Building a mini bike

Hi, I'm planning on building a mini bike which will be as close to a dirtbike as I can make it , but on a smaller scale, Much these mini bikes/mini dirt bikes found at

http://www.minidoodle.com/id32.htm

Here are the bikes.

http://www.minidoodle.com/2c3289e0.jpg

http://www.minidoodle.com/id297.htm

http://www.minidoodle.com/id251.htm

http://i21.ebayimg.com/01/i/04/fb/8d/e6_12_sb.JPG

I was wondering what kind of metal tubing should be used? My shop teacher thinks 1inch tubing 1/16th of an inch will be strong enough but personally I'd like ot go with 1/8th. What do you guys think? And also does anyone know what kind of tubing dirtbikes and mini dirtbikes are usually made of? Any help would be greatly appreciated. Thanks :biggrin:

I was wondering what kind of metal tubing should be used? My shop teacher thinks 1inch tubing 1/16th of an inch will be strong enough but personally I'd like ot go with 1/8th. What do you guys think? And also does anyone know what kind of tubing dirtbikes and mini dirtbikes are usually made of? Any help would be greatly appreciated. Thanks :biggrin:

No offense, but your shop teacher is nuts thing a .063 wall will work. You should you 1" to 1.25" with a .100 wall minimum DOM tubing. DOM is drawn on mandrel and is stronger than ERW, and easier to weld than chromoly. Dirtbikes and mini bikes (as I have designed a few) use DOM tubing. DOM is cold rolled steel ERW tubing that was then drawn over a mandrel to be stronger and more accurate. Most regulations require DOM tubing or Chromoly (with a heat treated certificate). I'd use DOM because chromoly cannot be properly MIG welded.

Is DOM expensive? Would I be fine with regular 1/8th of an inch regular cold rolled steel? I don't really have that much money right now. 1.25 is 1/8th right?

1.25 would be 1 1/4.

Oh I get it. 1 1/4 inch across. How thick is that metal though?

Oh I get it. 1 1/4 inch across. How thick is that metal though?

.100 wall. I said that in my first post. CRS tube (cold roll) will not work, it bends too easily. It needs to be DOM which is the same as CRS/ERW but has an extra process that makes it more accurate and stronger. It runs about $4.00/foot.

Dont forget to buy a tubing notcher and bender. The tubing nothcher is extremely important for a strong/sound weld. If the fit isnt correct, then might as well jump off a building as they both have the same likely hood of fatality. The joints need to be strong and secure.

I dont need to buy a notcher or a bender. We have them in shop. A hydraulic bender, and a mill to C notch tubing.

Oh yeah, I was also wondering what .100wall would be in like fractions. because I dont really know what .100 wall would look like but I do know what 1/16, 5/64,3/32 ect. would be. Thanks - Andrew

I dont need to buy a notcher or a bender. We have them in shop. A hydraulic bender, and a mill to C notch tubing.

Oh yeah, I was also wondering what .100wall would be in like fractions. because I dont really know what .100 wall would look like but I do know what 1/16, 5/64,3/32 ect. would be. Thanks - Andrew

Andrew, look for a decimal to fractions chart. It will definately come in handy. When talking to metal suppliers its all in decimal instead of fraction because of the tight tolerances.

.100 wall is smaller than 1/8" (1/8" is .125)

What do you guys think would be a better engine. A 6.5 honda gx200 or a 6.5 tecumseh power sport. The gx200 is an all purpose engine and the power sport is meant for go karts and stuff. I was thinking the tecumsuh one would be better because it's meant for a go kart or mini bike type set up but why is the honda 6.5 so much more money than the tecumseh. Is it better made?

I used to work in selling small 2 strokes so hopefully I can answer your question.

The GX200 is a much better engine, for one its Honda. Thats where the price difference comes in. Tecumseh is at the very bottom of the line. Honda's are at the top of the line, very durable and Honda stands behind them.

Heres the order of quality:
Tecumseh, Briggs and Stratton, Kohler, Honda

Chances are I'm going to go with the tecumseh because I really can't spend over 300 on an engine after buying all the parts and metal. Is the honda 6.5 faster or just built better? I was also wondering if there was anything I could do to the tecumseh engine that could improve it's preformance other than sending it out and getting it bored, or porting and stuff. Preformance air filter? Exhaust? Carb?

just hit up your local lawn mower place and pick up a motor for about $40, no need to waist that kind of cash on a toy.

Don't lawnmowers use engines with vertical shafts? For the bike I need an engine with a horizontal shaft.

depends, a 5hp briggs with some modifications should be more than enough power for play and 40+ MPH speeds (assuming you are using a Torque-A-Verter or some type of transmission and not just a cyntrifical chain clutch and sprocket)
you can grab a brand new one up for 200 some odd dollars, or a used one for 100 some odd dollars, modifications such as a new air induction, some porting on the intake manifold, getting heads with a smaller combustion chamber (high compression), messing with the flywheel/magneto to get a bit more timing, replacing all fittings in the caburator with brass ones and run an alcohol mix and removing the old muffler for a header or a custom exaust should give you 6-7bhp...and pushing 180-200lbs (assuming you dont eigh mroe than 140ish) it'll scoot pretty fast, building an all out race engine can yeild as much as 10hp but that requires some knowhow.
anoth engine i could sugges would be to salvage one off of a dirt bike, say a little 70cc 2 stroke...already equipt with a 3 or 4 speed. then you have to rig up a clutch and gear selector though.

Well, I weigh about 186. :rolleyes:
About the torque converter, no, but the bike is going to have a jackshaft which will be similar to a torque converter. It will look something like this.

http://i.timeinc.net/mx/content/images/mar05/030405/_Y5L4627.jpg

It will have a slightly larger sprocket connecting the clutch to the jackshaft and then a small one on the jackshaft to the larger rear sprocket on the tire. Another reason I'm doing this is becuase the chain might come off when the suspension compresses if i go off a small bump or curb or something. I want to ride this thing on dirttrails and stuff and not just the road. I already purchased used dirtbike suspension. A used pair for only 20 bucks. :iceslolan


BTW, I'm using these plans, but slightly modified to fit the suspension and I'm now using the makeshift transmision in those plans. I'm going to just go with the jackshaft as in the picture above.

gotcha. Torque-A-Veters are simply a good idea to get the good high speed gear ratio's but also have some low end grunt, but if all you care about is going fast even if it takes forever to get there... that'll work.
to give you an idea on how to set up the suspension take a look at a harley soft tail... instead of the conventional dirt bike suspnesion.