60cc chainsaw engine vs. predator 99cc on mini bike

[michaelr123]

hello everybody, i have a question i can't find any support for online... i built a motorized bike back when i was 15 and i found a 60cc chainsaw engine to use, after getting a hold of a professional shop mechanic to machine a few parts for me, its up and running and i was able to leave the de-railer and 7 gear stack on the back tire, making for a 7 speed transmission . but i wasn't able to get the ridiculous gear ratio, like 24:1 to have good acceleration even on the first gear, my ratio is is this: 12t on the motor, to a 10t compounded to a 60t which runs back to the gears on the tire, and the largest i think is 24t. the bike has 26" tires. i know how to calculate mechanical advantage in my classes, but this is a little bit over my head :/ . all i know is the chainsaw can only handle first gear and even when up to speed it can't go into 2nd gear, it just sorta bogs down.

any ideas on this? a typical start off requires me to push it along, it really can't get going on its own, it stutters around 16mph then all the sudden roars to life and gets me up to about 35mph... then i run out of street and i have to turn around haha. at the moment, its not practical to ride anywhere because of the acceleration...

my thinking is that the rpm on the chainsaw is too high, and i don't have a high enough MA to make good use of it. my idea was to replace it with a predator 99cc engine thats on sale right now at harbor freight. i assume that the predator will have more horsepower and also run better on my drive train set up, which should move the ideal gear more to like 3rd or 4th gear and give me a hill climber all the way on 1st to top speed on 7th.

any help on this matter would be much appreciated!

 

[fowler]

u could install a beter jack shaft

rather than having a 1.2 : 1 from the engine to the crank on the bike run a much greater ratio

reallly u need to drop the speed right down to bugger all to use the origonal gar set

most dont use the origonal gears as u get this problem

useing a differant engine wont help the issue much

the engine is putting out 4000rmp if not more as 2 stroke
the bike was designed to be human powered
our legs put out 50rpm plus + or - quite a few (a freind and a timer)

thats a ratio of 80:1

u really need to get rid of the origonal gear set and select 1 ratio

 

[michaelr123]

hmmm... alright, so is it better to just go with one gear rather then having 7? there is a compound gear in there, where the motor runs to a 60t that is on the same shaft as a 10t that goes back to the rear set, sorry if that wasn't clear... but that should be getting me into the ball park... how do guys that convert bikes typically do it?

 

[Kaptain Krunch]

your gearing is way off. Using the stock sprockets on the bike wont work. I suggest gearing it at the very least 15:1 maybe even 20:1, may need a jackshaft. ditch the bike sprockets, they are gearing for your legs, which obviously cant crank very fast, but produce a lot of torque, especially when you put your whole body into it.

This compound shaft your talking about is what we call a jackshaft. What size sprocket is on your engine? you say that runs to a 60t? if you could run a 10t on your engine, 60 on jackshaft, then another 10 on the other side of jack shaft and maybe a 30 or so on the rear sprocket, that would give you 18:1 which would be pretty decent i'd think.

What size rear tire is on your bike?

 

[michaelr123]

the engine has a 12t on it, which goes to a 60t which on a jackshaft as you guys call it, turns a 10t which runs to the rear cassette. and the largest gear on the cassette is a 24t i'm pretty sure, or very close to that. the wheel is approx. 26"

i know i'm in the ball park for ratios, i'm just wondering if i get a motor that has higher torque and less rpm if it might run better on my set up, because i can't really mess with the jackshaft because it was all machine for me, and i'd really feel bad asking him to make another one...

 

[landuse]

A bicycle cassete is not good enough to handle the torque that any engine is going to put out. What chain are you using. A bicycle cassete is made for bicycle chain, which could never handle the torque of an engine. You need to have the same pitch sprockets on the engine, jackshaft as well as the cassete for your chain to fit and work properly.

Are all you pitch sizes the same. What chain are you using?

 

[Kaptain Krunch]

Ratio is not correct, you have about 10:1 or so, which is way too high for a 26" wheel. your going to want 20:1 or somewhere around there for that chainsaw engine, may be able to get away with 16:1 if you used the 99cc predator 4 stroke.

So i suggest changing the jackshaft sprocket to a standard chain size (#35 or #40/41) and running a 10t to a 40t on the rear tire, or something similar.

 

[michaelr123]

hmm alright, well it uses #40 chain from the engine to the jack shaft, and then its bike chain from the jack shaft to rear wheel.

so if i want to replace my rear cassette with a single large sprocket, how could i do that? i have one from the original drive pedal that i could reuse, i think its 36t, would that do it?

as far as i can remember people some attach the sprocket to the actual spokes of the rear wheel, is that right?

 

[Kaptain Krunch]

Not sure how you could go about attaching a better sprocket. May need to machine it, may be able to bolt it to the existing sprocket.

36t might do it, but i would aim for 20:1 reduction between motor and tire, even then you may not have to greatest acceleration.

 

[landuse]

Not sure how you could go about attaching a better sprocket. May need to machine it, may be able to bolt it to the existing sprocket.

36t might do it, but i would aim for 20:1 reduction between motor and tire, even then you may not have to greatest acceleration.

with everything Kaptain Krunch says. Research how Happy Time engine sprockets bolt onto bike spokes. You might get some ideas there. Have a look at some YouTube vids as well. A machined sprocket might be the best though.

Just DONT use any sort of bicycle gear when using a fuel driven engine. It won't last more than a few seconds

 

[fowler]

come to think of it a 60cc motor is proberly putting out a little over or the same as a human can put out
but it can do it all day and the rpm is what will kill the bike gear
still dont use any bike gear

 

[beansdaddy78]

what kind, brand chainsaw were you planning to use? if you use a vintage chainsaw with 70+ cc's that huffy will haul im sure.

 

[mark880088]

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