HOOAA! Trumpy interesting stuff. So you mechanically bend PVC with out heat? The only way I know of bending PVC here in the states is with heat and I have made many a jig to keep it all straight during the time it takes to cool down.

Never had heard of a spring in the pipe but I have kinked my share of bends in larger 1 1/4" plus EMT.

I have always used the Benfield method except for box offsets. Have had a wallet sized Benfield calculator in my wallet for years. Don't bend much pipe these days.