@ M/B speeds the rear of the bike will flex in turnsI've seen a couple motorized bicycles that had frame failures of the seat post tube. There must be some serious stresses on that tube. Doh, forgot to buy cigarettes on the way home....back on the bike! Thanks All!
Side rails on my Whizzer cracked but it didn't happen until after 30k miles, So it's a really good frame
I welded the cracks once but it cracked again by the weld
Sometimes reinforcement is needed