Speaking of flex....some may say that sidecars do not create much stress etc on mounts in 'normal' use. This is really not true. Although one may not see a failure right away that does not mean it will not occur. Almost anything that flexes is prone to failure at some point. Bend a paper clip back and forth a few times and it will break. OVERBUILDING is far bette than underbuilding!! Good friend Hal Kendall used to say " build it stout out of things you know about" !! Not a bad thought.
Anything that is fabricated does have some chance of failure. All the cars do not finish the Indy 500 every year. In racing crashes are inevitibale but the ones that come from a failure of a part do get the attention of everyone right away. There ar epictures of some pretty good builds here at ADV. DON'T SKIMP!! The pictures just posted by D. Batchel are definately on the right track.
Again, I am not wanting to be critical but sometimes ya just gotta do what ya gotta do.