Why must there be a chain at the bottom of a truck​

Known as automatic tire chains and sometimes referred to by brand names like Onspot or Insta-Chain, these chains essentially hang listlessly from the vehicle's suspension until inclement weather arrives. When that happens, a driver can flip a switch that will lower the chains so they hang in front of the back tires.

Explanation:

(Happy to Help)

this is poiselle's equation from fluid mechanic in physics. mech eng