Originally Posted by Mr. Minecraft
So it has become pretty clear that the frame cracked because i had a 100 lb kid sitting on the back, puttin all the pressure right on the point where it cracked. Since this is true, doesn't that mean the frame should not crack under the weight on a normal single rider?
True, it should not crack under the weight of one young rider and it also shouldn't crack under the weight of two young riders.
Actually you don't know what caused it to crack. You're making an assumption that the extra rider's weight caused it. It might have and maybe it was already a poor weld there. Some riders of adult size weigh between 2 and 3 hundred pounds and have a motor on the rear rack or cargo and don't break their frames. When I was a kid we often had a second rider on back and I don't remember ever breaking a frame. Back then the bikes were made in the USA and most were well made to last. There is no getting around the idea that if you're going to motorize a bicycle which was not designed for a motor, then it is a good idea to have a well made strong frame. I call it peace of mind. One of my bikes was made in 1950, a Schwinn Panther... very well made in Chicago. Another was made in 1934 in Elgin, Illinois and is still a strong bicycle suitable for a motor. The frame is the foundation of your build. Let it be a good foundation you can trust.