Rock Chips & Your Car Heater


Most of us has experienced running a cold glass under hot water, or visa versa, and having the glass shatter. This occurs because glass is effected by temperature, and tends to expand or contract in the heat and cold.

Here’s what noted, Microbiologist, Ryan Carlyle says. “Like most substances, glass expands as it heats up and shrinks as it cools. It also has moderately low thermal conductivity. And it’s hard but brittle. These three facts are why glass can crack when suddenly cooled. And ironically, the same principles allow us to make tempered glass extremely strong.

Imagine you have a hot, thick piece of glass. It’s uniformly heated and thus is uniformly thermally-expanded. Now dunk it in an ice water bath — the surface will cool almost instantly on contact, but the middle has to wait for heat to conduct away before it can cool. That creates an uneven thermal profile, where the surface is cold but the inside is hot.

As a result, the surface shrinks! Or it tries to. The hot inner glass prevents the surface glass from shrinking. This creates a powerful stress profile through the glass — the surface is trying to shrink, but can’t, so it is forced into tension. The hot core is trying to stay the same volume, but the surface is squeezing in, so the core undergoes compression.”

The rock chip or small crack on your windshield has already weakened the structure of the windshield, regardless of how small the damage is. When it is cold outside and you run your car heater, you create the same circumstance as described above and it is much more likely the damage will quickly spread.

The same thing happens in the Summertime, only in reverse. In the Summer, the outside of your windshield gets very hot, sometimes well over 110 degrees hot! To cool off, you blast your air conditioner and the temperature differential causes the surface of the glass inside the vehicle to cool way faster than the middle layer of glass. Like running a cold glass under hot water, or visa versa, the glass is much more likely to shatter. So is your windshield.

Here’s the take-away: The rock chip has structurally weakened your windshield. Temperature, weather, condition of the roads all now play a part in whether your windshield will remain in tact long enough to have it repaired. As soon as you have a rock chip, dry off the windshield with paper towels and place a small piece of clear packing tape over the damage on the outside of the windshield, to help keep dirt, debris and moisture out of the damage. Contact us to schedule a repair. Do not use your vehicle air conditioner or heater until after the repair. 


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