Detecting Physic material on collision

Hello!

I have a problem. I want to make stuff happen depending on Physic material of a surface my wheel collider collides with,but I don’t know how to do it,and currently it gives me a very strange error with no code “Script error: OnCollisionEnter
This message parameter has to be of type:
The message will be ignored.”

What should I use?

Here’s my code so far…

using UnityEngine;
using System.Collections;

public class Wheel_Emmiter : MonoBehaviour {

	public GameObject DirtEffect;
	public GameObject GrassEffect;
	public GameObject SandEffect;
	public GameObject WaterEffect;

	public WheelCollider Wheelcol;
	public GameObject AsphaltSound;
	public GameObject DirtSound;
	public GameObject SandSound;
	public GameObject GrassSound;
	public GameObject WaterSound;

	public PhysicMaterial Asphalt;
	public PhysicMaterial Dirt;
	public PhysicMaterial Sand;
	public PhysicMaterial Grass;
	public PhysicMaterial Water;


	void Start () {
		DirtEffect.SetActive(false);
		DirtSound.GetComponent<AudioSource>().enabled = false;

	}

	void OnCollisionEnter(Collider other)
	{
		if(Wheelcol.rpm > 250)
		{
			if (other.GetComponent<MeshCollider>().material = Asphalt)
			{
				DirtEffect.SetActive(false);
				GrassEffect.SetActive(false);
				WaterEffect.SetActive(false);
				SandEffect.SetActive(false);

				AsphaltSound.GetComponent<AudioSource>().enabled = true;
				DirtSound.GetComponent<AudioSource>().enabled = false;
				SandSound.GetComponent<AudioSource>().enabled = false;
				GrassSound.GetComponent<AudioSource>().enabled = false;
				WaterSound.GetComponent<AudioSource>().enabled = false;
			}

			else if (other.GetComponent<MeshCollider>().material = Dirt)
			{
				DirtEffect.SetActive(true);
				GrassEffect.SetActive(false);
				WaterEffect.SetActive(false);
				SandEffect.SetActive(false);

				AsphaltSound.GetComponent<AudioSource>().enabled = false;
				DirtSound.GetComponent<AudioSource>().enabled = true;
				SandSound.GetComponent<AudioSource>().enabled = false;
				GrassSound.GetComponent<AudioSource>().enabled = false;
				WaterSound.GetComponent<AudioSource>().enabled = false;
			}

			else if (other.GetComponent<MeshCollider>().material = Sand)
			{
				DirtEffect.SetActive(false);
				GrassEffect.SetActive(false);
				WaterEffect.SetActive(false);
				SandEffect.SetActive(true);

				AsphaltSound.GetComponent<AudioSource>().enabled = false;
				DirtSound.GetComponent<AudioSource>().enabled = false;
				SandSound.GetComponent<AudioSource>().enabled = true;
				GrassSound.GetComponent<AudioSource>().enabled = false;
				WaterSound.GetComponent<AudioSource>().enabled = false;
			}

			else if (other.GetComponent<MeshCollider>().material = Grass)
			{
				DirtEffect.SetActive(false);
				GrassEffect.SetActive(true);
				WaterEffect.SetActive(false);
				SandEffect.SetActive(false);

				AsphaltSound.GetComponent<AudioSource>().enabled = false;
				DirtSound.GetComponent<AudioSource>().enabled = false;
				SandSound.GetComponent<AudioSource>().enabled = false;
				GrassSound.GetComponent<AudioSource>().enabled = true;
				WaterSound.GetComponent<AudioSource>().enabled = false;
			}

			else
			{
				DirtEffect.SetActive(false);
				GrassEffect.SetActive(false);
				WaterEffect.SetActive(true);
				SandEffect.SetActive(false);

				AsphaltSound.GetComponent<AudioSource>().enabled = false;
				DirtSound.GetComponent<AudioSource>().enabled = false;
				SandSound.GetComponent<AudioSource>().enabled = false;
				GrassSound.GetComponent<AudioSource>().enabled = false;
				WaterSound.GetComponent<AudioSource>().enabled = true;
			}
		}
	}
}

OnCollisionEnter passes a Collision, not a Collider.