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How to Strap a Leaf Blower Engine to a Bike and Go Fast

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Picture of How to Strap a Leaf Blower Engine to a Bike and Go Fast
This was one of those projects I couldn't get out of my head. I had seen, on the internet and in person, the small 2-stroke engine powered scooters which were becoming increasingly popular. Naturally I wanted one, but I'm not the type that would just buy a commercially available scooter that will work well and last for years and be happy about it: I would have to build my own.

This was also a project that kept changing. The initial design worked, but not very well (or for very long.) I kept redesigning as I went along, tweaking the bike for more reliable use. In its current condition it is quite effective at moving someone around and makes for a quick and easy to build project. My initial build lasted about 8 hours over one day. When I hopped on the thing and went flying down the road at speed, I was very thrilled and surprised to have gotten that much of a result out of one day of bodging. While you view this project, please keep in mind that some major improvements could be made to my design to fix various safety and performance issues. If you plan on building a similar design, make sure check out the lessons learned step before you build.

For more info about this project and a bunch of others, check out my website: thewidgetforge.com
 
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Step 1: Design

First, a quick disclaimer about the design: Since there is a lot of variation between different bikes and small engines, this design will likely need to be adapted to fit your equipment. It's important to figure out what you have before you commit to an engine or bike since some types won't work very well.

The design was constrained most by the motors available to me at the time and my lack of welding capabilities. I wanted to use my all-terrain style 12 inch tire scooter along with a clutch but I couldn't come up with the necessary equipment. My searches of local (and on trip that was not so local) yard sales and eBay returned few useful motors. Mounting the motor without welding also posed a challenge since that was the efficient and obvious way to fix everything together.

When I couldn't get a motor with a clutch, I got frustrated enough to haul off and improvise a spindle driven design for my bike from 5th grade. The bike is quite small for me, but still allows pedal starts and has coaster brakes which frees the handle bars for the gas and kill switch. I decided to make it front wheel drive for ease of construction: there's a lot more free room up front.

For the engine, I used a small 26cc McCullough engine which came from a hand-held leaf-blower. It is a half shaft motor (only one side of the crankshaft is supported) without a clutch, but it had a threaded shaft which allowed for easy attachment of the spindle so I was happy.

I initially went very simple with the design: just make some brackets and bolt it on. That worked for a few miles when the engine mounts loosened up and the spindle stopped transferring power. I eventually modified the bike to include a spring tensioning system to keep the motor firmly on the tire.

Step 2: Attaching the Spindle

Picture of Attaching the Spindle
The 1.25 inch spindle with a threaded hole that I used on my bike is a peg which came with my all-terrain scooter. The shaft was already threaded so the spindle was easily tightened onto the shaft and secured with two more nuts. In a spindle driven design, the diameter of the spindle is the only factor that can change the effective gearing of the bike (the tire diameter doesn't matter.) A 1.25 inch spindle works well on this bike as a nice middle ground between top speed and acceleration. It would still be interesting to try out some other sizes.

Step 3: Strapping on the Engine

Picture of Strapping on the Engine
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My engine came completely encased in plastic shrouds. Once the engine was free from its casings it was possible to lay out a pattern for the engine mount. I rough cut the holes for the flywheel and coil so I could layout the pattern of the screws. This engine has a cast bracket perpendicular to the shaft which made it easy to screw to the 1/4 inch plywood plate. The thickness of that plate was my first mistake. Thicker plywood would have been much better for this purpose for reasons I will explain further down the page.

Mounting Brackets
With the engine screwed securely to the plywood, I continued by making brackets to connect the plywood to the bikes front fork. These brackets bolt around the front fork, through the plywood and to a set of flat brackets with 5/16 inch bolts. The Brackets are able to tighten down on the fork so there is enough tension to hold the spindle to the wheel.

First Test Run
At this point, I took the bike on its first test run, which didn't last very long at all. Actually, more accurately, it never started. The 1/4 inch plate tended to bend rather than engage the tire. I couldn't get enough traction to kick over the engine. Hence why thicker 1/2 inch plywood, or even better 3/4 inch, would be much more effective. To stop the plywood from bending without starting over, I used more aluminum strapping bolted to the face of the plywood. Amazingly, this worked! With some furious pedaling I kicked the bike over and the motor started, rocketing me to the end of my street. This was very surprising and quite exciting to me since most of my poorly thought out, frustration ridden second attempts tend not to work out.

Step 4: Adding the Spring

Picture of Adding the Spring
I was able to ride the bike at this point, but I didn't get very far: after a few miles of high speed bumps and vibrations, the motor brackets loosened up. After a bit of time thinking up solutions, I ended up buying a spring from a local hard ware store. I rigged up a wire hook to a hole on the bikes fork and a brass hook on the far end of the plywood plate between which the spring can be tensioned. The spring allows for changes and movement of the engine while keeping plenty of traction between wheel and motor.

Step 5: Throttle

Picture of Throttle
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All of the parts for the handlebars were scavenged. The gas lever was originally a brake pedal on a free bike I found during one of my yard-sale searches. The irony of using a brake for the throttle should not go unnoticed. In fact, it's a borderline bad idea considering this bike has coaster brakes and I'm used to quickly squeezing brake levers for panic stopping. That could be a nasty surprise... The brake levers originally slipped over the ends of the handle bars. Since I couldn't slide the lever into its new position, I instead cut a section of the levers clamping attachment. It still allows for a tight fit without having to remove the handle bar grips. I was able to use the original cable form the brakes of the donor bike, which was plenty long. The end of the cable was secured to the engine in the same way as the original throttle cable. To attach to the throttle lever, I crimped on an electrical terminal with an eyelet and used a small 4-40 screw to bolt the terminal to the lever.

Step 6: Killswitch

Picture of Killswitch
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The kill switch is very handy. For example, you can let the engine cool down when it starts overheating and pinging. It can also let the engine assist with braking which is a helpful safety feature. I used a simple momentary, normally closed switch with a flush mount housing and basic copper wire for this task. The wiring just connects to the two terminals on the coil to allow the switch to interrupt the spark when pressed. The switch is mounted into a piece of plywood and fixed to the handle bars with an aluminum bracket and a single bolt.

Step 7: Results + Video

This video was taken on an improvised oval track. I was holding about 20 mph throughout the run.



Here's a video of a "high-speed" run (about 25 mph) down my street. I hadn't added the hand throttle yet so I had to pull the engine's original throttle cable to accelerate, which is why I'm awkwardly leaning over the front of the bike.



Starting Procedure
To start the bike, I use the standard priming and choking procedure. This usual includes a priming until the bulb fills with fuel followed by a quick pedal start with the engine fully choked until it kicks over. Once their is fuel in the lines, I open up the choke to the middle setting. The motor starts when the bike is pedaled to around 8 mph, which can take some furious pedaling, but isn't too bad. The bike is even easier to start when it's hot. At about 12 mph the bike has enough power to start accelerating up to speed.

Performance
Acceleration is very sluggish at low speeds, but over 15 mph the bike zooms along with adequate acceleration. According to the bike odometer I have attached, it has hit about 29 mph on a slight down hill. It can go up significant hills and still hold 15 mph. Average speed riding around the neighborhood is about 18 mph. If you think of the machine as a bike with a power-assist rather than a mini-bike, it is very reasonable. With a little gas it is effortless to cruise at 20 mph, a speed barely attainable on a downhill when pedaling this bike. Since your rear is near the ground, it feels even faster than it is.

Is it worth it?
For all of the frustration that comes with keeping the machine running, the question you have to ask yourself is: Is it worth it? Well, as long as you enjoy hot exhaust blowing on leg and the wind in your hair while flying down the road at obscene speeds with a small two-stroke between your legs screaming at 7000 rpm, it is definitely worth it.

Step 8: Lessons Learned

Overall, the bike could have been much better built. With the modifications, it does an adequate job, but a bit more forethought would have helped make a better bike with fewer headaches. The bracket system is not a bad method for attaching an engine without welding, but using a tensioning mechanism is essential. A better mechanism than the spring that I used (I've been thinking about using a turnbuckle) and a sturdier mount for the engine would show the greatest improvements in usability, reliability and safety. In many ways this project should be used as much for how not to build a motor bike as how to build one. Here are some things I would do differently next time (some have already been mentioned):

  • Use at least 1/2 inch thick plywood for the plate (1/4 inch thick aluminum or steel would be even better it you can cut it)
  • Add a tensioning mechanism from the start.
  • There is definitely a bracket geometry that would work better. A stiffer set up that connects on the other side of the wheel would make for a much stronger and more stable platform.
  • The rims that came with the bike are not nearly strong enough to take the 25 plus miles an hour the bike can achieve. Mine have come out of alignment and been seriously bent a few times already.
  • Steel brackets would be better to attach the motor plate for added strength and safety. One of the aluminum brackets I made broke at one of the bends. A set of store-bought u-bolts would probably be the best solution and they would be even easier to use as long as you can find the right size.
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TheBirdManV10 days ago

I made a bet with a mate, who could make the fastest motor powered bicycle by the end of the year. I'm losing at the moment, with him having completed his build using a chainsaw motor and with me having not started yet, but i think i think i can leave him in the dust (literally) with this build. Thank you so much man, you have blown new life into a bet i would have lost and given me a fighting chance. now all i need to do is find a bike, motor, assorted bolts, nuts, springs and god knows what else, but at least i have a reference. thanks again bro, heaps of respect.

buck221727 days ago

cool, are you still alive?

Jnkyrdguy (author)  buck221725 days ago

Yes... it isn't that dangerous ;)

Well where's the fun in that then!!?

ImJopel1 year ago

good if you only make right turns

BTW it look unsafe because the blower is on one side and it will be heavy on the left so you will fall just saying because I just saw the picture.

rabad32 years ago
I made few of them using weed wacker engine way back 1996, mounted on the rear using a converted bike rear rack hinging the engine so you can have a clutch. I used hard small diameter skateboard wheel as a driver on the shaft. We mounted them on our mountain bikes and had lots of fun.
Bikebudy2 years ago
I think you did great ! Ya lots of bugs to work out when building like this.
But, man that is half the fun. I would try mounting on the rear of the bike.
Most engines run the same direction. That is why mini bikes use a jackshaft
to run the chain on the other side. As a rear mount, do a jack shaft or make a lever
to pull the engine off the tire. The spring to keep tension is a must.
If useing a jackshaft, you could use the rear gears. But, I'm sure that will
open a hole mew set of bugs to work out. lol

All in all, Great job....
Wesley6665 years ago
You said you hit 29mph? My pedal bike speedometer says I've gone 168 km/h...No joke! I didn't do anything at all. Its set for the correct size tire and everything...I know when it happened, but I didn't think I was going that fast...That was a bigger hill then I expected! If you don't believe me look at the photos!
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I think what the bottom number is, is the total distance you have travelled.
I may be wrong, but that is how mine used to work.
Ya cause my max speed is 0! WTF! no the odometer reads 600km. The top number is always the speed you are currently traveling. the bottom number is whatever the letters next to it say. If you look it says Mx for max. and if you press the button again it goes to Average which is 20km/h then it changes to calories and so on. That is the actual Max. speed!
So what you're saying is that on a mountain bike you have gone faster than world record holders:
Sam Whittingham 2009 133.28 km/h (83 mph) Flat surface, unpaced, (recumbent) and
Markus Stöckl      2011 164.95 km/h (102.50 mph) Downhill on a volcano, on a serial production bicycle.*

I agree that your speedo shows a max of 168.4, but I'd say it wasn't reading right. Still, I'm open to being wrong - you should perhaps look at having a go at the world record particularly for "downhill on a volcano, unpaced, serial production, upright". 

*For other speeds see: http://en.wikipedia.org/wiki/Cycling_records 
Guess so! Haha! I don't think it was reading properly. I know that I have hit 100km/h on that bike, downhill, as it is calibrated properly for tire size and it is fairly accurate, but something must have gone wrong to get 168km/h. That and I have hit 170km/h on my motorbike, and I can tell that I have not hit that on a pedal bike. If my some amazing chance I have and didn't notice/realize, I hope I could figure out how to do it again! Wouldn't mind holding a world record really...
I'd say re-calibrate that sucker!
Never had to, after seeing that I got a friend on my bike (motorcycle) to ride next to me at 10, 20 and 30 km/h to see if she was off, was dead on. I think what it is, since it works off a magnet on the rim that goes past a reed switch or coil of wire on the forks, my best guess is at high speed there isn't enough time between one pass of the magnet and the next and the computer goes nuts and gives you monstrously high value for speed.
I believe you. I've hit 45 MPH on a mountain bike when i was 13 years old. Could've gone faster, but the hill was really windy.
wenis016 years ago
i notice from your pics that you didn't reattatch the pull starter. To start it you just pedal until the engine turns over?
Jnkyrdguy (author)  wenis016 years ago
Yeah, no pull starter. A few quick pedals and the motor starts and at about 12mph I stop pedaling and let the motor take over.
will it go up hill
no, once the engine tilts above three degr- YES IT GOES UP HILLS YOU TARD ITS AN ENGINE NOT LARGE ROCK
buffersam says: Nov 28, 2010. 12:00 PMReply Bwah Ha Ha Ha Ha - still laughing after reading this post - love the "YOU TARD" reference............... :D
Yeah me too! :]   I especially like the way you interrupted yourself at "degr-". You are one funny guy!!
With due respect to dragonsniper, of course, the engine may not have been powerful enough to pull uphill...
Bwah Ha Ha Ha Ha - still laughing after reading this post - love the "YOU TARD" reference............... :D
Yes, you do need more to do, ha ha. It's a legitimate question. There are no shiftable gears and the engine used to move air, not a person.
Anyways, in step 7 he says it will and you can always help it out by pedaling.

Nice instructable Jnkyrdguy! I have been wanting to do this for years now, but my designs always involved attaching something to the spokes and using a belt or chain drive which is more difficult. This seems like a good solution.
I think he means how much horse power.
Did you have trouble getting the spindle centered? i stuck a peg on with a washer inside the peg as well and it is about estimated 1mm off center. i started it without putting it on the bike and it vibrated like crazy.
Jnkyrdguy (author)  Whitedude07284 years ago
I got lucky! The spindle I used had the same threads as the shaft on the motor so it's just bolted on. Otherwise, I'm not sure how I would have attached it without welding or somesuch.
I got the metal peg on but it was too big (1.25) and had no torque so i finally ended up using this pvc pipe. i also stuck a bearing in the far end of the pipe and attached it from the side opposite of the engine. Haven't gotten a throttle to stay attached though :(
Forgot the Pics :o
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ProjectPAPI3 years ago
I hope to make something along the lines of thsi if i have any questions could i email you? if you want check out my blog at www.projectpai.blogspot.com
Jnkyrdguy (author)  ProjectPAPI3 years ago
You can PM me on here if you have a question and I'll try my best!
cool thx
afleming24 years ago
do you think its possible with the housing still on
1august114 years ago
ya, i see that your mount is burnt
msullivan54 years ago
I made it just about the same way as you, except for instead of brackets i used zipties andl screw locks(not sure if thats the name), but my spindle keeps slipping on the tire. Please help.
Jnkyrdguy (author)  msullivan54 years ago
I had the same problem with mine until I added the spring to hold the spindle down to the tire. Otherwise the brackets would loosen up and the spindle would loose grip over time. If you already tried a spring, maybe changing its position or adding a heavier spring could help
11richie214 years ago
I meant do all gas motors spin in the same direction because i dont know which side to mount the motor on
11richie214 years ago
does the gas motor always spin in the same direction?
when the engine is on idle doesn't it still spin? because if you want to stop you have to shut it off!
Jnkyrdguy (author)  RocketPenguin4 years ago
Yes, you have to shut it off to stop! It's not convenient, but it works
well thanks for replying!
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