Need help with physics materials.

So i'm pretty much complete novice at using Unity and have started by trying to make a basic driving game.
At the moment i've got all the physics down and it's working pretty well.
But i want it so when the wheel colliders contact a certain object/material the car will slow down as if it's driving on a high friction terrain. I tried using physics materials but to no avail, they simply didn't do anything to the friction of my wheel colliders.
Am i doing something wrong?
Is there a better way of doing this?
I so how?

 

Wheel Colliders are not affected by physics materials. This is because friction for wheels needs to be calculated separately. Wheels use the slip-based friction model.

A way to get around this would be to change the friction of wheel itself. Maybe you can have different triggers over different types of ground. When the car comes in contact with those triggers, change the friction values of the wheels.