Anyone tempted to use cheap motorbike engines?

[5-7HEAVEN]

try1897, are you in the right thread?

 

[5-7HEAVEN]

my cheap pocket bike engine arrived via air mail today. box weighed 9lbs/3oz. quality is MUCH better than "happy times." picked it up during lunch hour,so not much time to eyeball it. seems lighter than my chinese 80cc.

for $79.95+shipping...i'm buying another one for my SECOND twin-engine project.

 

[5-7HEAVEN]

more engine specs:
weight:approx 7lbs.
length:9.50"
width-9.25" with "transmission",maybe 8.25" with sprocket clutch
height-5.25"
five mounting holes under engine
"transmission" is actually a jackshaft. 8t at the crank, to 26T, for 3.25:1 ratio. external gear is 17t, needs a 77t rear sprocket to replicate "happy engine" and 36t sprocket. one option is to use 36t rear sprocket and change jackshaft gear from 17t to 8t.

too wide for twin engine/midframe installs.

sorry, no pix until i can get my daughter to help post them.

 

[Dockspa1]

I've been following this thread and it's like listening to a mad scientist radio show. I love the enertia involved with the minds of great inventors.
I too am getting ready to mount a Liffan 2.8 HP 4 stroker to what is going to have to be a very modified frame. I think I will stretch the frame in order to mount the engine and leave the standard crank set inplace. The frame will have to be streched behind the crank set. Do you guys think that a 1.12:5 ratio will work with a standard centrifical clutch on 26" wheels? Thats a 11 tooth drive to a 50 tooth rear sprocket.
Thanks
Doc

 

[5-7HEAVEN]

Doc, isn't that like 4.55 gear ratio, like happy time engine and an 11-tooth rear sprocket?!

totally undergeared. in comparison with a happy time engine and 36-tooth rear sprocket, your engine would need a 165-tooth rear sprocket, the size of a GEBE sheave rear pulley.

JMO, you definitely need a jackshaft with extra gears, and that 50-tooth sprocket is unusable. to get it to work, you'd need a 33-tooth sprocket connected to the 11-tooth engine sprocket, which calculates to 3:1.then an 10-tooth sprocket chained to the 50t rear gear factored to 5:1. your ratio would then be 15:1, which is what happy time engines have with a 36-tooth rear sprocket ratio(14.94:1.and that would be with a 33t sprocket spinning next to your left calf, if you could find that much room near the downtube.)
unsure what your usable rpm range is with your lifan engine. if, for the sake of discussion it had the same range as happy time engines, you could get by with an 11t engine sprocket chained to a 15t gear at the jackshaft, then an 8t sprocket to a 72t rear sprocket. this gives you overall ratio of 14.85:1, which compares to happy time's 36t rear gear(14.94:1).

Myron

 

[Dockspa1]

Myron, this being pedal assisted " 4 stroke" not 2, and with the 2.8 HP, the power band is more steady at lower RPMs and still tops at 6k RPM. Look at your average mini bike with a 10 tooth drive on the centrifical clutch going to a 24 tooth on a 14" tire. No jack shaft. The biggest differance being the 26" tires and I was thinking by going to the 50 tooth sprocket and pedal starting, it should be able to hit some fairly high MPH runs and run smooth like a tractor on the lower side. Am I wrong with this theory?
Doc

 

[Dockspa1]

BTW this 2.8 HP Lifan is 97cc displacment.

 

[Torques]

I think those small engines (used for rc) might not be good candidates because of the size of the crankshaft bearings. They may not be beefy enough to handle the weight, seems like they may burn up.

 

[5-7HEAVEN]

Doc, i know less about minibikes' engines than you do.

however, with a happy time engine, there's a built-in gear reduction from crankshaft to engine sprocket of 4.15, multiplied by the rear gear ratio for some serious leverage.

a more obvious comparison would be with a GEBE setup and a stock ROBIN 4-stroke engine. since the engine sports a small sprocket, see how large the rear pulley is?

one option might be to run what ya got, and submit your report here.

 

[Dockspa1]

5-7, I hear what you are saying and it makes good sense. When it comes to 4 stroke motors there is a very smooth low end powerband. Not like a 2 stroke, where you take off real slow and then at the perfect time it sort of spikes in the power band. I concur as far as the ratio being spanned out and yet if you look at the old time mini bikes, you will find a straight 11 tooth to a 24 - 34 tooth sprocket and even with a large person on a 3.5 HP motor, it would take off just fine.
Any how, worse case scenario would be to go to a belt drive like you sugested if this doesn't pan out or find a 186 tooth sprocket.
I am still working on another project for my wife's bike now but will make a point to get back to you on my findings.
Thanks for the very good advice.
Doc