The windshield is a vital component of your car that quietly and invisibly does its job. It's often taken for granted until it gets cracked. Sometimes the cause is obvious, such as when a stone flies into it while driving. Other times, the crack mysteriously appears: it wasn't there when you left your car and is there when you return. What causes windshield cracks and how can they be prevented?
Cracks Caused by Projectile Impacts 
Cracks of this kind happen as the result of gravel or stones getting kicked into the windshield from the tires of a car or truck in front of you on the road. A stone can even get thrown into your windshield from oncoming traffic. Depending on the size of the projectile and the speed of impact, a crack can vary from a small pit to a large crater-like hole. Sometimes smaller cracks radiate out from the central hole. These types of impacts are best avoided by allowing plenty of road space between your car and the vehicles in front, especially when you are following a truck.
Hail storms often cause cracks and can be quite destructive because your windshield may receive multiple impacts over an extended time. Keeping your car garaged and checking the weather forecast before driving are the best preventative measures for avoiding hail damage.
Cracks Caused by Stress
Stress cracks are often caused by an extreme temperature difference between the inside of the windshield and its exterior. If you use a strong defroster on a very cold morning, the inside of your windshield will expand relative to its outer cold surface. If this expansion is too extreme, a stress crack will form in your windshield.
Temperature extremes can also occur when your windshield gets very hot on a warm day and you use your air conditioner at its full setting. In both cases, you should turn on your defroster or air conditioner at a low setting to allow your windshield to warm up or cool down more gradually.
A stress crack can also occur when using hot water to melt ice off of your windshield on a winter day. Sometimes a manufacturing defect leaves residual stress in the glass that results in a crack.
Whenever you notice a crack in your windshield, prompt repair or replacement is recommended. Cracks can grow over time and interfere with your ability to see the road. They also weaken the structural integrity of your windshield. Coverage of your windshield may or may not be included in your auto insurance policy. Contact your independent insurance agent to find out if you have coverage. If not, it should be possible to add the coverage to your policy.
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