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I didn't know what part was missing from the first bike and it was a little frustrating the author didn't explain! I have an idea now from comparing to real bikes, but I am not confident that this bike would "immediately break." From forces on the rear wheel?

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It's missing the horizontal struts between the pedals and the back axle. Those make strong triangle shapes, and without them the back fork is going to be quite flimsy and unstable. The person's weight on the seat would put a lot of torque on the welds connecting the back fork struts to the frame below the seat.

correct, this is known as the chainstay in case you want the jargon

Thank you. The author really missed the big reveal at the end, I was also wondering this same thing.

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