ok, uuu: i liked the idea of finding a pre-existing motorbike engine so much that i went "hmm, what about 50cc scooter 2-stroke engines?" and well that led me to look on ebay and i accidentally wandered across a polish company that makes "big bore cylinder kits" for £32 + £15 shipping, each.
well, now, i don't know about you, but i think i would be a bit of a jack-ass to go doing all the work of making something that's near-identical to what i can get from poland for £47, don't you? :)
although it feels like cheating, i'm all for tackling one thing at a time. so i found a CNC shop that's happy to make the cams for me, he found it hilarious that i sent him a kid's-style picture originally. also i think he thought i must be mad, or something, and i could tell he was thinking "this is money for old rope, here, i ain't complaining if this guy pays me to do something nuts!". i've since beefed up operations and written the computer program that generated this:
http://lkcl.net/engine/trilobe.png
surprisingly it only took me a couple of hours. it's a horrible algorithm, order n-cubed, because i couldn't work out the maths. the bearings that roll along the surface of the cam, you can't just use a sine-wave cam surface because the point at which the bearings meet the surface as it's going up and down is *not* on the line between the centres of rotation. argh. so rather than do some maths equations i just tested all possible rotational combinations, hence the truly dreadful algorithm.
eyy, it only took about 2 minutes to run, what am i complaining about? :)