Does this sound ok?


Local time
5:37 AM
May 18, 2008
Perth, Australia
Hey i got a mountain bike and a 30cc 2 stroke engine from a wipper snipper ( weed wacker ). Im thinking of mounting the engine in the frame under the seat area like most people do then welding or fitting a small sprocket on the shaft of the motor then running a chain from one of the wheel sprocket to the engine sprocket. This way i can also change gears with the bike too as it has gears on the back so it works out well. Then ill take off the front brakes and use the lever and hook up the throttle cable to it. Then ill just have back brakes.

Does this all sound good? With the weed wacker engine it doesnt have separate Fuel and oil containers u mix it together then put it in. Does this mean u can tip the motor on its side or upside down and it wont affect it?


I think im the youngest here, 14 yrs old!!!

From Brendon.v
if only it were that simple. :(

you need some serious gear reduction to get the weedeater's powerband spinning your tire at ~25 mph. The weedeater output is in the many thousands of revolutions per minute; a bike crank is around 80 RPM.

Some chainsaw carb/fuel tank setups are set up for use at various angles. I had a weedwhacker with a "heavy pill" attached to the end of the fuel intake inside the tank, the pill would sink on whatever side of the tank and always be sucking fuel.

Incidentally I wouldn't jettison a complete working brake system just to rob the lever-- rather grab one from a junk bike at the dump or mail order a twist throttle grip, they're $20 or so.
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A good way to go about this would be a tire roller drive, which means that you can skip a gearbox and a chain or belt drive. It would mean that you would have to mount the engine over the back wheel, attatch a roller to the engine shaft. The roller would directly rub against the tire and propel the bike. Do a search on tire rollers.

Right. You'll need a rough surfaced roller, approximately 1.25 inch in diameter, to transfer power to the tire. The tire is just an 'idler' in this case, transferring the power from the roller to the road.

Take a look at the friction-drive kit pics and mounting instructions at for ideas as well.
the friction drive ( been researching ) seems to hard to mount, its a lot easier to do the chain method. I got 5 gears on the back so i can change it to the biggest gear then put a small gear in the shaft of the motor. Ill take the whipper snipper apart now and see what the engine looks like. Ill take of the pole bit so i just got the motor and the shaft. ATM i still got it all in one peice so i need to take the engine out of it n stuff.
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well i got told not to take it apart, we will fire it all up make sure it all works properly as its been sitting in the shed for years. My dad says if it works we could also sell it for 150 then buy a better motor for it with the money.
I'm all for building from scratch- but this calls for a little math- as noted above, you need reduction of 18 or 20 to 1- your bicycle gears are not nearly large enough for that reduction. You will need some sort of primary gear reduction.

this is why people are proposing a friction drive. With a 1.25" roller and a 26" wheel, you end up with almost a 21 to 1 reduction without any gears.

If you really want to use a chain, let's say you have an 11 tooth engine sprocket- you need a 198 tooth sprocket to get 18 to 1 without a primary gear reduction of some sort.....that would probably be bigger than the back wheel of the bike. It can be done- just takes some planning. A primary reduction of about 4 to 1 would allow you to use a standard 50 or 56 tooth rear sprocket and get where you need to be. Good luck!
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firstly a two stroke shouldnt have any problem running inverted but you need to know the rotation direction at the output shaft. Setting everything up and then realising that you can do 25-30mph in reverse is not a good thing.

you will need to fit a freewheel part to the crank which will allow you to run the engine without being de-shinned by a pair of pedals doing a fair approximation of local light-speed - trust me, that will hurt like all ****.

I agree with others regarding brakes - DO NOT cripple the bike and possibly yourself and others just for a cheap part. Get hold of a twin brake lever and run that for both brakes (back brake to the top lever for graduated braking) - using the freed up single lever for throttle control.

If you can get a 10-tooth sprocket fitted to the engine and have the chain run from that via a freewheel crank to the 5-ratio drive at the back you should have reasonable total ratio on the bike.

alternatively there is a jackshaft kit available that will allow you to use an engine that rotates the wrong way - it will allow you a drive through to the geared wheel from one side of the bike to the other.

By the way - when you have done all that be sure that you keep an eye on the mechanical elements of the machine - throwing a bolt or a fixing is often enough to destroy more than you'd ever expect...

hope that helped

Jemma xx
yeh ive gone with the friction drive, ill probably chuck a bike peg on the motor but if it wont fit my dads friend has a machine that cuts out metal objects like screws and things so i could get him to make me a roller. Then ill just make up a mount and make it so i can adjust height so i can ride my bike without the motor on as well.