Discussion at Motorized Bicycle Engine Kit Forum in the Motor Bicycle Safety forum. I was rideing on the beach and my crank broke. Could have gotten hurt if I was on the street. ...
I was rideing on the beach and my crank broke. Could have gotten hurt if I was on the street. I was pedaling to get it started, and it just snapped. Probably good idea to inspect for crackes because there was a dark spot that looked like it was cracked for awhile before it snapped
Do not know how to post pictures hope this is okay
The fact you have noticed a 'dark spot' might indicate a stress fracture failure from vibration...
could be the engine is setting up a resonance that is travelling back through the drive system and stressing the metal of the cranks - considering the stress a crank is usually under they wouldnt be made of very strong or tempered metal...
I broke the crank on my Next bike- I was trying to do a burn out (no engine, just me). The third time I tried to "lay a patch" the crank broke off of where it's mounted to the sprocket. I welded it and it seems fine now.
__________________ If it ain't broke, and you mess with it long enough, it will be.
I got lucky that it did not cut me. When me and my brothers were kids, and into BMX. My brother broke some Shimono 600 cranks the aluminum ones, and cut his leg so bad that he had to get 5 stiches.
Cranks could break 
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