If you drive about 10-12,000 miles per year and your tires are three or four years old, then it's probably just about time to replace them. Replacing old, worn down tires is an important part of keeping your car both safe to drive and performing as it was intended to. Getting new tires can drastically change the handling and ride characteristics of a car, but what actually happens as your old tires wear out and why is changing them so important?

It's All About the Contact Patch

The traction that allows your car to accelerate, brake, and corner is generated by an area of your tire known as the contact patch. The contact patch is simply the portion of your tire that is in direct contact with the road surface at any given time. The size and shape of the contact patch is dependent on the type of tire being used, with completely smooth tires (known as racing slicks) providing the best possible traction in dry conditions. For street tires, the grooves in the tread exist to direct water away from the tire in order to maintain traction in wet conditions.

How Tread Wear Works

As your tire rolls over the road, the heat created by the friction gradually wears down the tread on the tires. Assuming your tires are properly inflated and your vehicle does not have any outstanding issues with its suspension or steering, the tread on your tires will generally wear evenly. For street tires, this wear reduces the height of the tread blocks, in turn reducing the depth of the grooves. Modern tires are equipped with wear indicators which become visible once the height of the tread blocks is low enough.

The Effects of Tread Wear

It is important to keep in mind that the tread patterns that are commonly found on street tires exist entirely to improve traction in adverse weather conditions. This means that wear on the treads primarily impacts wet handling. This is one of the ways that worn tires are exceptionally dangerous. A tire with dangerously low tread may seem fine in dry conditions, only to cause a sudden loss of traction once the road turns wet. This makes tire replacement especially important if you live in a wet or snowy region.

In addition to a loss of adverse weather traction, worn treads also increase the risk of a catastrophic failure. The grooves on your tire help to provide cooling for the rubber and tread provides protection for the tire itself. As your tread wears down, it becomes more likely that your tire will blow out or that debris on the road will puncture the tire. Tires that are very old and worn can often suffer from a sudden tread separation as well, which can be especially hazardous if it occurs at high speed.