barrack]

Ask Jason he's real good with chainsaws he's down in Texas.

who's Jason?

 

[5-7HEAVEN]

Alright thanks. Just to be clear, that means that the gear on the engine needs to be 20x smaller than the one on the bike?

Yes. However, it's highly impractical to have a 12-tooth engine sprocket and a240-tooth rear wheel sprocket.

You'd need a jackshaft and another set of gears, like the link I posted.

Surprisingly, even the builder in this link is undergeared. You can tell by the sound of the engine, and how long it took to get up to speed. He' prolly has less than 14:1 gears. He just has to enlarge his driven sprockets or decrease the size of his driving gears.

 

[inspectorcritic]

who's Jason?

Jason like in Jason Friday the 13th Texas Chainsaw Massacre I lived it for real when I was a kid the cop took the report what happen then he wrote the script I told him to write me out of the script or I'd kill him don't want anymore peole chassing me with a f***ing chainsaw!

 

[5-7HEAVEN]

To make your chainsaw engine scream, you'll need two sets of gears and/or pulleys and a jackshaft.

I believe a 20" sheave (pulley) at the rear wheel would be most effective.

A sheave is basically a bare bicycle rim. You bolt it to the spokes of your rear wheel to act as a driven pulley.


With a 2" jackshaft pulley and the 20" sheave, that's 10:1 gearing so far,
(20÷2=10).

With a 2" engine pulley and a corresponding 4" pulley at the jackshaft, that's 2:1 (4÷2=2).

Then, 10:1 x 2:1 = 20:1 gearing.

Your engine would scream.

Not as quick to scream as it would with a shift kit.....

but that's another wild suggestion of mine for another time.

 

[Matt Barrack]

Jason like in Jason Friday the 13th Texas Chainsaw Massacre I lived it for real when I was a kid the cop took the report what happen then he wrote the script I told him to write me out of the script or I'd kill him don't want anymore peole chassing me with a f***ing chainsaw!

oh ok lol

 

[Matt Barrack]

Yes. However, it's highly impractical to have a 12-tooth engine sprocket and a240-tooth rear wheel sprocket.

You'd need a jackshaft and another set of gears, like the link I posted.

Surprisingly, even the builder in this link is undergeared. You can tell by the sound of the engine, and how long it took to get up to speed. He' prolly has less than 14:1 gears. He just has to enlarge his driven sprockets or decrease the size of his driving gears.

What if I were to attach the chain from the saw to the sprocket in the centre (by the pedals) and then chain that to the rear sprockets?

 

[5-7HEAVEN]

What if I were to attach the chain from the saw to the sprocket in the centre (by the pedals) and then chain that to the rear sprockets?

And that, my friend, is the basis of the venerable shift kit.0

HOWEVER, one must be very careful of the sizes of
the drive sprockets and the driven sprockets.

One wrong calculation and the clutch would disintegrate
without nary any movement of the bicycle's rear wheel.

Basically driven sprockets (usually large) are divided by the
drive sprockets (usually much smaller).

The rare exception is where the drive sprocket is in the centre (by the pedals).

Normally on pedal bicycles, the drive sprocket in the centre is larger than the driven sprocket on the rear wheel.

This setup decreases the overall gear ratio.

If the pedal sprocket has 36 teeth and the rear wheel sprocket has 18 teeth, then 18/36 = .5:1.

If the gear ratio is 6:1 when reaching the pedal sprocket,
it'll total 6 x .5 = 3:1 at the rear sprocket.

In this case, your clutch would disintegrate and the bike would never move under its engine power.

However, if you had 18 teeth at the pedal sprocket and 36 teeth,
then you'd have 6 x (36/18) = 12:1.

Your bike would run sluggishly under engine power
and still burn out the clutch.

Now if you had 12:1 upon reaching the pedal sprocket,
then you'd have 24:1 at the rear sprocket.

Your engine would love these gears and scream at top end.

The trick is to attain 12:1 gearing, before you chain it to
the sprocket in the center.

Sooo, a 12-tooth engine sprocket linked to a 24-tooth jackshaft would yield 2:1.
Then a 9-tooth jackshaft chained to a 54-tooth sprocket in the centre gets 6:1.

2 x 6 = 12:1.
36/18 = 2:1.

12 x 2 = 24:1.

FYI, you want as small a jackshaft sprockets as you possibly can.

Better a tiny buzz saw at your ankle than a full-size buzz saw blade.

 

[FurryOnTheInside]

What if I were to attach the chain from the saw to the sprocket in the centre (by the pedals) and then chain that to the rear sprockets?

You have to replace the normal bicycle pedal cranks with a special freewheeling crank otherwise you would need to keep pedalling all the time, and if your foot slips then it's difficult to get back on the spinning pedals, also going around corners you risk pedal strike because you are unable to drop the outside pedal and raise the inside pedal.

 

[5-7HEAVEN]

Hey 5-7, you got a link to your latest build? I have moved on to scooters since procedure RS-68 was introduced in Florida.

Thanks for asking, Bonefish:
https://motorbicycling.com/threads/twin-pocket-bike-engines-build.62871/

It's a slow, but mentally rewarding project.