Detecting collisions without rigidbody

I have a car with the following aspect:


As you can see, colliders are locally static, because they are separated from the original rigidbody component.

Well, as you can suppose, I have a problem, because MonoBehaviours with an OnTrigger(…) or an OnCollision(…) method won’t work. (It needs an rigidbody and a collider on the same object)

I thought about Raycasting:

But I have another problem, I have to specify a direction, so, using this: isn’t an option because this is too slow?

What can you suggest me?

as long as those colliders are children if the rigidbody, it will receive the collision calls because they combine to a compound collider: