that's not a very long question, but.... requires a very ... long answer....
if you want my honest opinion, i'll give it...
buy a frame... don't build it. UNLESS... you are not only a very good welder, but good at figuring out precise geometrical measurements to calculate stresses on the frame in various situations..
What i mean is, i don't care how good of a welder you are, if you take a jump on a bike that isn't designed specifically for taking jumps, you can bend it in half or brake it... and that SUCKS when you're coming down from a 10 foot jump at 25 or 30mph...
The handlebars go through your chest and you end up in the ER..
BUT.......... if you're going to design a frame and just go put around on it, it's a little different.
the other thing is this... given what i just said, you'll be likely to compensate for that by reinforcing the hell out of the frame, which of course adds a LOT of extra weight.. a bike that was designed using CAD, 50+ years of knowledge, and experience etc is a lot better for that as well..
finally,
with the cost of materials to build your frame, you could buy an old bike and put a new motor in it.. you can probably buy an old bike for a few hundred bucks.. how much does steel tubing cost?
OK............ aside from that,
if you do build your own frame, here's what you need..
first of all, you need to have enough knowledge about bikes to know where the center of gravity SHOULD be for the bike... you should also have an understanding of how the height as well as the length or wheelbase of a bike affects handling.. things like fork slope, ground clearance, where you sit on the bike, and everything else take a HUGE toll on how the bike operates.......
i.e. safely or ... not safely...
a good example of that is this..
in the 60's and 70's they had a bike called the Suzuki T500 (hence my name) a LOT of people raced them, and STILL race them today... but the wheel base made it rather sluggish in turns, so most people bought rear ends off of the smaller brother T350 and put them on the T500 to shorten the wheel base.. by doing so, they found that it made the bike much more agile in corners.. where the wheel sat played in where the riders center of gravity was as well as the potential turning ratio of the bike..
as far as the front end, choppers with really long forks don't turn nearly as well as sport bikes ..
so you need to know all of that...
once you figure that out and DO build a frame, one important thing is to note that, if you don't like the motor up PERFECTLY with the rear sprocket you will wear out sprockets and engine parts with a quickness.
so you need to figure out how to line them up when you weld the mounts... there's a lot more to it than what i just said, but it's something to think about.... the width of the engine plays part in that... it also plays a part in the size back tire you use...
lets say you do build the frame... and you do mount the motor successfully and you spend the hundreds of dollars on shocks, a seat and all of the other stuff, and are ready to get the engine running....
for the most part, any motor you buy should have a magneto system in it that will work WITHOUT a battery...
they are typically points systems and have exciter coils on them, therefore all you really need is a toggle switch.. click it on, turn the gas valve, kick it to start and it goes... the electricity from the motor runs itself just like a lawn mower...
however, even with an electronic ignition (CDI) you don't necessarily need a battery... typically when you buy a motor it will come with most everything you need though...
i was trying to find a link for you for a good place to buy motors.. i've talked to the guy several times and he does a good job with motors.. i'll see if i can find it later..
anyway, there is a lot to think about in building a bike of any type..
seriously though, i'd recommend buying a frame somewhere it would be a LOT cheaper and a LOT easier...
they have already gone through all of the trouble that i mentioned to design bikes for specific purposes.... so, you know their going to be much safer than one you build..
of course, its not as much fun to buy one, but you can still alter it as you need....
good luck either way...
seriously though, you can buy a -running- one for cheaper than what it's going to cost you to build one...
but.... like i said, thats no fun.
i build bikes too by the way....
