Often drivers are lulled into a false sense of security by a small rock chip. They imagine that a small rock chip is somehow less likely to grow in to the dreaded crack that makes its way all the way across the windshield if left untreated. To begin with, most cracked windshields start with a small rock chip which later expands into a long crack. The crack may spread immediately, over night, or even in a matter of months to years. There is simply no way to tell when the damage is going to spread. Often customers inform me that the rock chip I am about to repair has been there for several years. I always tell them the same thing, which is that they have been on borrowed time, and very lucky at that. A rock chip may seem stable for a long time and then circumstances align causing it to suddenly break. Here’s some of the factors and physics behind what causes a rock chip to spread.
Glass tends to expand and contract in the heat and the cold. Usually, the rock chip you can see with your eyes also contains several invisible micro cracks which extend further out. When the glass expands in the heat or contracts in the cold, already weakened by the impact of the rock, it may give way, cracking sometimes up to 2 feet or more.
The impact of the rock hitting the windshield has removed a small piece of the surface glass, weakening the structural integrity of the windshield. Vibrations and bumps in the road can easily cause the chip to crack.
Differences in atmospheric pressure can also cause a weakened windshield to crack. Often drivers report that a rock chip that had been on the windshield for months, suddenly cracked when they went into the mountains.
Last, running the car heater or air conditioning can cause an unstable rock chip to crack. Like running a cold glass under hot water, or hot glass under cold water, the difference between the temperatures of the outer and inner layers of the windshield can cause the glass to shatter.
The size of the rock chip is immaterial. A tiny rock chip is just as likely to crack as a larger one. The same physics behind large rock chips are in play with smaller ones. That’s why I say that drivers are often lulled into a false sense of security when they have a small rock chip. I can’t count how many times I’ve heard the same story. It always starts something like this: “I had this little rock chip on my windshield that happened a few months ago and I thought, it was no big deal. Then I came out yesterday and it had spread to over a foot long. Is there anything you can do?” And of course, the answer is, “no”.
Rock chips are a pain in the neck no matter what size. My advice is to have it repaired immediately so it doesn’t become a much bigger, much more expensive, pain in the neck.
Glass will always behave in a relatively predictable way. There are a few “enemies” of windshield repair. Extreme temperatures is one of them. Glass tends to expand in the heat and contract in the cold. The windshield is comprised of three layers, UV glass on the outside, UV glass in the interior of the car and a synthetic in the middle, laminated together to create a strong bond. When it is very hot outside, especially when the car has been sitting in direct sun, the windshield can heat up to well over 110 degrees F. More than likely, drivers will run their car air conditioner on a hot day and if there is a rock chip or a small crack on the windshield, the variance in the temperatures from the outside of the windshield and inside the car, can easily cause the damage to grow in size resulting in the need for a costly windshield replacement. Here’s why: The outer layer of glass is expanding in the heat. The middle layer is fixed, and the inside layer is contracting from the cold air blowing from the air conditioner. This causes a certain amount of pressure, and to relieve the tension, the glass cracks. It is the same principle that cause a hot glass to shatter if it is held under cold water or visa versa. In the Winter, the same thing can happen, only in reverse. The outer layer of the windshield is icy cold and when the car heater is turned up, the inside layer begins expanding with the heat.
The take away from this, aside from having your windshield repaired asap when you have a rock chip, is:
Avoid extremes in temperature. If you are still using the vehicle before you have a chance to have it repaired, try to park it in a shaded area, out of direct sunlight.
Avoid using sun reflectors that are shiny. Shiny reflectors will shield the interior of the car by reflecting sunlight away, but they are also known to heat the windshield to temperatures that can easily cause a small rock chip to spread. Instead, use opaque, or satin-finished sun guards.
If you are using your air conditioner, keep it on low, and keep the setting adjusted so that the air flow is focused mainly on your legs and not on the windshield.
-Simple tips that can help to save your windshield.
Repair results can be significantly enhanced by reporting and scheduling an appointment immediately after damage has occurred. When a rock from the road hits the windshield a small piece of the top layer of glass is literally sheared off. At the same time, some of the glass directly under the point of impact is propelled inward. From that point on, as you are driving, the small opening into the middle layer of glass is collecting and compacting dirt, debris and moisture in the break. The longer you wait before having the windshield repaired, the more difficult it will be to remove what ever has accumulated in the damaged area. In short, the sooner the repair the better the result.
Above is a magnified photo of a common type of rock chip. this is a combination break showing a bull’s eye and a star break. Magnification show many micro-cracks that extend out in all directions. Without magnification the damage may appear less extensive giving the impression that it does not need immediate attention. One significant bump or extreme changes in temperature can easily cause this rock chip to crack out. The “legs” extending outward are unstable and can easily spread resulting in a costly windshield replacement. If the damage is repaired soon after it occurs, the resin injected into the damage area will totally fill the break structurally restoring the windshield to pre-damage condition. The visible damaged area will be enhanced between 75 – 95%. The sooner the repair is made the better the results.
This is a very common type of rock chip. Notice the point of impact in the center of the damage on the left side. You can see the dark area which extends out from the damage point on each side of the bulls eye. When the windshield is damaged, air, dirt and debris are forced into the break causing it to appear darker in color. The repair process consists of alternating pressure and vacuum cycles. The vacuum cycle draws out the trapped debris, air and moisture while the pressure cycle injects resin into the damaged area eventually filling it. The “after” photo shows little trace of the original damage, leaving a small, faint gray dot at the original impact point.
A quality windshield repair should result in an 85-90% reduction of the visible damage to the windshield. There will always be a slight imperfection usually in the form of a faint gray “dot” at the point of the original impact. Also, depending on the extent of the damage and that amount of time that has passed before the windshield is repaired, it is not uncommon to see visible remnants of small cracks which occurred at the time of the original impact.