Tires: snow traction, snow traction, concrete pavement

Question
I read one of your responses from 2005 where you said a larger size tire 20" vs 17" will improve dry and wet traction(except hydroplaning) but degrade snow traction and hydroplaning resistance. Explain to me why wet traction gets better but snow traction is degraded. I am trying to decide between 18" and 20" for a new Mazda Cx-9. I have heard the ride is rough on concrete pavement on the 20". DO you have a recommendation?  

Answer
Raja,

Let's start with the last question first:

I am not a fan of plus sizing unless you plan to race.  The only real benefits are traction, and there are a whole lot of negatives.

Traction?  Wet traction - excluding hydroplaning - is a function of how much the tire can penetrate the road texture, just like it is for dry traction.  The problem is that hydroplaning becomes a factor fairly early in some locales and most folks don't distinguish between the two.

Snow traction can be divided into 2 components - touching the road surface, where any bit of touching generates more traction, and this is the most common case - and "floating" on the snow, which is the low traction arena.  It is always best to reach the road surface if you can.

Hope this helps.