I don't know why you wouldn't weld to the frame. You won't be ruining any existing heat treating if the bike isn't heat treated, and for the money you're looking at it won't be heat treated. I wouldn't want a seatstay weld coming undone and you getting the seatstay shoved up your [leg] by a 7hp motor so I
feel it needs a gusset adding, personally. I can't picture what you're going to do about making the motor mount inside the frame.. is it really THAT small? I had a 27cc off a goped(copy) motor, never managed to mount it in my Hoffman because of the difficulty getting a sprocket for it (goped chain was tiny, not a BMX chain to make things simple) but it was never going to fit in the front triangle of the frame. I'd love to see some pics of your build once you get it going, this is turning out to be really interesting!
Idk MBs, though I am more familiar than most about BMX and MTB.. only meant to chat with you about "general bicycle questions" as I know BMX bikes so I can actually talk in this part of the forum without talking out my armpit.
I still say go for a 24"(wheel) frame though, you get longer toptube and seatstays that way, and even if you stick 20" wheels in there could well still have clearance for pedalling as long as you don't try to pedal around corners, lol. A 24" fram could use a (adult) mountainbike fork, suspension fork, disk brake compatible fork (check my album), slackening head angle a little by raising bars but without ruining BB height as much as on a 20"(wheel) BMX. Bottom bracket height affects looping out more than anything when messing about with a BMX frame, a slight increase in BB height means quite a few degrees higher from your rear axle.. hope I put that clearly.. I'll draw up a pic if you like though.
Mounting the tank further forward would just help a little with (not) looping out, that's what I was thinking.. gives you a little more knee room too, not to mention would be barely visible hidden inside there so the bike would look really cool. The little bar mounted tanks on the Team Boxer bikes (check the Gallery section) that I've seen look ideally sized for this and shouldn't affect your steering too badly if your bars are mounted "correctly" parallel with the fork leg (not wayy dangerously forward as a lot of kids have them set up).
But am I correct that BMX frames are stronger than regular bicycle frames?
Well that's a huge sweeping generalisation. Short answer is no, but they have short tubes so lower
leverage forces to deal with, same as with the wheels' stiffness. "stronger BMX frame, cheap or expensive?" was the original question.. I'd say twice as expensive as the Diamondback you were looking at, and you won't find a real BMX freestyle frame browsing classifieds same as you won't find a dirt jumping/downhill MTB in classifieds, only 26" wheel road/shopping "mountainbikes".. You would find it by chatting with older BMX riders, the owner of the local indoor skatepark, or maybe putting a "
wanted: strong 1990s BMX frame & fork" poster in your local indoor skatepark.
The majority of
all "stunt" bikes are made for kids to ride on the road and feel like their "stuntman" hero, not actually ride like one. You could just weld gussets onto a Diamondback I expect, since you're not going to need to jump it or pedal it at jumps. Or are you going to jump it?? If your motor is truly something special, then a cheap frame would waste it
IMO.
