I was wondering how critical strike chance stacks? Say I buy a Phanton Dancer with its 30% crit chance. Now if I buy another Phanton Dancer, what is my critical chance?


Question: Using a ski helmet for winter biking
Answer: 
  Are skiing helmets designed to protect you from the same kind of acidents that you would have on a bike?


Here is an article, for what it's worth, about the kind[s] of accidents that skiing helmets are designed to protect you from: The Science Behind Helmets.

On another (but related) subject, according to the web sites referenced in this answer there is even, apparently, some controversy over whether bicycle helmets will protect you from the kind of accidents that you would have on a bike.