While waiting for a flat tire to be repaired, I spent an hour in Les Schwab tire salon looking at tire treads, and I noticed something odd: none of the treads were symmetrical, not quite.
Am I seeing things? Was it just those tires, at that salon? Is there an engineering reason for this?
My WAG would be that it prevents resonance in the tire/wheel and cuts down on the road noise of the tire. Also, if the grooves in the tire all lined up exactly, it would create a ring of weakness along that groove.
My search-fu is lacking, but I recall seeing on TV many years ago that a significant part of the design of car treads is to remove water to ensure a better grip. So you want to direct the water.
There are two types of asymmetry.
A performance passenger car tire may be asymmetrical when looking at the tread from the inside to the outside. This type of tire will have an arrow on the sidewall pointing in the direction of rotation, and must be mounted so that the arrow points forward when on the car. This is called a unidirectional tire. Needless to say when rotating unidirectional tires, they do not get rotated in an X pattern. This type of asymmetry is done for the best handling.
The second type of asymmetry is found on all passenger car tires and most truck tires. If you look at the angle of the sipes (the little tiny channels cut into the tread), and then look again a little further around the circumference You will see the angle is different. This is done for noise. Each angle creates a different tone. If you have a bunch of different tones, each of them is quiet enough not to be heard. If all the sipes were at exactly the same angle, the noise is LOUD.
If you have ever driven near a jacked up pickup truck with giant mud tires, some of those don’t have any variation in the tread, and the noise of the tires at freeway speed is impressive. And very annoying.