Here are plans to build a small motorcycle from any old BMX bike. The tools that I user were a welder, die-grinder, crescent wrench, and other smaller hand tools that are common around most peoples 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:

Video:

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

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)

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!!!!

LOOK RIGHT BEHIND THE ENGINE TO GET AN IDEA OF WHAT I MEAN

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.

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.

Misc:

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

8 Comments to “BMX Cycle”

1. antwon — August 21, 2008 @ 3:15 pm
   

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

   [Reply]

   miles reply on August 21st, 2008:

   What do you mean? The clutch is on the engine not the sprocket.

   [Reply]

2. Matt — July 31, 2008 @ 8:50 am
   

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

   [Reply]

   Miles reply on July 31st, 2008:

   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.

   [Reply]

3. david — June 10, 2008 @ 10:14 am
   

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

   [Reply]

   miles reply on June 10th, 2008:

   A scrap yard or a thrift store might have some cheap bikes, or craigslist.org may have some free ones in your area.

   [Reply]

4. Gdfense26 — March 25, 2008 @ 2:19 pm
   

   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

   [Reply]

   miles reply on March 25th, 2008:

   For a motor I would check http://www.craigslist.org/about/sites.html or possibly http://www.ebay.com. 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.

   Thanks,
   Miles

   [Reply]

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