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.
As the weather begins to cool around the country rock chips that have been unattended to become particularly vulnerable. The short and simple of it is that glass tends to expand and contract based on ambient temperature. If there is a rock chip which has not been repaired, your windshield has already been weakened, even if it appears to be stable and even if the rock chip has been there for ages. The physics involved is also simple. The small micro crack caused by the rock chip will do its best to find a path of least resistance to the furthest point on the windshield to relieve pressure. This is why a spreading crack often appears more like artwork as it swirls around the windshield searching for a final resting place -which usually ends up in an auto glass replacement shop!
All of this is actually avoidable most of the time. While the laws of physics will eventually hold up, all you need to do is have the rock chip repaired before physics makes the choice for you.
Your windshield is comprised of three layers that have been laminated together. The top layer is UV glass, the middle layer is a soft PVB material similar to plastic and is the layer that allows the windshield to crack into many smaller pieces on impact instead of breaking into sharp shards. The bottom layer is similar in composition to the outer layer. When a rock or some other type of debris hits your windshield causing a familiar bulls eye, or star like break, it leaves an open hole in your windshield, exposing the soft PVB layer. Debris hits with such force that the object often breaks apart leaving even more debris in the damaged area.
Simply placing a piece of adhesive tape directly over the damage right after it occurs has many benefits.
Covering the damaged area prevents more dirt, debris and moisture from accumulating in the break.
The resin used in the repair process does not mix with moisture (water) and can lead to a cloudy looking repair.
Covering the damaged area reduces some of the pressure placed on an already damaged windshield. Wind pressure causes the top and soft middle layers of the windshield to pull apart which in turn can cause the damage to spread.
Its a simple solution that has a multitude of benefits.
Why You Should Repair A Rock Chip Soon After It Occurs
A windshield chip can crack at any time. Increased pressure could turn the chip into a crack at a moment’s notice – including while driving.
Repairing a chipped windshield can take as little as 30 minutes.
Windshield chip repair saves money. The cost of windshield replacement far exceeds that of repairing a chip.
The longer you leave a windshield chip, the more likely it is to get worse. In cold weather conditions, windshields contract. As a result, windshield chips and dings are more likely to spread in a horizontal fashion. According to a study conducted by the Motor Industry Research Association (MIRA), chips were likely to spread more than 80% of the time at temperatures of 14 degrees (-10° C). Windshield chips tested at 32 degrees were nearly 60% likely to crack.
Warm weather conditions cause windshields chips to spread into vertical cracks. The cooling effect of the air conditioning unit makes chips even more likely to crack.
Driving over a speed bump, pothole or other uneven terrain can cause a windshield chip to crack. The force radiates through the car and puts additional pressure on the edges of the chip, which can quickly lead to a crack.
Although it is not possible to say how long it will take for a windshield chip to spread, simply driving will eventually cause or lengthen a windshield crack.
Getting a your windshield repaired early can stop the damage from spreading and prevent the need for a windshield replacement.
You car windshield is not just “another piece of glass.” It is one of the most important vehicle safety features and the only thing between you and everything else on the road. Your windshield is comprised of 3 laminated layers. The top layer on the outside is typically UV glass as is the bottom layer, inside the car. The middle layer is soft and comprised of PVB (Polyvinyl butyral (or PVB) a resin usually used for applications that require strong binding, optical clarity, adhesion to many surfaces, toughness and flexibility.
Trained windshield repair technicians inject an acrylic resin into the damaged area on your windshield using special tools and high pressure. The process involves both pressure and vacuum cycles. This is where do-it-yourself kits fall short and usually fail. Do-it yourself kits only allow for one pressure cycle. In other words, you get one shot. If it fills, you are lucky and what ever the result, you are stuck with it because once the resin is injected and cured (hardened) there is no do-over. The do-it-yourself kit does not allow for a vacuum cycle so there is not way to remove trapped dirt, debris and moisture from the damaged area. The results are usually cloudy at best and unpredictable. In short, you risk ruining your windshield to save money and in the long run, home kits lead to the need for a costly windshield replacement.
While on the topic, I might also mention that attempting to repair a damaged windshield with household products, such as, Crazy Glue, and nail polish will ensure the need for a replacement windshield.
There is no substitute for using a trained professional when it comes to something as significant as one of your vehicle’s most important safety features.
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.