How to Power Your Bike With a Whippersnipper Motor

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Introduction: How to Power Your Bike With a Whippersnipper Motor

This Instructable will show you how to modify your push bike with a small motor so you wont have to pedal as much and look a whole load cooler! I made mine using a really old kawasaki whipper snipper (weed whacker for you Americans) motor. I could get about 40kmh along a flat with no pedalling and an easy 55kmh down a slight slope, I could go faster than traffic!

this is intended for the Instructables Bike Month of May contest, so please vote, I would really like to get a t-shirt. they are cool.

I'm not responsible if you hurt yourself/others/things/stuff in anyway. Also if you make something that is illegal and get caught don't come running to me, its up to you to check the regulations in your area for motorised pushbike.

This is also my 2nd Instructable so please be constructive and supportive.

Step 1: Stuff You Need

I made this bike modification with few tools, materials. You just need to think about what you doing and what you want to achieve.

you will need:

  • a bike (any will do, but the simpler is best ie. no rear shocks and stuff because it just complicates things, although still possible)
  • a motor of some kind, preferably small but with a fair bit of power, a clutch is nice because you can ride it normally without the motor on, although still possible without. and if it has mounting holes or something that will make it easy to strap on your bike. I used a small 2 stroke whipper snipper motor made by kawasaki, bit gutless as I'm 85kgs but I had a bigger carburettor on it before when I was mucking around with it.
  • a bit of thick plate to use as the mounting plate/bracket. thick stuff is good because it hold your motor nice a steady.
  • a roller/driver shaft to run on the back wheel to transfer power to the wheel. size is important as it affects gearing and therefore final speed. I did the math and figured out it needed to be bout an 25mm - 40mm in diameter to get around 40-50kmh
  • welder
  • grinder
  • miscellaneous, tooling, ie screwdrivers, spanners, persuasion tools (big hammer and shifting spanner)
  • an oxy-acetalyne set is nice to bend your plate if it needs it

Step 2: The Motor Mounts

This step involves making the mounting brackets for the motor, this is probably the most crucial step and also the longest, not because of the fabrication (takes bout 30mins) but the thinking and fiddling around. This took me about about an afternoon and night wondering whether I needed more or less or stronger supports. I then just came up with a 2 support plan and stuck with.

it basically would hold the motor by the 2 bolts that also held the clutch on and in its previous life (as a functional whipper snipper, the wire stand). I forgot to take a photo but its pretty simple.

Threes so many different bike shapes and motor mounting designs, so I'm not going to tell you how to do yours, (unless yours is same as mine) so you'll have to figure out the shape and/or mounting style for your bike/motor.

when you have your motor mounts ready to be fixed to your bike. you should just "tack weld" them on first because you will probably have to adjust them as you want a fair amount of pressure from the drive shaft onto the wheel but not too much that it hard to turn.

Step 3: The Drive Shaft/roller

this is the piece that transfers the power from the engine to the wheel. I had lots of ideas about this, including using bike pegs, making my own, scooter wheels etc ( I saw someone use a socket and it slipped against the wheel and didn't work at all if he went through a puddle)

I had sum bike pegs I was going to use but they had imperial threads and my clutch used a metric thread and I didn't want to make an adapter so I decided to make my own.

I needed a lathe to make my own but so dad knew a bloke with one and he made it for me. it fit nicely into my clutch housing and threaded right into it.

the size of the shaft/roller matters as it affects the gearing of you bike. you will need to work out revs of your motor, size of your back wheel, how fast you want to go etc, to decide on the size of the roller.

Step 4: Mounting the Motor

this is the most satisfying step as u can see the bike starting to come together. it is also the easiest step if you have done everything else correctly. (i had luckily done everything correclty and it went together like a charm) this is simply bolting the clutch to the motor to the mounts and it shld all line up.

if you motor is sitting at an angle to the bike this is wen you may need to heat your mounting brackets and give them a bend to make it sit straight. or if u really screwed up, u can cut the mounts off and reweld them straight.

make sure that if you are holding the bike in a vice like i was that the bike is the same level as if it was sitting on the ground!. mine was tilting back so when i leveled the motor and then sat it on the ground the fuel tank was leaning back and i had to chock it so that the fuel would flow throught the tap.

Step 5: Mount the Throttle

I only just had enough throttle cable to have a squeeze lever throttle like on jetski's, if you don't have enough throttle cable as most of the newer whipper snippers use arm linkages and not cable, you will have to get a longer cable and rig it up. I imagine it would be a pain to do and thats why I made do with my short cable.

Step 6: Riding

This is the fun bit. If u used an old motor like mine that is very unreliable and crappy, it would be best to carry out some maintenance on your engine. Also unlike me it would be wise to put brakes on your bike!!! I got quite a scare going into a busy intersection with thongs (flip flops, sandals) as my only brake.

Also check the legality of these kinds of mods, in NSW, Aust, to be legal it must be under 800watts of power and under 25cc or something. I’m sure cops would love to book someone causing trouble on a motorised pushbike.

My bike when the engine was running nice did 40kmh easy on flat. I put nitro fuel (15% nitro methane, the stuff used for R/C cars) in it one day and it went ballistic, it scared me.

The clutch on mine wore out pretty quickly coz its old n I’m heavy n didn’t want to pedal at all. But if your light or have a more powerful motor u don’t have to pedal ne time except for start off and really steep hills.

Thanks for reading my instructable and please vote for it in the Park Tool Bike contest!

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167 Discussions

I am building one of these, but i ran into a problem. how do you keep the bike peg from unthreading itself when its on the bike?

2 replies

if its going the right way it will tighten itself

thats how it is on minethat i built

is mounting the motor where you r putting it efficient? does it work as good as other ways? will it last without breaking? and for how long?
thx help will be appreciated :) :)

Althought this is an old post, it is for anyone else considering the same thing.

In short, no. Because motors used in "most" ( I say most because i have not seen every single lawn mower in the world) run direct- drive to the rotating blade.

The blade is spinning on the wrong axis, and although some simple gearing may solve this issue, i do not recommend it without proffesional machinery.

Basically, the lawn mower's carb is set 'sideways' (in comparison to the direciton of driver) , and because "most" lawn mower use float carbs, it will not work if you flip it to over to drive your wheels.

Did that make any sense?

Country Dude's idea may work but it seems sort of like a lop-sided baseball pitching machine...

First of all great comment! And it was wise of you to say "most". There are some Honda push-mowers with blade clutches and heavy flywheels so you can turn off the blade on back on again whenever you want. (without turning off the engine)

This past weekend I was looking at a rear drive self propelled lawn mower thinking "I cna make this work!" The main shaft has a belt wheel mounted on it, belt connects to the back of the mower to the gearbox by the rear wheel. All mechanical parts are there, would just need to be re-worked for the bike.

You can if it is a 2 stroke but the problem is some carby's only work at one angle, so the fun part is trying to rotate it so it works with the motor running a horizontal axis.

all you hafto do is put a rubber disk on the shaft and mount it so the rubber disks are on the left or right side of the wheel (dependent on which way the engine rotates) and use a spring to apply pressure horizontaly not vertically

Would this work better on a mountain bike with gears?

Cheers,
Lemon

Just a dumb question here; Why did either of you just adapt a chain drive from the motor down to the stock bicycle chain? You would get much better speed, choices of gearing (depending on the bike's sprocket set), and less tire wear! An adjustable idler sprocket would keep the chain tightened (add a weight & it could become self adjusting). Nice Instructable, but these might be hints for a "New & Improved" version.

3 replies

I thought of that to but then i realized that the pedals would be spinning at like a zillion miles an hour.

ross bicycles used to make a ten speed that had a freewheel clutch in the front cranks. it was so you could shift while not pedaling. if you could find one of them, they would let you hook an engine up and still use the shifters. i put a 3hp briggs engine on one when i was a kid in the '80's. i could get that 10 speed going faster than i really wanted to go, wanting to live and all.

If you run it on a chain drive, you can take the pedal drive parts out of the project.