Test 2 of sandcar effect

Here is my second attempt at producing the sand ca effect, i managed show the car driving off while some sand particles are attached to the car and some falling off. What i want to try and fix is the origin of when the sand particles start falling off from the car. Im going to try and get the sand to stick on the car for atleast 4 seconds to let the audience know that what they are seeing is the Audi R8 sand version before the particles explode