Smoothing Groups Problem

Mohamed_Salah_Bchir

I tried all angle thresholds, can't get rid of the distortion, i think adding control edges would solve the problem but i don't want more edges. can i fix it as is? i think the problem is that smoothing groups don't work on flat curves?

Mohamed_Salah_Bchir

I fixed it by giving each polygon (before the curve) it's own smoothing group