Depends how far below 40 & how much wind. Down to mid-thirties, I don't like to layer at all & prefer a tight with a built-in chamois (Etxe Ondo). Below 35 down to just below freezing, or mid-thirties with nasty wind-chill, same thing but I want the tight to be a bib with a wind-stopper front (Gore or Assos). Seriously below freezing & I want fleeced bib-knickers (Assos is the only maker I know of), and fleeced tights over the knickers (Shebeest wind-pro, or Sugoi sub-zero). I have learned with layering bottoms in really cold weather that it is much more comfortable to have only one waistband, and to not have that waistband against your skin, which is why I prefer the underlayer to be a bib style. That works for me.