how to reduce the speed of bike when collide with side walls.

Hello guys, i am stuck in one place actually i am working on racing game in which i have a bike and on the road side i have fence. so when my bike hits the fence in speed it flies in air. i just want to reduce the speed of bike when it collide with the fence.

i have FixedUpdate() method in which i have all my physics. and OnCollisionEnter() method in which i am detecting the fence. where i set the speed to 0. but i want to reduce my speed. how do i send message to FixedUpdate method to reduce speed when it hits the fence.

thanks.

var hittingFence : boolean = false;

function OnCollisionEnter( other : Collider){
 if (other.gameObject.CompareTag("fence"))
{
 hittingFence = true;
}
}
function OnCollisionExit(otherObject : Collider){
if (other.gameObject.CompareTag("fence"))
{
 hittingFence = false;
}
}

function FixedUpdate(){

if(hittingFence)
{
Bike.AddFoce(Vector3.forward *-1)
}

}

something like that.