BMX To Motorcycle Conversion

Here are plans to build a small motorcycle from any old BMX bike and get close to 100 miles per gallon in the process. The tools that I used were a welder, die-grinder, crescent wrench, and other smaller hand tools that are common around most people’s homes. My finished product had a gear ratio of 12:72 or 1:6 this carried me (150
lbs.) at 30+ MPH!

Here are some pics of my bike right now:
Finished Bike 1

Finished Bike 2


Required Parts:
1. BMX Bike
2. Steel tubing about (4 ft.)
3. 3/16″ or thicker flat stock (4 ft.)
4. 3 H.P. engine
5. Clutch (mine was 12 tooth)
6. Sprocket (mine was 72 tooth)
7. Chain compatible with your clutch and sprocket (mine was #35) (5ft.)

How to Build:
1. Remove all of the pedal assembly

Bike with no Pedals

2. After removing the pedal assembly measure the inside of the frame and compare it to the height of the tallest point on your motor. (most likely the spark plug)

3. Now we must redesign the frame to hold the engine. (It is quite simple all you need to do is bend the front support down till you have your desired height and finish it with a motor mount.
4. Build a motor mount to make the bottom part of your frame. (I welded two pieces of 3/16″ flat stock together)

Motor Mount

Frame Mockup


Here is a picture of the front frame after being cut and rewelded to make room for the motor. While I had the front apart I also cut the top part of the frame and took about an inch out of it to tilt the front end back and give it a board track racer look. I also welded a piece of thick sheet metal to the bottom of it to improve structural strength.


5. Once the frame is mocked up and you have fitted your engine to the motor mount and have the clutch on the motor (hint hint put the clutch on) Bolt the motor in and weld up the frame nice and tight. In my final version I put a support from the top of the frame to the bottom to prevent the frame from flexing. This is a must!!!!

Bike with Frame Support-Done



5. Build the sprocket and chain drive assembly. For this I put the rear tire on backwards because the motor needed to be mounted with the chain on the other side of the bike that what is normal for a pedal bike. Basically I moved the chain sprocket from the right side to the left. I then welded my 72 tooth sprocket onto the one used when it was a BMX bike. Make sure to line the sprocket and clutch gear up as close as possible to avoid complication with the chain falling off. Just the other day I was riding mine up a hill and the rear sprocket came loose. Upon further inspection I realized that the clutch I had welded the sprocket onto (BMX gear) is a left hand thread so when the bike is used normally the gear is tightened onto the tire as more forward force is applied, but after putting the tire on backwards I was unthreading it as I put more force on it. As of this article I have not fixed it but hope to have it working again soon.

Sprocket Welded to Old Gear

Sprocket View 2

7. Now all you need is brakes. I chose to use the stock bike brakes, but I highly recommend something better. The added weight and increased speed melts the stock pads and takes a good 25 ft to stop at full braking.

8. Now try it out and you will probable realize something isn’t as strong as you thought or something needs to be tweaked. Don’t feel bad it took me 3 weeks after building it to get it to the point where I could ride it up the street without breaking down and having to walk it home.

Seat Mount

Flatstock for more Weldable Surface on Motor Mount

I am not responsible for ANYTHING and I mean anything that results from the use of any part of this article.

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Tuesday, June 26th, 2007 at 11:44 • BuildsRSS 2.0 feed • leave a response or trackback

31 Responses to “BMX To Motorcycle Conversion”

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  2. feather down says:

    Nice, its inspiring me really. Its really work? I mean can it run smoothly and how much it cost to build one?

    • miles says:

      The cost really depend on how resourceful you are. I only had to buy the engine, clutch, and spur gear. I got them all used for about $100 total. Yes it really works.

  3. It looks like you have the engine mounted right on the tire so my offer is that you squash the tire about 3/4 of the way and remount the motor on the tire as hard as you can without hitting the rim and fill the tire up with air again that should give you a lot of abrasion and if that doesn’t work you should get a tire with big tread

  4. Jon says:

    Awesome! I want a go on that! Great project…

  5. Marquetto says:

    I will try it. Congrats for the project, you nailed to give the machine a cool and agressive look.

  6. Lee says:

    what about if the drive shaft was at the bottom not the side ? how would you change it becuase cant turn the engine over on its side?

  7. luke says:

    I’d love to do this but I’d probably kill myself not to mention its not street legal so I would have nowhere to ride it anyway.

  8. john29302 says:

    dude…you said you “Upon further inspection I realized that the clutch I had welded the sprocket onto (BMX gear) is a …..” then ya said “What do you mean? The clutch is on the engine not the sprocket.” sooo…ya musta meant you welded a #35 sprocet to your rear bike sprocket?? i told em twice down at the battered womens shelter…but they already been told twice……good luck and thanlks for great pictures but get the tech writing stuff sane please

    • miles says:

      Sorry for the confusion. What I meant by clutch that the sprocket is welded to is the one way mechanism that allows you to pedal the bike and then stop pedaling (or moving the pedals) with the bike still in motion. You must weld this mechanism solid before installing the sprocket. The only real clutch is on the engine drive shaft.

  9. JimYoungbird says:

    Nice job, but… need to weld in some gussets at the neck joints. Thats an awful lot of welding up there and the gussets will keep it all together. Much safer too. Otherwise….MOTOR ON!!!!

  10. Scott says:

    hi, just woundering, does he bike has a clutch and accelorator like a real motorbike. (on the hanle bars)?????

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  12. I was wondering about the gear ratio. Does this have enough torque to climb decent hills? Also, what is the max RPM’s for that engine (im comparing it to my 3hp 2-stroke snowblower engine)

    • miles says:

      Yes it has enough power to climb reasonable size hills. I used an old briggs and stratton flathead and I believe it only revs to 3500 rpm. I am sure a 2-stroke can go much faster than that.

  13. Adrian says:

    In the one photo it looks as if the chain rubs on the tubing that connects the left hand side of the wheel if what can be done to improve the design. Where can i look for a rear sproket

    • miles says:

      I got my rear sprocket from just search “go kart 72 tooth sprocket”. Yes the chain does rub on the left hand fork for the rear tire. To solve this on my bike I just cut a section of the tubing out there. You can see this in this picture . In the picture the chain is in front of the section cut off but that is because the chain is off the rear sprocket. When the chain is on the rear sprocket it runs smoothly in that cutout. I am pretty sure this greatly weakens that support, so I do not recommend that solution. To fix this correctly you would need to move the location where the chain goes around the motor (clutch) down so when the chain is tight it is at a less steep incline going to the rear sprocket and thus running under the rear fork. You might be able to achieve this by using a shorter motor or a larger bike and lowering the engine in relation to the rear sprocket. But you will need to determine the correct height based on you setup/bike. You could also experiment with running the chain inside the fork rather than outside it. This would eliminate the need to cross the rear fork at all.

      Feel free to ask me something else if I didn’t make it clear.


  14. Kevin says:

    where can i find a welder? for cheap and expensive.

    Can you do this stuff in your garage or do you have to go somewhere?

    • miles says:

      You could probably find one on, but you will need to get instruction on how to use it before you will be able to weld correctly. You can do it at home, but you need to be in a well ventilated area and use protective clothing. Thats what I do.

  15. antwon says:

    where can i find a clutch that i can install on to my engine?

  16. antwon says:

    where can i find a clutch that i can install on to the sprocket
    is there anyway u can install it without a welder?

  17. Matt says:

    Man dude….who taught you how to weld? LOL.
    Bike looks like a Death Trap.

    • Miles says:

      Ya I know. 🙂 Normally I am pretty handy with a welder, but I was having trouble with my new welder and it was sputtering like crazy. I brought it back the next day and they said it had a defective transformer in it so I got it exchanged. I did make sure they have good penetration and they are pretty solid. Just not cosmetically pretty.

  18. david says:

    any suggestions on where to get a good but REALYREALY cheap bike for this project

  19. Gdfense26 says:

    Hey, I have always wanted to build some kind of motorcycle thing but I’ve never had a motor. Where are some places i might be able to get a cheap motor. Also i was wondering how much this costs to build (just an estimate). If you could help me out here that would be much appreciated. Thanks

    • miles says:

      For a motor I would check or possibly I got mine from my local craigs-list page. The guy wanted to get rid of all his small engines so I got two 3HP and one 2HP for $25. As for the total cost it is going to depend on how much a motor costs. If you buy a new engine it will be way more than $25. I can’t remember what the clutch and sprocket cost but it was around $40 from ebay. The flat stock and other small parts I had on hand, but I can’t see it costing more than $25 from your local hardware store. I am going to say that it will be around $100 if you have a bike to use.

      If you have more questions or comments I would be glad to answer them. Hope this helps you.


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