i've heard of a few frames breaking, but never saw one.
personally, i would never do it, and i would never advise anyone else to do it.
the downtube of a bike is a structural member of the frame. normally, if they crack, it's where the downtube meets the headtube. but if you drill a hole in it, you just made an even weaker link.
if you want to visualize a worst case scenario, picture your average rider on an average bike. now, think about the downward pressure the rider puts on the bars, the seat, and the cranks. now think of the upward force from hitting bumps or something.
now imagine having a big weakspot, almost in the center of the bike, at the bottom.
i can picture the bike buckling in half as that downtube breaks.
i don't wanna picture what happens to the rider at 30-40mph.
in the last year or so, upgrades and solutions to the various problems the chinese didn't care about have come up. there's so many better ways to mount a motor than drilling a hole, in my opinion, the only people who would still do it are the people with no mechanical ability and are out to make a quick buck and don't care what happens to the actual rider.
they can always claim, "but that's how the instructions said to do it..." if someone gets hurt.
If i go over the handlebars at 24mph (300Lbs LOL) from a cracked frame, pride wont stop me from giving y'all a full report.......After my broken fingers heal.
