At some point prior to this moment in your life, you likely discovered the windows in the back doors of most cars don’t go down as far as the front ones. Most likely, this realization came at a young age; while your parents or older siblings sat up front, you were exiled to the aft row, forced to decide between staring at the back of the seat in front of you or risk car sickness by gazing out the side. In hopes of nipping the latter in the bud, you might well have tried to roll down your window and pop your arm on the sill, the way all those adults do…
...only to find the window only goes down part of the way.
Given a child’s inquisitive nature, you may well have asked those bigger, more experienced humans in the front seats why their windows open up so much more than yours do. Perhaps they offered up a simple “I don’t know,” or maybe they just pretended not to hear you. But there’s a fairly decent chance they offered up some sort of explanation along the lines of: “Well, that’s so you don’t fall out of the car.”
If so, well, I have some tough news for you: you were misinformed.
At face value, of course, the idea that a car’s rear windows are designed to only slide down so far in order to keep kids safe seems to make sense. Children, of course, are rambunctious creatures, second only to cats in their ability to create chaos out of order; the idea of one slipping out of their seat belt and accidentally falling out of an open window at 75 mph feels both horrifying and realistic. Why wouldn’t carmakers try and prevent such an occurrence with a simple fix?
Think about it for a moment, however, and that idea falls apart. After all, most rear windows still go down more than far enough to allow a child to squirt through. (They might be effective against adults, but if you have a problem with grown-ups trying to squeeze out of your car while it’s moving, the FBI might like to have a word with you.) And cars have other, even more effective means of keeping children from escaping the back seat: specifically, window lockout switches that give the driver complete control over the aft glass and child locks that disables back seat occupants’ door handles, much like in a cop car.
So why then do those rear windows only go down part way in most cars? Well, it’s actually due to the design of the rear doors. Simply put, there’s no place for the glass to go when the window rolls down.
While the bottoms of a car’s front doors stretch just about all the way to the vehicle’s belly along their entire length, the lower edge of the rear door of most cars tends to curve upwards along the back axle’s wheel well. A car’s windows are larger than you probably think — and unlike, say, a convertible top, there’s no compacting them when they stow away. They need as much vertical space in the door as possible to hide away...
...which, in turn, generally means a window can only go down so far as the tallest point of the bottom of the door.
Carmakers have found some ways to get around this, to an extent. Some new vehicles (i.e. the Mercedes-Benz A-Class) have bisected their back-door glass into two parts: a squared-off forward section that rolls up and down, and a rear part that's fixed in place. In this scenario, the section that rises and falls is only as wide as the lowest part of the door, enabling it to completely open the way the windows in front do.
So there you have it. Rear windows don’t go down only part of the way as a safety measure; they do it simply because they physically can’t go down any further.
