Why do Cars Have Lines on The Rear Window?

There are so many things we take for granted when it comes to our cars. Thousands of little details are overlooked every day. Sometimes, you may see something on your car and not really know why it’s there. If you’ve ever wondered what the little lines that run across your back window are, let’s fill you in!

The lines are wires built into the window to quickly defrost it

Rear window defroster lines on 2016 Ford Fiesta ST with trees and shop in reflection
Rear window defroster lines on Ford Fiesta | Braden Carlson, Motorbiscuit

According to Miracle Auto Glass Center, the lines that run through your rear window are actually wires that heat up to defrost it. The official name of these lines on the rear window of your car is secondary defrosters.

Though many cars have vents that defrost the front windshield as well as the front windows, you’ll find that most vehicles don’t have vents to defrost the rear. Sure, the window will eventually defrost or de-ice due to ambient air temperatures rising in the vehicle with your heater on. However, the secondary defrosters can defrost your rear window in a matter of minutes without relying at all on the vehicle’s heater.

The secondary defrosters are usually a grid of wires that are all connected to one power source. So, once you activate them, all of them begin heating up immediately. However, unlike the heating system that provides warm air to the vents in most cars, the secondary defrosters do not rely on engine and coolant temperatures to heat up. So, even if the rest of your car is not warmed up yet, the secondary defrosters will still do their job.

Having properly functioning secondary defrosters can be a lifesaver on cold winter mornings. Think about it; how many times have you been running late and didn’t have time to wait for your car to warm up before you needed to leave? Sure, you still have to scrape most of the windows on a frigid morning. However, the secondary defrosters can do their thing while you clear everything else and save you a bit of time.

Moreover, they can provide much-needed rear-window visibility.

Why don’t front windows and front windshields have secondary defrosters?

Rear window defroster lines on 1991 Volvo 740 with trees and trailer in reflection
Rear window defroster lines on Volvo 740 | Braden Carlson, Motorbiscuit

Naturally, one might think that it would make sense to use this technology throughout all the glass on the vehicle. That way, the secondary defrosters could quickly clear all of your windows without having to exert your precious energy with a window scraper. However, there are a couple of reasons why that’s not a great idea.

Most obviously is visibility. While rear-window visibility is definitely important, having the secondary defroster lines in your rear-view mirror isn’t all that distracting. Think of the last time you had a rock chip or big bug on your windshield, though. That can be quite distracting. Now, imagine having two dozen black lines running the length of your windshield and side windows.

Additionally, front windshields use two-pane laminated glass with a layer of plastic between the glass. This way, if the windshield brakes, it doesn’t shatter into a million pieces all over the cabin and its occupants. Attempting to insert secondary defrosters into windshield glass would require a lot of re-engineering.

However, some companies are actively developing invisible electric glass heaters for this very application. Though there are currently no vehicles with this technology on the market, it may not be far off.

Additionally, some vehicles do have wires in the windshield at the base to warm frozen windshield wipers.

So, next time you’re with someone who’s curious about what the lines do that run through your back window, you can tell them all about secondary defrosters!

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